TCAS philosophies

[ATC Watcher]

I decided to start this thread as a follow up of the “ Ueberlingen trial “ thread which was used by some to re-open what I call the “ TCAS Philosophies “

If a mod would like to move the posts in the Ueberlingen trial thread here, welcome, but not necessary.

The TCAS system is very complex, there is no more than a dozen persons in Europe and in the USA who fully grasp everything, and no more than a few more dozens that understand more than 50% of it . ( I classify myself in the second category )

What is below is not a lecture, this is basic stuff to help you understand where we are .

A few facts :

The Ueberlingen collision revealed to the public ( but not to the specialists) a few flaws in its logic , mainly that the RA sense reversal did not work. (That is when one aircraft follow the RA , but the second maneuvers in the opposite direction of its RAs and both aircraft end up towards each other ).
There was request made from Eurocontrol to re-open the work on TCAS and make a software change to correct this flaw, but the FAA refused initially as version 7 was “ final” in their eyes and that the TCAS Team had been disbanded. Nevertheless though RTCA , discussion were restarted and what became known as Change Proposal 112 ( or CP112 ) became a fact, later it became 112E ( enhanced) , and , since we were re-opening the Pandora box, we added a few more flaws corrections mainly one ( CP115) on the replacement of the RA ‘ Adjust vertical Speed ,Adjust “ to :” Level off , Level off “ and another one (CP116) on weakening RAs at low altitudes .

There is a chance now that we will get a new version 7.1 in the” future” . I do not put “ near future ” as the date is still fought between Eurocontrol which would like 2010 for all, and the FAA which says “ much later” for CP 112E ( sense reversal ) and is against CP 115 , the “Adjust/ level off “ change .

In case you are lost in the TCAS / ACAS versions here is a short recap :
TCAS is a brand name. ICAO call its Specs ACAS
2 current sofware versions of TCAS flying around : 7 and 6.04 A
Udate to 6.04A was free, 7 was not . 7 was pushed by the Europeans, FAA was against it. 7 is RVSM compliant , 6.04A is not.
If a new version comes ( 7.1 ) , will it be mandated ( i.e free) or will it be recommended ( at a cost ) is a good question to which I do not have the answer.
Initially there were 3 TCAS intended , TCAS I : TAs only , TCAS II RAS in vertical plane, TCAS III RAS in both vertical and horizontal plane. Work on TCAS III stopped as the logic proved too complex, and vertical plane RAs were more effective at avoiding a collision. So there will most probably never be any TCAS III .

This is the situation today .

Now questions asked were : is a new Ueberlingen possible today :

With TCAS sense reversal logic flaws : yes , nothing has changed since 2002. same logic. Same version flying around (7.0).
With procedures : maybe : ICAO has revised its documentation and most training material have highlighted the way to follow in case contradiction between ATC clearance and RA. But incident statistics so far both from USA and Eurocontrol show that some controllers still issue clearances contrary to RAs and that pilots still choose to follow ATC instead of RA.
( a recent case in France, with US trained and experienced 767 pilots from a major US airline , so no need to think it only affect exotic or Russian pilots )
Main reason : lack of understanding on how humans and automated machines function in reality. Prof Ladkin in his excellent paper http://www.rvs.uni-bielefeld.de/publications/Reports/SCSS04.pdfon causal analysis after Ueberlingen call this “ Sociotechnical systems “ .

Now a last statistic : ( coming from Bill Thedford, of the Boston MIT LLC, one of the top 12 in my first classification earlier) Ueberlingen type encounters ( sense reversal ) occurs at 4.7 x 10 to the minus 6 per flying hour , or 58 events a year in Europe .If we expand this model, Bill predict that we could have 1 Ueberlingen –type mid air collision every 4 years due to the logic flaws.

The conclusion of this all :
Do not philosophy about TCAS : It is too complex a system. My advice : if you are a pilot : follow the RA, and if you are a controller and you have to give anti collision instructions , give a turn as well, in case TCAS comes in , it will complement your instruction instead of nullify it.

If you found this too long , this was the ( very) short version.
Safe flying.

[airmen]

We have very clear company procedures which are to follow the TCAS and not the controller, and I do follow the orders, they saved already my live once at least!
However, do not use the TCAS screen as a radar, target position are innacurate.
I have seen too much pilots using their screen for avoiding and or criticise the ATC...
The thing is to use the system as it should be used and the training and recurrent given actually will tell you exactly that.

[ray cosmic]

Thank you for the information. Actually I am quite astonished to hear conflicting situations still exist. Wasn't THE lesson of Ueberlingen that pilots shall follow TCAS alert in preference of ATC instructions?
I don't exactly know how much technical info one needs to have to operate in unison with the system. Its more like in flightschool: don't argue, just do it!

[berkut]

first of all, thank you very much for the information.

In my opinion it should be very clear. FOLLOW THE TCAS COMMAND. If you see, that a conflict can arise in a couple minutes, because you see it on the screen, than an advice from the ATC is helpful. If the "target" has already reached the TCAS warning area its clear. Follow the commands. The ATC controller is not a good help at this time as he has in the worst case a position indication on his screen which is 6 or 12 seconds old, depending on the radar.

There are only two scenarios which we have to ignor the TCAS command. In the event of a EGPWS call out (hard warning) or a windsheer warning.


Überlingen was a tragic and I hope we never have it again.

The guys on the flights DL 37 / CO 25 on 1987 July 8th had a lucky day, the missed each other by centimeters over the atlantic.

With the new technique (TACS III) and a proper pilot education it should be possible to avoid such scenarios in the future.


Happy flying and trust your avionics (with good airmenship) :)

[joernstu]

@ATC Watcher

Depicting the situation today, you said:

But incident statistics so far both from USA and Eurocontrol show that some controllers still issue clearances contrary to RAs and that pilots still choose to follow ATC instead of RA.I think that one can argue about the controller in the Ueberlingen accident, if he really has issued a clearance contrary to the TCAS RA.
He had to maintain separation of the aircraft, which (if I have read the report correctly) normally meant 5NM and - due to inactive STCA - at the night of the accident had been set to 7NM.

The TCAS RA came before the aircraft underrun separation (at 7.11NM) and the controller had just finished wording his clearance when the aircraft under-run separation.


Do you (or anyone else in this threat) know, if there is work going on to give the controller feedback on the RAs issued by TCAS?

[Capt Pit Bull]

OK,

Pre Ueberlingen, many operators had ops manual phraseology allowing the crew to disregard an RA under certain circumstances. Post Ueberlingen that option no longer exists.

Much comment has been made regarding the differences in training philosophy that led to the accident.

The thing is, even before Ueberlingen, every source I ever saw gave specific advice that an aircraft should NEVER be manouevred in the opposite sense to an RA.

Loss of flexibility about following an RA (pilots discretion) is, imho, a bad thing. I've talked about this before on several threads, and I am not going to rehash the arguements or get into a pissing contest with anyone who slags me off for saying that following an RA may not be the safest course of action. The simple fact is that TCAS has good info about some threats to your aircraft, but there are a bunch of things it doesn't know about. As such 99.9% of the time its a good bet to follow the RA but as Pilots we should retain final control of our aircraft to cover the 0.1%. Thats why they pay us the big bucks, if you don't like it get off the jet and let them replace us entirely with computers.

At Ueberlingen, what did the Tupolev crew need to do to avoid the collision?

Well...... nothing in fact. Thats right, not a thing.

All they had to do was NOT manouevre in the opposite sense to their RA. Unfortunately they did, not once, but twice.

Ueberlingen type events had happened before. They were standing out in the JAA TCAS transition program newletters (but fortunately with horizontal separation saving the day). The simple fact is that most TCAS nasties contain at least one, and often several, manouevres opposite to the RA. This is the key point, focussing on the need to follow an RA is missing the point, and sooner or later somebody will find themselves in a situation where they can't follow it, or to so so will be unsafe, and then they'll be in a double bind.
ATC Watcher: Are there still 6.04 users flying around?

I agree with the 'add a turn' concept. It used to be part of the training offerred to controllers at LATCC (I spent a fair bit of time talking to their terminal training team when I put our companies TCAS program together). However I believe that they don't suggest this anymore.

Airmen: Absolutely. Over use of the traffic display is a common error.

joernstu: Mode S contains the basic architecture and message formats to downlink RA data to ATC. Apologies if you already knew that. Last I heard (which is a couple of years at least out of date) it was being trialed at a couple of places. I don't know what the current state of play is, anyone know?

pb

[ATC Watcher]

I think that one can argue about the controller in the Ueberlingen accident, if he really has issued a clearance contrary to the TCAS RA.

No, he did not. In Ueberlingen , the controller did not issue a clearance contradictory to TCAS. He issued the clearance BEFORE the TCAS annunciated the RA.
But , unknown to him , TCAS " choose " (for various reasons too long to explain here )to make the TU154 Climb and the 757 descend.
That put the TU154 crew into conflict/doubts . The crew choose to follow ATC.
Had the RA configuration be different ( TU154 to Descend and B757 to climb, there would have been no collision, as the RA would have been in the same sense as the ATC instruction.

is there is work going on to give the controller feedback on the RAs issued by TCAS?

Yes, but on down-linking the RA message to ATC only ( Advanced studies made by Eurocontrol , called FARADS ( Feasibility RA down Link) and RADE (RA Downlink simulations in Bretigny)
Consequences of D/L RA to ATC are complex and many think ( inlc. me ) that it may induce more problems that it will solve.

[PBL]

they saved already my live once at least!

There is a phenomenon here worth thinking about. When I voiced some of my concerns about TCAS on a pilot's mailing list some years ago, I was deluged with affidavits from pilots who claimed that TCAS had saved their lives. If all of these claims had been true, that would have amounted to some 20 or so midair collisions in a sample of a few hundred pilots (the members of the mailing list) over the course of, say, ten years. Let's assume 10 "life-savers", that is, otherwise-midairs, for 500 pilots in 10 years. That is an average of one "midair collision" per 500 pilots per year.

There are about 15,000 aircraft from Airbus, Boeing, MD, Lockheed still in service (calculated from figures in Flight International, 24-30 October 2006). So that's not counting the "commuters", or the Tupolevs. Those aircraft are in service, let's say, 14 hours per day, for a total of 210,000 hours per day, or 5,880,000 hours in 28 days. Duty rosters take 100 flight hours for a pilot in 28 days to be relatively high, but let's use this figure, since it will lead to a conservative estimate. It means there are about 59,000 captains needed to fly those 5,900,000 hours every 28 days. With one "midair" per 500 captains per year, and accounting for the fact that it takes 2 captains to have a "midair", we come to a figure of 59 "midairs" per year.

Compare this with actual midairs amongst airline transport aircraft. 1959 Grand Canyon, 1976 Zagreb, 1978 San Diego, 1985 Cerritos. And then, since the advent of TCAS, 1995 Namibia (involving a German military machine which I do not believe was TCAS-equipped), 1996 New Delhi, 2002 Überlingen and 2006 Amazonas. That is 4 before the advent of TCAS (mandated 1991, I believe) and 4 since (16 years).

Those crediting TCAS with all these "saves" need to explain why, before the advent of TCAS, there were a statistically-negligible number of collisions in the history of airline flying, and upon the introduction of TCAS there are suddenly almost 60 per year. Even considering the growth of air travel, that is an unbelievable jump.

The obvious answer is, of course, that most of those "saves" would not have resulted in collisions, despite what their pilots thought or think.

Consider the following, as a thought experiment. Up to the Congressional mandate for TCAS, there were three collisions involving airliners in the U.S.: 1959 Grand Canyon, 1978 San Diego and 1985 Cerritos. Suppose TCAS initiated RAs, not for 2 TCAS-equipped aircraft, but only when one had TCAS and the other only Mode C. That would have satisfied the issue worrying the Congresspeople, namely incursions of GA aircraft into zones of intensive airline operations, and it would have saved one of the midairs since (Überlingen). The other three midairs in the TCAS era occurred to non-TCAS-using aircraft.

Let us compare TCAS with another highly-lauded system for avoiding accidents, GPWS/EGPWS. Also a system introduced primarily by one manufacturer (again Honeywell). And in contrast to TCAS addressing a real, continuing problem which still leads to many aircraft losses per year, namely CFIT. EGPWS is essentially a private development (stemming from the work of Don Bateman and colleagues).

It is interesting in this regard to notice how TCAS is self-advertising and EGPWS not. A pilot can say "TCAS saved me" without it reflecting on himher self, but rather on some air traffic controller establishment somewhere. No pilot can say "EGPWS saved me" without someone else asking how heshe lost situational awareness in the first place (with rare exceptions in which it is clear how, such as with British Mediterranean at Addis Ababa).

So the statistics were never there to say "TCAS is a good thing", and it is self-advertising, through pilot "save" stories. But there are smart people in aerospace who, no matter whether they think TCAS is a net gain or a net problem, are able to look at the facts and the situation just the same as I am now. Airline CEOs, for example. So the question is: why are collision avoidance mechanisms still being promoted, researched and supported to such a great extent?

I think there are two reasons. One is that they are one attempt at a solution to a problem which no one knows yet how to assess accurately, namely the avoidance of midair collisions, and no one is willing to risk misjudging (underestimating) the likelihood of those. The other reason is that effective on-board systems make collision avoidance a contract between the two (or more) aircraft involved, which transfers the costs of collision avoidance away from large ATC organisations. Since the airlines and their passengers ultimately fund these organisations, one could see the financial benefits to airlines of bringing this function in-house (after the initial cost of installing the equipment, which has n any case been mandated). Add to this that collision avoidance is one of the brakes on moving to "free flight" (self-routing IFR at high altitudes), which airlines believe would bring them great cost reductions, and one can see that TCAS+free flight is a politically sellable package which could reduce the costs of air travel even further. Without some ATC-independent collision-avoidance system regarded as effective, this move could not happen, and TCAS is the only technology in town.

PBL

[brain fade]

Would it be possible to have the TCAS generate the radio calls? Or at least comunicate to the controller in some way what is actually happening.

Seems to me that it can be quite tricky to get the call off especially as there are, for obvious reasons, often other calls being made at the time. Even a short duration chime or tone might do the trick.

[Beakor]

Those crediting TCAS with all these "saves" need to explain why, before the advent of TCAS, there were a statistically-negligible number of collisions in the history of airline flying, and upon the introduction of TCAS there are suddenly almost 60 per year. Even considering the growth of air travel, that is an unbelievable jump.
PBL

You also need to consider the increase in navigational accuracy and altimetry. Events which would previously have been close airproxes due to kit inaccuracy could well now be collisions with everyone within a few metres of the airway centreline and a few feet of their FL.

Even so, still agree with PBL, it is an unbelievable jump

[PBL]

Since people here are also advising always follow the RA, without exception, it might be appropriate to point again to my paper, which ATC Watcher referenced, in which I note that

* the TCAS algorithm is known to resolve all conflicts between two aircraft (Lynch, Lygeros, MIT 1997)
* It is still not known whether the TCAS algorithm resolves all conflicts between three aircraft

and I have offered (since 2002) a PhD to anyone who can definitively answer the question whether the TCAS algorithm resolves all conflicts between three aircraft.

I also pointed out that, in the Überlingen collision, the Russian commander believed they were in a possible three-aircraft conflict (I wondered about that in a paper I put on the WWW in August 2002, and it was confirmed by the report in 2004). Further, in the decision situation they were given, it was rational to choose to descend (I also went to some pains to point out that saying "it was rational" and supposing that that was the reason he continued to command a descent are not the same thing at all).

Capt Pit Bull suggests that following the RA is appropriate some X percentage of the time, and maybe inappropriate some other percentage (100-X). (He chose X = 0.1, but I presume he was illustrating his point, rather than suggesting a real value for X.) One of the main issues with TCAS, for me, is that no one knows what the value of X is.
What if it turns out to be 50?

PBL

[ATC Watcher]

Capt Pit Bull :
excellent remark about focusing on the fact that an aircraft should NEVER be manouevred in the opposite sense to an RA. . If this only would be understood and followed by all pilots, most of our problems will be solved ( except for encounters with SSR-only intruders of course)

Quick reply :
ATC Watcher: Are there still 6.04 users flying around?

Still a lot in the USA to start with. In a recent publication ( March 2007) the FAA ATO reported that :
" US aircraft were not mandated to equip with Version 7, but Europe elected to mandate this version. Therefore, US aircraft flying into European airspace must have TCAS Version 7 avionics. Currently, two thirds of the US commercial fleet is TCAS Version 7 equipped.."
Which means 1/3 of commercial fleet is still on 6.04 and some of General aviation jets and turboprops on top of that I guess .
Rest of the world : status unknown on the domestic fleets. All in all , still a fairly large number, I would say.

[Capt Pit Bull]

Capt Pit Bull suggests that following the RA is appropriate some X percentage of the time, and maybe inappropriate some other percentage (100-X). (He chose X = 0.1, but I presume he was illustrating his point, rather than suggesting a real value for X.) One of the main issues with TCAS, for me, is that no one knows what the value of X is.
What if it turns out to be 50?

Peter,

You're quite correct, I was not attempting to be scientifically accurate but rather speaking conceptually, (I'm not even remotely qualified to try and put a precise numerical value on it).

regards,

pb

[antic81]

Hi there

When looking at collisions, or near collisions, in the modern, largely TCAS II equipped era, we must also remember that routes are now being flown through far more precise and exacting corridors of airspace using GPS and IRS navigation systems, and these routes are being flown through some very poorly manned and ill equipped airspace (i.e. Africa) therefore increased chances of colliding with other aircraft is higher.

I am of the opinion that when looking at post-TCAS vs pre-TCAS collisions, we need to take that into account as it does not give an accurate portrayal of its effectiveness.

Cheers
Anthony

[Kit d'Rection KG]

1. Make the legislation fit the technology;

2. Accept that it's an imperfect system driven by greed (for cheap travel, higher share prices, etc);

3. 'Accidents' will happen;






















4. Invest in Kenyon

[Kit d'Rection KG]

Oh, and by the way, there's at least one very well respected academic in Europe who can show (by 'proof') that the TCAS probabilities are bunkum anyway, and providence is adjusting the fatality rate without our knowledge...

[alf5071h]

Re Überlingen, one of the alternative views of this accident was that it involved ‘poor’ worldwide/industry communication.
ACAS / TCAS evolved from a US concept (to solve a specific US problem) into something that the industry found to be of great value and thus was approved by ICAO. The ‘failure’ was that the procedures (operational assumptions and crew actions) that evolved in the US were not communicated worldwide – they are now via ICAO. I suspect that there are still many operators / countries who do not appreciate the need to follow the system’s instructions. I recall that at the time of the accident one PPrune poster suggest that in his country (Africa), every pilot would turn when responding to TCAS!

I wonder if the semi-circular / quadrangle flight level system would have prevented Überlingen? Was the lack of this feature a failure to ‘defend in depth’, - over reliance on radar systems, need to handle more aircraft in a crowded airspace, or the human desire to help others by giving them direct routing?

I note similarities with the TAM overrun accident re worldwide communications; i.e. the knowledge of, adoption, and training for the revised crew procedure for the MEL single thrust lever operation.

Re EGPWS; several ‘Saves’ are in the paper “Celebrating TAWS ‘Saves’: But lessons still to be learnt”. (www.icao.int/fsix/_Library%5CTAWS%20Saves%20plus%20add.pdf) To my knowledge none of these events have been claimed as saves by the crew, primarily as the pilots were unaware of the severity of situation that they encountered. Many of the HF contributions / human behaviors in these incidents can be seen in TAM and other overrun accidents, and the issues of situation unawareness and the failure in ‘the last defense’ - the crew - failing to react or reacting inappropriately to warnings, arises in the discussion in this thread.

: is a new Ueberlingen possible today : alas ‘yes’ as humans are part of the system.

[airmen]

TCAS is designed to work according manufacturer manual, it is stipulated here that pilots have to follow orders to escape, so why argue with that and prefer to follow ATC orders?
Maybe those guys need to have a good Sim session to understand by doing it wrong to be able to learn something?

PBL wrote: The obvious answer is, of course, that most of those "saves" would not have resulted in collisions, despite what their pilots thought or think.

I saw the conflicting aircraft (a Beech Baron) at the last minute (sun in the back) during the escape manoeuver, he was very close and was flying opposite course at the same altitude (controller error) and I can tell you that the controller went by himself to meet us after landing, he told us that he saw nothing and as such gave us no instructions. Off course we followed the TCAS but I can not tell you what the other pilot did...

[4Greens]

There are many analogies with GPWS warnings. If you get either of them go. Both these responses need to be trained in the simulator so that they become automatic. The warnings of course need to be unexpected.

[PBL]

Perceptive comments as usual from alf.

one of the alternative views of [the Überlingen] accident was that it involved ‘poor’ worldwide/industry communication.


I wouldn't characterise the view as "alternative"; it was thoroughly investigated and all but proved in the report.

ACAS / TCAS evolved from a US concept (to solve a specific US problem) into something that the industry found to be of great value and thus was approved by ICAO

While this is generally true, I am not sure that TCAS was introduced to solve a specific problem. I was around in California at the time of the 1978 PSA midair and already flying around the state at the time of the 1985 Cerritos accident. TCAS had been under development then for quite a while, by the U.S. research contracting corporation Mitre, on government money. The precise history eludes me at the moment, but Honeywell claims on their WWW site (where there used to be a history) "over 40 years of TCAS development experience". That would make at least 18 years up to Cerritos. In other words, there was a system there waiting to be sold.

So there are two polar-opposite ways of construing developments. One is that some far-sighted industrialist knew that an opportunity would arise, developed a system, and grasped the opportunity with aid of the appropriate lobbying. Another is that some far-sighted industrialist anticipated the future need for some such system two decades before it became necessary. The difference between the two views is only that of whether one believes that such a system was/is necessary. The evidence was obviously lacking at the time of Cerritos; I don't know that there is any way to decide the question now.

The feature of TCAS which most disturbs me, and which I believe should continue to disturb anyone, is that we now have two institutions whose (main) function is collision avoidance, TCAS and ATC, and the former works *against* the latter. With 2,000 ft vertical separation, 1,500 fpm vertical manoeuvring (at least on first RA) and 30 seconds to CPA, the math says that there is no obvious conflict. However, with 1,000 ft vertical separation, a TCAS-manoeuvring aircraft could get potentially within just over 100 vertical feet of another, thus initiating a further RA. This used not to be true at cruise altitudes, but is so now with the introduction of RVSM.

Eurocontrol did a study, called ACASA, which amongst other things looked at simulated RVSM/TCAS interactions. They massaged about two days' worth of radar data to give aircraft RVSM-type vertical separation, and scanned it for potential TCAS interactions, and saw none. Eurocontrol then claimed in their safety case for RVSM that there were no identified risks with TCAS and RVSM (citing the ACASA results as proof, in personal communication). The problem is that they were aiming for a far higher TLS than looking at
the statistical equivalent of two days' radar data would allow . (General considerations from statistical theory suggest that it will be impractical to assess TCAS interactions statistically to currently-required levels of safety.)

In contrast, EGPWS, while requiring a manoeuvre, does not operate in conflict with ATC, because an aircraft responding to an EGPWS warning is already well below minimum altitude and therefore well out of airspace in which there are other controlled aircraft.

PBL

[ATC Watcher]

One of the main issues with TCAS, for me, is that no one knows what the value of X is.

Indeed , and that debate is hindering the obvious solution to our problems : making the RA a command, i.e. an obligation to follow in all circumstances.
In the US, at its conception ,the MITRE/FAA Lawyers prevented this and insisted we call the TCAS avoidance resolutions ' Advisories " and not " Commands" therefore it is called an RA.

But besides this " value of X " technical issue , there are other aspects as well.
I always refer to the " German glider " one , following a presentation by a famous German 747 Capt in ICAO : to explain why he will never agree that following RA a should be re mandatory in all cases he said :
,I am in IFR in class G airspace and I get a climb RA , but just above me are gliders without transponders , my first duty as Captain is to maintain the safe operation of my aircraft and stay below the gliders , therefore I will elect not to follow the RA in this case.
And he is right. So an advisory it remains.

TCAS is designed to work according manufacturer manual, it is stipulated here that pilots have to follow orders to escape, so why argue with that and prefer to follow ATC orders?

Indeed why argue ? Mainly for 2 reasons : one is human : people react differently to automation that others and culture also plays a role. PPRuNe is full of discussions on this issue . Studies shows that even as we speak today 30 to 40% of the " Adjust vertical speed " RAs are not followed by pilots. ( it used to be 60% 3 years ago ) . So it is not only ATC " interference".

The other reason is the risk , the " value of X" mentioned above. If it would have been all without risks, , coupling the RAs to the auto-pilots would have resolved the issue and would have been done.

[PBL]

TCAS is designed to work according manufacturer manual, it is stipulated here that pilots have to follow orders to escape, so why argue with that and prefer to follow ATC orders?


Please read my analysis of the decision problem presented to the Bakshirian crew at Überlingen. They were faced with an "intruder" at 10 o'clock which they saw, and an unknown conflict at 2 o'clock which they didn't see, and for which they had an advisory to descend (that is, he was at or above their altitude).

What would you do? Climb towards another conflicting aircraft that you don't see but ATC does? Or avoid him, descend towards an aircraft that you do see, and hope to avoid him using visual means?

Please give some good reasons for your answer that will also be good reasons for any other pilot in this situation.


I saw the ...Beech Baron.. at the last minute ..... during the escape manoeuver ...... he was very close and was flying opposite course at the same altitude ..... the controller ....... told us that he saw nothing and as such gave us no instructions. Off course we followed the TCAS but I can not tell you what the other pilot did...

That's very interesting and I agree with you that it counts as a save (but of course it doesn't contradict my observations on claimed "saves"). I have a number of observations.

First, Beech Barons don't have TCAS (I don't know that the avionics are approved for installation in a Baron, does someone?). So he is painting Mode C, otherwise you wouldn't have picked him up on your TCAS. If he is painting Mode C and you picked him up, then how come he is not showing up as the usual box as big as yours on the controller's screen? I think there is a lot more to this incident than you are able to tell us. If the controller said he saw nothing, and he was telling the truth, then it must have been a system failure rather than controller error, and his going to meet you and apologising was someone else's way of covering up a technical system failure.

PBL

[PBL]

" German glider " one , following a presentation by a famous German 747 Capt in ICAO

Very interesting. The difference between the "German glider" decision problem and the Bakshirian one is that the B747 knows the non-painted conflicts are above him. Bakshirian doesn't know that.

The "German glider" decider is therefore able to maintain altitude without conflicting with non-painted traffic, and equally able to hope that the intruder is TCAS-equipped and responds to a descent RA. The "Bakshirian" decider is not able to maintain altitude without conflict with non-painted traffic, as far as he knows.

The German-glider decider can follow the dictum not to manoeuvre contrary to an RA. The Bakshirian decider cannot follow this dictum without risk.

PBL

[Capt Pit Bull]

Peter,

In contrast, EGPWS, while requiring a manoeuvre, does not operate in conflict with ATC, because an aircraft responding to an EGPWS warning is already well below minimum altitude and therefore well out of airspace in which there are other controlled aircraft.

That might be stretching it a bit.

You can have EGPWS warnings when the aircraft is physically in the correct part of the sky. A good example is a that map shift on final approach can easily lead to an escape manoeuvre. We had a spate of this on one type I flew. Now, flying a go around from that point in the sky shouldn't (in theory) unduly tax ATC but at a busy multi runway international airport..... well, you get the picture.

Not that I disagree with the broad thrust of what you are saying.


pb

[PBL]

You can have EGPWS warnings when the aircraft is physically in the correct part of the sky. A good example is ..... map shift on final approach

Thanks, pb, you are right. I had forgotten that as I was writing.

PBL

[joernstu]

I wonder if the semi-circular / quadrangle flight level system would have prevented Überlingen? Was the lack of this feature a failure to ‘defend in depth’, - over reliance on radar systems, need to handle more aircraft in a crowded airspace, or the human desire to help others by giving them direct routing?

Yes, Überlingen would have been prevented, if both crews had made a semi-circle to the right (or the left). If promoting this to a standard resolution to a collision threat though could be problematic in crowded airspace. Note that the issued TCAS Resolution in the Überlingen cause itself would have prevented the collision - as would have the descend order to the Tupolev issued by ATC.

ACAS III was planned to implement horizontal RAs.

[Andu]

PBL, I see others have answered your question (back on page 1) already, but let me add my tuppence worth to agree with them Those crediting TCAS with all these "saves" need to explain why, before the advent of TCAS, there were a statistically-negligible number of collisions in the history of airline flying, and upon the introduction of TCAS there are suddenly almost 60 per year.The answer, as already given, is GPS and IRS. Anyone who remembers reading Ernest K. Gann's classic "Fate is the Hunter" will remember the incident that is described in the very first pages of the book - where he and his co-pilot see, far too late to avoid it, another aircraft pass 50' overhead and disappear into the night.

I know it won't fix crossing conflicts, but offsetting, which others here on PPrune have been calling for for years now, (ever since the 1996 New Delhi mid-air, I think) WILL help mitigate the problem (because it is a problem) of ultra accuracy in GPS when the conflict is between opposite direction traffic on the same air route, as in the recent Brazilian tragedy.

I find myself wondering how it is that some ambulance chasing lawyer hasn't latched on to this fact years ago and tried sueing (sp?) someone - (ICAO?, the FMS manufacturers?, the tea lady at Boeing?) over their not doing sometrhing to fix what everyone should recognise is a quantum leap in loss of separation standards.

I think Ernest K. Gann's incident proves these really close calls have been happening for many years, and we've in many cases been totally unaware that many such incidents have occurred. It's only now, with ultra accurate GPS and RVSM-compliant altimiters that many such incidents now have such potential to be "hits" rather than the rather confusing, almost contractory term "near misses".

[PBL]

Those crediting TCAS with all these "saves" need to explain why, before the advent of TCAS, there were a statistically-negligible number of collisions in the history of airline flying, and upon the introduction of TCAS there are suddenly almost 60 per year.


The answer, as already given, is GPS and IRS

Really?

That might be a factor in explaining an increase in en-route near misses. Most TCAS activations occur in other phases of flight, off the airways, as far as I understand it, so your proposed factor wouldn't affect those statistics.

In any case, your factor wouldn't affect the *sudden* jump in apparent "saves" which occurred with the introduction of TCAS, since nav equipment precision has evolved, rather than jumping spontaneously and coincidentally with the mandate for TCAS.

PBL

[GlueBall]

What would you do? Climb towards another conflicting aircraft that you don't see but ATC does? Or avoid him, descend towards an aircraft that you do see, and hope to avoid him using visual means?
Please give some good reasons for your answer that will also be good reasons for any other pilot in this situation.
To be sure, I wouldn't be maneuvering opposite the RA! I might do nothing and depend on the other crew's compliance with their RA for separation; to mitigate the hazard, I would probably climb 300' just to offset my flight level. It would be an instinctive, reflex action . . . nothing to think about.

[PBL]

To be sure, I wouldn't be maneuvering opposite the RA! I might do nothing and depend on the other crew's compliance with their RA for separation; to mitigate the hazard, ..

You are not addressing the scenario. If the other aircraft had TCAS, then you'd be painting him. In the given situation, you are not, but ATC is. He doesn't have TCAS, ergo he has no RA nor (as far as you know) does he even know you're there.

I would probably climb 300' just to offset my flight level.

Right into where he might well be?

You may well not want to manoeuvre against the RA, but then it also appears that you don't want to try to avoid the unpainted intruder at all. Your choice, but it wouldn't be mine!

PBL

[Capt Pit Bull]

PBL

In any case, your factor wouldn't affect the *sudden* jump in apparent "saves" which occurred with the introduction of TCAS, since nav equipment precision has evolved, rather than jumping spontaneously and coincidentally with the mandate for TCAS.

I see what you are saying, however (in Europe anyway) whilst nav equipment precision has been evolutionary, adoption of it has not.

At the lower / middle of the commercial air transport sector the B-RNAV mandate caused a sudden step change improvement in accuracy. On the fleet I was flying at the time we went from blundering around on VORs 100 miles away to using GPS with observed typical ANPs of .07 miles (or better).

E.G. If memory serves that was '98? and I went straight from being on the team introducing our FMS (for B-RNAV) to being the lead guy on our TCAS implementation for the ACAS mandate, which was in and operational by the end of 2000 ish

Hence, my (admitably insular) viewpoint is that we had a radical improvement in nav accuracy almost concurrently with getting TCAS.

pb

[bugg smasher]

The difference between the two views is only that of whether one believes that such a system was/is necessary

Despite all of the problems associated with TCAS, many of which I was unaware of until now, it has generated a greatly heightened alertness in the cockpit as compared to the pre-TCAS days. In the majority of cases, we didn’t even know a potentially conflicting target was out there.

I think, however, the fait-accomplis of increased airspace utilization using TCAS as the all-encompassing anti-collision solution is premature.

Clearly, it is not ready for that function.

[oceancrosser]

Compare this with actual midairs amongst airline transport aircraft. 1959 Grand Canyon, 1976 Zagreb, 1978 San Diego, 1985 Cerritos. And then, since the advent of TCAS, 1995 Namibia (involving a German military machine which I do not believe was TCAS-equipped), 1996 New Delhi, 2002 Überlingen and 2006 Amazonas. That is 4 before the advent of TCAS (mandated 1991, I believe) and 4 since (16 years).
I think PBL is actually missing a few midairs in the above count. According to www.airsafe.com, there were quite a few others in the pre TCAS period, namely:
1960, AF Caravelle and light a/c, Paris, France.
1960, UA DC-8 and a Lockheed Constellation, New York, USA
1967 TWA DC-9 and light a/c, near Urbana, OH, USA
1967 Piedmont 727 and light a/c, near Hendersonville NC, USA
1971 Allegheny DC-9 and light a/c, near Fairland IN, USA
1971 Hughes Airwest DC-9 and US Navy fighter, nr Duarte CA, USA
1971 ANA 727 and jet fighter, Honshu, Japan
1973 Iberia DC-9 and Spantax CV-990, Nantes, France
1978 Nigeria Airways F28 and light a/c, Lagos, Nigeria
1992 Libyan Arab 727 and fighter a/c, nr Tripoli, Libya
Nearly 800 people were killed in these 10 collisions. PBL, don´t you think you should have included these, instead of only the 4 examples you cite in your argument?

[firstchoice7e7]

and also BA476 which collided with a inex DC-9 in 1976, 176 lives lost.

[joernstu]

@oceancrosser:

I believe that the list of mid-airs could get very long. In Germany one could include a lot of collisions between gliders (which is of no concern on pprune, though).

In your list, 8 mid-airs have involved light a/c and fighter a/c, which don't have to be equipped with a transponder or may have
turned it off. I venture to doubt, that these collisions would have been prevented if the aircraft were equipped with TCAS.

The TCAS approach on avoiding collisions does not include ATC, it only works with a/c carrying painting transponders. It may be possible to solve the issue of informing ATC on TCAS RAs, but I don't believe that it is feasable to solve the issue of including non transponder a/c with the current TCAS devices.

[PBL]

pb,

If memory serves, TCAS was mandated in the U.S. in 1991 and, as far as I can remember, most of the correspondents to whom I was anecdotally referring were pilots flying for U.S. carriers. The kit required for precise RNAV came a little later, as I recall.

Anyhow, I was making an observation from anecdote, and I don't see any way of defnitively settling the issue that it raises, namely whether increased "save" rates reflect true save rates caused by other changes in environment, or whether they reflect changed perceptions due to TCAS. As bugg smasher astutely points out, there has been an inevitable increase in awareness of one's neighbors through implementation of TCAS, not just in near misses.

don´t you think you should have included these, instead of only the 4 examples you cite in your argument?

Yes. Thanks for doing the work to identify these. Is it a complete list?

I agree with joernstu that the civil/military collisions fall into a different category. There are still problems with civil/military near misses in Europe, where the airspace is largely divided vertically.

PBL

[FullWings]

If this thread was about a crew's response to a GPWS alert: "Whoop Whoop PULL UP!", which involved pushing the stick forward instead of back, would there be much in the way of technical discussion required? After all, to manouevre against an RA is the worst possible case... doing nothing (anything!) is a better option.

I agree that it is difficult/impossible to "prove" TCAS logic for multi-aircraft encounters but do you think you'd do any better as a human with only a limited view out of a small window and seconds to make a decision? We start off at a profound disadvantage with our senses (the one that's going to hit you is the one that's most difficult to spot, amongst other things) and often just don't have the information available to be able to reach a calculated conclusion - it's instinct, right or wrong. We're not fighter pilots in highly manouevrable aircraft with excellent all round visibility - and they get it wrong, too.

[PBL]

If this thread was about a crew's response to a GPWS alert: "Whoop Whoop PULL UP!", which involved pushing the stick forward instead of back, would there be much in the way of technical discussion required?


I would have thought it was a straightforward exercise to list the differences between a ground proximity warning and an RA.

1. Ground proximity is based on radio altimetry, not on self-advertisement from another electronic device.
2. That altimetry is measuring something definite and very close
3. What it is measuring will not move and requires no projected CPA that must be monitored during the manoeuvre in case a different advisory is required
4. You are way out of legal airspace when you get that warning, so the likelihood that there is someone else around you is just about zero and you can pretty much do what you like.


After all, to manouevre against an RA is the worst possible case... doing nothing (anything!) is a better option.


I introduced a specific scenario. Maybe if airmen doesn't want to address it, then you can? Why is manoeuvring against the RA the "worst possible option" in the specific scenario I gave?

I agree that it is difficult/impossible to "prove" TCAS logic for multi-aircraft encounters but do you think you'd do any better as a human with only a limited view out of a small window and seconds to make a decision?

Those are not the only two options. Another arises when you have an advisory to manoeuvre to avoid a conflict from same-level or higher, and you have an aircraft below you in sight. What would your decision be and why?

PBL

[alf5071h]

Peter, the ‘GPWS’ situation is more complex if you consider EGPWS which uses a database and navigation position in its computation. You may have meant this but your response related to GPWS; as you realise, it is important to distinguish between the two. This is particularly so for the crew as the later system (EGPWS/TAWS) is more capable and reliable than previous systems, but then opens greater opportunity for error due to the range of alerting and warning modes and the terrain display, e.g. during an approach a pilot may pull up in response to a warning and incorrectly level at MDA, but due to a (gross) error in range, MDA is not safe.

Also note that some versions of EGPWS use GPS altitude as a gross error check of altitude, mainly incorrect pressure setting. GPS (geometric) altitude does not replace the rad alt (there are exceptions) as its accuracy is less than that of normal altimetry, and therefore at this time it would not be a candidate for ACAS altitude as suggested earlier in the thread.

As for the solution to the three aircraft ACAS problem, this is done reasonably successfully by combat pilots in 1 vs 2 situations. My experiences and existence today is relevant, but based on the same experiences I would be less confident for situations involving 4 vs 4, due to the human limitations in continuously computing a 4D (space – time) solution, that’s assuming that you can see everyone!

Your ACAS problem relates to a similar 4D situation and thus might be modelled in the form the 4th power.
Considering three aircraft avoidance, x, y and z, then a solution could be in the form of x^4 = y^4 + z^4.
This form of equation for powers greater than 2 has been proven to have no solution (Fermat’s conjecture). With appropriate deductive logic, ‘no solution’ relates to a collision condition, thus I claim my PhD !

[ATC Watcher]

If this thread was about a crew's response to a GPWS alert: "Whoop Whoop PULL UP!", which involved pushing the stick forward instead of back, would there be much in the way of technical discussion required?

Interesting point .Because , if we go back in time, there are similarities indeed . If one remembers the introduction of GPWS, the early systems were prone to many false alarms, which worked against crew acceptation. ( remember for instance the Air Inter decision in France with their new A320 fleet, and the discussions that followed after their Mt St Odile CFIT )
I do not have precise archive data at hand , but I remember there were quite a few CFIT accidents in the late 80’s /early 90s , where GPWS alerts were ignored by pilots. ( the Independent air Boeing 707 in Santa Maria /Azores in 1989 , or the Thai International A310 in Katmandu in 92 are just two that come to my mind for instance )
So the “ philosophies ‘ ( how to deal with automation ) do apply for both systems, in their early phases.
In 2007, GPWS , and especially EGPWS could be considered mature. Not TCAS.
TCAS is, despite its age, still in its early phase , as we now on version 7.0 of its development, and that after a collision and a good dozen of very,very near calls , we are most probably going to have a version 7.1 in a few years. That might resolve partially the flaws that contributed to those accidents/incidents.
What we do in the meantime( i.e. until that new version comes ) , is part of the debate here.

[FullWings]

PBL,

I would have thought it was a straightforward exercise to list the differences between a ground proximity warning and an RA.

1. Ground proximity is based on radio altimetry, not on self-advertisement from another electronic device.
2. That altimetry is measuring something definite and very close
3. What it is measuring will not move and requires no projected CPA that must be monitored during the manoeuvre in case a different advisory is required
4. You are way out of legal airspace when you get that warning, so the likelihood that there is someone else around you is just about zero and you can pretty much do what you like.

I agree with 1. to 3. but would you agree that 4. is likely to happen near an airport (you have to approach the ground when landing and get near it when taking off) and airports are well known as places where aeroplanes like to congregate... Anyway, the points I'm trying to make are:

a) GPWS and TCAS (and to a lesser extent EGPWS) are last-defence systems, conceived to possibly stop/reduce the chances of an accident occurring. They are not designed or certified to replace ATC or good piloting - except at the last minute when either or both have been lacking.

so,

b) As the alerts/advisories/warnings given by the above equipment are somewhat time constrained in their period of usefulness (!), then a consistent response over a short period is required to allow these systems to function as intended, i.e. you can't pontificate at great length about what action to take: it must be a memory drill - "recall item". And you have to perform it correctly.

I introduced a specific scenario. Maybe if airmen doesn't want to address it, then you can? Why is manoeuvring against the RA the "worst possible option" in the specific scenario I gave?
I assume that is the Überlingen scenario? I say that it was the worst possible because it led to a collision; almost by definition any other action would have produced a "miss". I know this is slightly simplistic but if you think about it in reverse, to generate a "hit" both aircraft have to occupy the same small space over the same very small time period. Any changes to the trajectories of either will quickly disrupt this meeting. Finally, it's because the manual says: "NEVER MANOUVER AGAINST AN RA". :p

Those are not the only two options. Another arises when you have an advisory to manoeuvre to avoid a conflict from same-level or higher, and you have an aircraft below you in sight. What would your decision be and why?
Follow the RA. The danger in aviation comes mostly from the aircraft you can't/haven't seen or aren't aware of. I would posit that you are unlikely to hit an aircraft that you have in plain sight as you can tell if it is on a constant relative bearing or not; also you have the option of a lateral manoeuvre to de-conflict. If the a/c below has TCAS, there might be coordination going on that you are unaware of. If it doesn't, then they'll (like you) probably follow the rules of the air to avoid a collision (if they've seen you!)

I find detailed technical discussions about most things to do with aviation absolutely fascinating... but the job has shown me that for some scenarios, you have to have a fairly rote response prepared for immediate use. I applaud scientific examination of the limits of aircraft systems but when you get to 99.99% confidence (or whatever) in a particular one, especially if it involves time-critical warnings, you just have to say: "It works, do what it says" as there isn't any room left to do a risk analysis on an alternative response.

I have had several RA's in real life, one caused by an aircraft directly above deciding to descend at a great rate through our level. Did TCAS save our lives? Don't know but it was f***ing close when we did eventually see it.

[PBL]

Thoughtful posts from alf, ATC Watcher and FullWings.
I'll respond to alf and ATC Watcher separately from FullWings, since I am having trouble getting the SW to let me jump between pages to copy quotes, without losing my reply window.

your response related to GPWS

That's right. FullWings posed the GPWS warning as a comparison. I believe the decision problem with GPWS is fairly straightforward; also, one has apparently only about 12 seconds to respond, so even if there are uncertainties, there is no time to formulate a decision: your reaction must have been formulated in advance.

I am also talking about the algorithms, assuming that the equipment functions as intended. And I realise that can be a big "if". ATC Watcher's point about maturity refers partly to this issue: can we get the kit to function as intended?
If so, let me call the kit "refinable".

Let me call a system "adequate" if, when it functions as intended, its goal is achieved.

Let me call a aid system "optimal" if pilots can adequately follow the required behavior.

Then the GPWS algorithm is refinable, adequate, but not optimal. Whereas TCAS is refinable, not adequate, and questionably optimal if at all.

Comparing with EGPWS is a different kettle of fish. For example, Capt Pit Bull's point about map-shifts show that it is not adequate (in the terminology above). I haven't worked on EGPWS long enough to have as good a grasp of the issues as I would like.

As for the solution to the three aircraft ACAS problem, this is done reasonably successfully by combat pilots in 1 vs 2 situations.


Yes, but they are using 3-dimensional avoidance, whereas TCAS is a 1-dimensional algorithm. The ACAS problem and the TCAS problem are not identical.


Considering three aircraft avoidance, x, y and z, then a solution could be in the form of x^4 = y^4 + z^4.
This form of equation for powers greater than 2 has been proven to have no solution (Fermat’s conjecture).


What are x, y, and z here?

Three aircraft follow trajectories in 3-d Euclidean space, so
their position at time t is given by functions
f(t), g(t) and h(t), and the values of these functions are points in 3-D space. So there are coordinate functions (in your favorite coordinate system) f1, f2, f3, g1, g2, etc. Easiest is to pick one coordinate as the direction of the TCAS movement: say the third. Then either there are some trajectories f,g,h such that, with accelerations as specified by TCAS in direction f3, g3, h3, at least two of those trajectories come sufficiently close, or for all trajectories f,g,h, none of them come sufficiently close.

The quartic equation you gave has no *integer* solutions. That was shown by Wiles and Taylor. If has lots of solutions; indeed, its solutions form nice shapes in 3-space.

I claim my PhD !
Sorry, not quite there yet :}

PBL

[PBL]

FullWings,

I agree that GPWS alerts are most likely to happen during take-off and landing phases, and I obviously agree that airplanes congregate around airports. However, if there is rising terrain then either you are in "mountainous terrain" and the next legitimate airplane is 2,000 ft above, which at < 256 kts true leaves everyone lots of time, or you are near a protected approach channel (that you should be on) and there is no one close above. And you don't follow a GPWS pull-up for 30 seconds.

With TCAS, at more than 256 kts true, and airplanes legitimately 1,000 ft above (under RVSM, say), you have a required 30 second manoeuvre time, and you are in that other guy's legitimate airspace before your manoeuvre is over.

I can guarantee you that this situation has not been analysed thoroughly with regard to the interactions.

So I don't regard the situations as comparable.

Concerning the scenario I posed,
Follow the RA. The danger in aviation comes mostly from the aircraft you can't/haven't seen or aren't aware of. I would posit that you are unlikely to hit an aircraft that you have in plain sight

You seem to me to be giving conflicting answers here. "Follow the RA" says climb into the aircraft you are not painting. But you then say he's the danger. Exactly my view. The "aircraft in plain sight" that "you are unlikely to hit" is DHL. So that suggests you would manoeuvre contrary to what you suggested in the first sentence.

PBL

[joernstu]

@FullWings:

You base your descision, which reaction to a collision threat is good, which is bad on the manual and the outcome of a situation.

As the outcome of a collision threat and the actions to resolve it can only be judged after the threat is resolved (or not), a crew cannot use this criteria for its descisions. Leaving only the manuals. If all manuals around the world contain the same instructions, all would be fine. But in the Ueberlingen accident, this was not the case.

Finally, it's because the manual says: "NEVER MANOUVER AGAINST AN RA". :pThe Tu154M Flight Operations Manual had the following passgae (at least around Jul 2002) and, as you will note, "never manouver against an RA" was not in it.

"For the avoidance of in-flight collisions is the visual control of the siituation in the airspace by the crew and the correct execution of all instructions by ATC to be viewed as the most important tool.
TCAS is an additional instrument which ensures the timely determination of on-coming traffic. the classification of the risk and, if necessary, planning of an advice for a vertical avoidancy manoeuvre."

So the "never manoeuver against an RA" at least was not in the TU154M FOM. Instead the crew was supposed to follow ATC, get visual control of the situation and descide for itself, which action best to take.


So, if the out-come and the manuals are not usable for deciding on good and bad reaction to a collision threat, what criteria do we have left?

[FullWings]

PBL,

However, if there is rising terrain then either you are in "mountainous terrain" and the next legitimate airplane is 2,000 ft above, which at < 256 kts true leaves everyone lots of time, or you are near a protected approach channel (that you should be on) and there is no one close above. And you don't follow a GPWS pull-up for 30 seconds.
Hmmm. In the widebody I fly, a full-energy pull-up will generate sustained rates of climb of 5,000fpm+, when down to average landing weights. This is enough to cause an RA for someone 2,000' above you. Also, most airlines SOPs would require that a "hard" GPWS is followed by a max angle climb until out of immediate danger, then possibly further to MSA, especially in IMC. This is because if you trigger the GPWS you are probably not where you thought you were, so instead of finding out exactly where the ground is by using the aircraft structure as a probe, you get the hell out of there into a known environment. This may take some time, possibly a lot longer than 30 seconds. :ooh:

One of my recent details involved flying north over Lake Geneva at 1,500' in landing config. until a full EGPWS was triggered. It took some time to get to 7,000' to clear the top of the mountain (as you keep the config. the same during the escape) but the angle of climb just overtook that of the slope. :)

You seem to me to be giving conflicting answers here. "Follow the RA" says climb into the aircraft you are not painting. But you then say he's the danger. Exactly my view. The "aircraft in plain sight" that "you are unlikely to hit" is DHL. So that suggests you would manoeuvre contrary to what you suggested in the first sentence.
I think you've misunderstood my response. When I said "follow the RA" I wasn't thinking about whether it was climb, descend, monitor, increase, decrease, etc. I meant follow it whatever it says. I've done many sim exercises involving multiple aircraft encounters and it's interesting being non-handling and watching the whole thing unfold out of the window whilst monitoring what's on the cockpit displays. With "crossing climbs" and "squeezes" you get pretty close but there is always a "reality check" with relative bearings - if it's moving across the windscreen, you're not going to collide (unless it's filling it as well :ouch:). If for some reason the TCAS has failed to resolve the conflict then there is always the last-ditch option of a lateral or vertical change of course. This is likely to be an exceedingly rare event, so an early attempt to manoeuvre outside/against the TCAS guidance will diminish safety margins pretty much all of the time.

In your original scenario: "an advisory to manoeuvre to avoid a conflict from same-level or higher, and you have an aircraft below you in sight" there are at least three aircraft present. As far as I'm aware there were only two aircraft involved in the accident we're talking about?

joernstu,

If all manuals around the world contain the same instructions, all would be fine. But in the Ueberlingen accident, this was not the case
I'm sure they do now. I think, although I may be wrong, that the Tu154 Flight Manual for that airline was out of step with SOPs in pretty much every major carrier, even in 2002

[hedgehog-in-fog]

This is an extract from the
FCOM3 A-319\320 Operating Limitations 3.01.34 p1
of one of Russian airline.

"It is not recommended to use TCAS II (ACAS) TA/RA mode in flight in former USSR airspace. When ТА mode is selected, pilots should coordinate actions with the АТС. In the former USSR airspace, aircraft can only fly on routes covered by АТС ground facilities, using RBS mode."

Looks strange (at least).
Every flight I brake this "limitation".:ugh:

[CDN_ATC]

I wonder how close ATC "Traffic Alert" and "Conflict Alert" parameters are to TCAS RA's, I know they have greater margins built in to ATC alerts, so that a TCAS RA can be avoided if possible.


How much lead time does a TCAS RA give? Or is it dependant on the situation?

[ATC Watcher]

Oh dear oh dear !
hedgehog-in-fog:
This is an extract from the
FCOM3 A-319\320 Operating Limitations 3.01.34 p1
of one of Russian airline.It is not recommended to use TCAS II (ACAS) TA/RA mode in flight in former USSR airspace
I sincerely hope that the date on top of this FCOM page is before 2002 ..
CDN_ATC,:
How much lead time does a TCAS RA give? Or is it dependent on the situation?
I do hope that you are not working as a controller in the UAC I am thinking about, seen your profile location. If you do, then we we still a lot of education to do...
In short :
ATC Short term conflict Alert (STCA) is depending on ATS systems, there is no fixed parameter worldwide, but is typically 2 minutes, while ACAS audio warnings are dependent on altitude, but in the upper airspace , they start around 45 seconds before CPA. There are no coordination whatsoever between the 2 systems.

[hedgehog-in-fog]

ATC Watcher
I sincerely hope that the date on top of this FCOM page is before 2002 ..

2006
Sorry
:ugh:

[punkalouver]

"Please read my analysis of the decision problem presented to the Bakshirian crew at Überlingen. They were faced with an "intruder" at 10 o'clock which they saw, and an unknown conflict at 2 o'clock which they didn't see, and for which they had an advisory to descend (that is, he was at or above their altitude).
What would you do? Climb towards another conflicting aircraft that you don't see but ATC does? Or avoid him, descend towards an aircraft that you do see, and hope to avoid him using visual means?
Please give some good reasons for your answer that will also be good reasons for any other pilot in this situation."

Reading part of the report quickly, I couldn't find where this second intruder is mentioned. Please show the chapter for me.

Section 2.2.2 says that the DHL plane was to the left of the 154 and first seen on TCAS at 27 miles. The board says in this chapter that the crew knew that the intruder aircraft was at their altitude based on the captain saying "Here it is showing zero". The 154 crew then got a traffic alert and after being instructed to descend by ATC, they got a climb RA. They followed ATC and the rest is history.

Faced with your scenario above, I would follow the RA. It is most surely correct that it is giving accurate information about a high risk of conflict. Ignoring an RA because there are some other lights out there is very risky. There are many optical illusions, especially at night and those other lights may be very far off or not even an airplane. I would suggest that the risk of a collision with that aircraft is likely less and even if you were to intentionally try to hit it you would most likely miss. On the other hand thinking that you can visually avoid a conflict by going against the RA can lead to disaster as this JAL captain nearly found out, by almost creating the world's worst aviation disaster.

http://aviation-safety.net/database/record.php?id=20010131-3

As an example of an illusion of a risk of collision leading to evasive action that actually caused a collision leading to airline passenger deaths, read this.

"TWA Boeing 707 N748TW and Eastern Constellation N6218C approached Carmel VORTAC at FL110 resp. FL100. Due to an optical illusion caused by the up-slope effect of cloud tops, the Eastern 1st officer thought they were on collision course with the TWA jet. He pulled up the Constellation. The reaction of the TWA crew was to roll to the right, followed by a left roll down. Both aircraft collided. The Boeing made a safe landing.

PROBABLE CAUSE: "Misjudgement of altitude separation by the crew of EA853 because of an optical illusion created by the up-slope effect of cloud tops resulting in an evasive manoeuvre by the EA853 crew and a reactionary evasive manoeuvre by the TW42 crew.""

http://www.prop-liners.com/midair.htm

Avoid the danger that is being warned to you by reliable technology, sometimes it is dangerous to try to outthink it all. In the end, its an odds game with TCAS in your favour.

As an interesting tidbit in this report, read section 1.1.2. The aircraft was proceeding direct to Trasadingen VOR. After the captain of the 154 said "Look here, it indicates zero", and more than a minute before the collision, with the autopilot pitch channel and roll channel still engaged, the airplane turned at a bank angle of 10° to the right a total of 10° heading change. Was this using the TCAS as a lateral conflict avoidance tool? Would they have missed each other if this slight turn had not been made? Perhaps not.

[PBL]

I couldn't find where this second intruder is mentioned. Please show the chapter for me.

If one doesn't know that the Russian crew were concerned with two possible intruders, one is missing a large part of what can be learned from the accident.

Since the thread has split since some of the observations were introduced, let me first refer to it:
Section 2.2.2 of the (english version of the) report, which says "According to CVR data it can be concluded that the instructor searched the airspace in front and to the right in vain."

Reason is that ATC had given as a reason for immediate descent that traffic was at 2 o'clock. Bakshirian already had visual contact with DHL, who was at 10 o'clock.

The Überlingen-trial thread went two ways, almost from the start. The first was to, well, celebrate the rehabilitation of Peter Nielsen (which event I applaud!). The second was to discuss the accident again. I plead guilty to contributing to the second. ATC Watcher split the thread.

A good place to start reading the previous thread is at a contribution from 30.05.2007 by 120.4. I brought in my study of TCAS on 05.09@10:40, currently post #78. The study can be found at
http://www.rvs.uni-bielefeld.de/publications/Reports/SCSS04.pdf
and is published in
http://crpit.com/Vol47.html,
specifically
http://crpit.com/confpapers/CRPITV47Ladkin.pdf
in which the paper by Ed Williams is also pertinent:
http://crpit.com/confpapers/CRPITV47Williams.pdf

the airplane turned at a bank angle of 10° to the right a total of 10° heading change. ......... Would they have missed each other if this slight turn had not been made?

We have been trying to figure out this turn for a couple of years, without success.

I think they would have missed.

PBL

[punkalouver]

Thanks I found the referance. I would still play the odds in my favour and follow the RA. After all. If ATC is giving you an immediate descent due to traffic they are showing here that they probably have already made an error of some sort. A big reason for TCAS is to prevent a collision due to ATC error.
Follow the RA. I don't think many of us on a line check will get top marks for saying we wouldn't(not counting an overriding stall or GPWS warning at the same time).

[PBL]

If ATC is giving you an immediate descent due to traffic they are showing here that they have [probably] already made an error of some sort

Or they are handling a military aircraft with an emergency, or a military aircraft with whom they have lost communications and who is manoeuvring. [Added after punkalouver's edit: I don't think anyone has a good grasp of what probabilities may be involved here.]

You are right about it being an odds game. Profis call it a "decision problem under uncertainty". One of the main issues for me is that none of the experts who study and devise strategies for such "decision problems under uncertainty" would go so far as to make a strategy compulsory. But that is what the BFU recommended about TCAS RAs, and that is what other organisations are promoting.

PBL

[punkalouver]

I edited my last post to say thet they have PROBABLY already made an error. As a minor side note, at least some aircraft are designed so that if a GPWS warning is activated, the TCAS will go into TA mode and TCAS audio alerts are muted.

[CDN_ATC]

ATC Short term conflict Alert (STCA) is depending on ATS systems, there is no fixed parameter worldwide, but is typically 2 minutes, while ACAS audio warnings are dependent on altitude, but in the upper airspace , they start around 45 seconds before CPA. There are no coordination whatsoever between the 2 systems.

I'm aware that TCAS and ATC CA do not co-ordinate, wasn't my question sorry if it came off that way.


There has been numerous accounts however of TCAS going off and issuing an RA even when there's no airplane within 100 miles of their location.

Also TCAS going off and issuing an RA for "Traffic above" when tapes prove neither airplane infringed on the 1000' separation.

If TCAS/ACAS is giving RA's for airplanes that don't even exist, whos to say it will come up with the right answer every time when they do?

Obviously TCAS is an invaluable tool, and would have likely saved the Gol disaster had the Legacys transponder been on, but one has to wonder just how well it works.

It's not exactly easy to "test" it on a regular basis real world without endangering lives.

[PBL]

Also TCAS going off and issuing an RA for "Traffic above" when tapes prove neither airplane infringed on the 1000' separation

This is an ACAS versus ACAS-II specification incompatibility. It is also a kit problem, as follows.

This is a known problem with Honeywell's TCAS 2000 V 6.4, which is not ACAS-II-compliant, and is fixed with TCAS 2000 V7, which is ACAS-II-compliant. The issue is that the RVSM altitude-measurement requirements allow some margin for error, and if the aircraft below errs allowably above its true and the aircraft above errs allowably below its true, the vertical separation is less than that required to trigger a V 6.4 RA (but not less than that required to trigger a V7 RA).

I hear rumors that it is taking longer to get away from V6.4 than originally envisaged. Anyone have data?

PBL

[FullWings]

One of the main issues for me is that none of the experts who study and devise strategies for such "decision problems under uncertainty" would go so far as to make a strategy compulsory. But that is what the BFU recommended about TCAS RAs, and that is what other organisations are promoting.
But do their "decision problems under uncertainty" include a very limited amount of time for assessment and action? It's a bit like the "rules of engagement" that military personnel operate under - if they're made too complicated, people on both sides end up being killed unnecessarily.

There are finite probabilities that normal actions taken on board aircraft may have diverse undesired catastrophic results, through system failure/design/build or just plain bad luck. These probabilities are low enough for the failure modes to be discarded by the operator as unlikely and confusing and are not included in SOPs for this reason. This is how I view a TCAS RA. Yes, there may be a small chance that the software has failed to provide an adequate solution to a complicated multi-body problem - but what do I do about that, especially as I know that incorrect action (or inaction) will lead to a "sub-optimal safety scenario" 99.99% of the time?

IMHO the authorities have it right this time. Bluntly, either you do what it says or you might as well not fit it. To be effective, TCAS relies on at least one interested party doing something close to what it is suggesting; to inject an element of doubt at this stage does not help matters much and may lead to a reduction in the chances of a successful conflict resolution.

[PBL]

But do their "decision problems under uncertainty" include a very limited amount of time for assessment and action?

Yes.


There are finite probabilities that normal actions taken on board aircraft may have diverse undesired catastrophic results, through system failure/design/build or just plain bad luck. These probabilities are low enough for the failure modes to be discarded by the operator as unlikely and confusing and are not included in SOPs for this reason.


This is blue-sky hypothesising. As far as I can tell, none of these "low enough" "finite probabilities" have ever been estimated; neither is it clear that one could do so.


Yes, there may be a small chance that the software has failed to provide an adequate solution to a complicated multi-body problem - but what do I do about that,


If you wish to make it compulsory, you devise a provably-correct algorithm, prove it correct, and put the proof in the public domain for peer review.

And if you can't do that, you let it be advisory and let people choose what to do. And respect them for making a choice which, while different from yours, is still rational.


especially as I know that incorrect action (or inaction) will lead to a "sub-optimal safety scenario" 99.99% of the time?


You don't "know" that; in particular, you don't know about that 99.99%. By the way, that is far too often for the typical TLS's required to be assured in similar cases.

PBL

[FullWings]

This is blue-sky hypothesising. As far as I can tell, none of these "low enough" "finite probabilities" have ever been estimated; neither is it clear that one could do so.
Exactly. So for practical operation of the aircraft, we ignore them.

If you wish to make it compulsory, you devise a provably-correct algorithm, prove it correct, and put the proof in the public domain for peer review.
That doesn't solve your problem. You need a provably correct implementation of the algorithm. Have the manufacturers "open-sourced" their code yet?

And if you can't do that, you let it be advisory and let people choose what to do. And respect them for making a choice which, while different from yours, is still rational.
The Tu-154 crew chose, or should I say "elected" to do something different. People died. I'm not saying they didn't have reason (their SOPs seem to have been at odds to everyone else's, for one thing) but the end result was disaster. An avoidable disaster, one that TCAS was invented to prevent. You can "choose" to drive on the opposite side of the road to other people but I don't think you'll get much respect for it.

You don't "know" that; in particular, you don't know about that 99.99%
That leaves us in a bit of a stalemate as you don't "know" either... That gives me a (miniscule) chance of being absolutely correct with my random typing of decimal places. :p

Point is, we, the airline industry, have been given a collision avoidance tool that has been approved and accepted, even mandated, by many national aviation authorities. For effectiveness it relies on timely responses to given commands, so it loses much of its appeal if these commands are ignored or flown against. Thus the need for legislation.

As a pilot I'm paid to consider possibilities... The FAA, CAA, LBA, etc. could be making a huge mistake and exposing us all to increased risk from this system and I shouldn't trust it... But where is the evidence? Well, it's all proprietary stuff so looks like it'll be some time (if ever) before any non-affiliate can come up with any data one way or the other. So I'll just have to go on the information available which seems to be: it does what it says on the tin and you'd be foolish to ignore it.

[bsieker]

This is blue-sky hypothesising. As far as I can tell, none of these "low enough" "finite probabilities" have ever been estimated; neither is it clear that one could do so.

Exactly. So for practical operation of the aircraft, we ignore them.

I think you misunderstood. One cannot tell that these probablilities are "low enough" to disregard them. Since we do not know them, perhaps never will, they may be quite high.

That doesn't solve your problem. You need a provably correct implementation of the algorithm. Have the manufacturers "open-sourced" their code yet?

A provably correct implementation of a flawed algorithm won't help. First and foremost the algorithm needs to be proven correct. Methods to make a very-high confidence implementation of a specified algorithm are today available and well-understood, although they are neither trivial nor cheap.

The Tu-154 crew chose, or should I say "elected" to do something different. People died. I'm not saying they didn't have reason (their SOPs seem to have been at odds to everyone else's, for one thing) but the end result was disaster. You can "choose" to drive on the opposite side of the road to other people but I don't think you'll get much respect for it.

joernstu will remember the details by heart, and correct me if I'm wrong, but I think the Tupolev crew got the ATC instruction slightly before the RA. A case which is not considered in the ACAS specifications, as it assumes it is needed only when ATC separation has already failed. It does not take into account late ATC action, which I imagine happens frequently.

An avoidable disaster, one that TCAS was invented to prevent.

And yet, the Ueberlingen accident would not have happened, if neither aircraft had been equipped with TCAS. ATC was there to separate them. (Perhaps later than usual, but early enough.) Thus, the presence and operation of TCAS was a necessary causal factor in that accident, as has been shown clearly by Stuphorn's Why-Because Analysis.


Bernd

[PBL]

FullWings,

I think everyone understands by now that you choose to follow a TCAS RA no matter what. But you have not given anybody else a reason to do so until you can answer the decision problem to which I referred in post [email protected] (currently #22) and explained again in post [email protected]
(currently #43).

You offered a response in [email protected] (currently #41) which gave decision criteria which had you going in two different directions in my problem, which I pointed out in [email protected]. You responded ([email protected], currently 45) that I had misunderstood your response. Maybe, but you still have to offer a resolution, along with reasons, of the decision problem, and it's been 11 days now. The Bakshirian crew only got 30 seconds.

bsieker has responded as I might have to some of your other points, so I won't repeat those.

PBL

[FullWings]

PBL,

Please read my analysis of the decision problem presented to the Bakshirian crew at Überlingen. They were faced with an "intruder" at 10 o'clock which they saw, and an unknown conflict at 2 o'clock which they didn't see, and for which they had an advisory to descend (that is, he was at or above their altitude).
The (sadly last) communication from the Tu-154 to Zurich was: "'Ja', we have traffic at your... 2 o’clock now at 360". Where does the idea of an "unknown conflict" come from? :confused:

What would you do? Climb towards another conflicting aircraft that you don't see but ATC does? Or avoid him, descend towards an aircraft that you do see, and hope to avoid him using visual means?
What does the TCAS have to say about it? Follow the commands to resolve the conflict. Done. To reverse the question, why would I not follow the RA? You have 18 seconds to reply...

May I respectfully suggest that you're trying to find a complicated answer to a simple problem? The sky is often full of aircraft, some you can see, some you can't (try an approach to EWR or JFK on a busy day). The sort of scenario you're talking about is not uncommon; in fact, I would go as far as to say, unremarkable. Installation & use of TCAS is compulsory in the USA and Europe in most airspace that airliners use; if we all went about 'doing our own thing' in reponse to RAs, I don't really want to imagine the consequences. Until someone proves it otherwise, I shall be 'staying out of the red', thank you!

bsieker,

I think you misunderstood. One cannot tell that these probablilities are "low enough" to disregard them. Since we do not know them, perhaps never will, they may be quite high.
Given the amount of flying hours logged and the number of incidents (and I'm talking generically about systems) one should be able to set an upper bound on the these probabilities, given certain confidence limits, just like you would with any other experimental data?

And yet, the Ueberlingen accident would not have happened, if neither aircraft had been equipped with TCAS.
Not possible I'm afraid. We all have to have it. Therefore we need a common standard in how we respond to it (which is where I came in on this discussion).

All,

I'm continuing to partake in this discussion because I don't want any impressionable pilots reading this thread and thinking that it's a good idea to start tinkering with standard responses to RAs (especially if I am in the other aircraft). I'm all for academic arguments and theories as that is the way we progress science but in many cases there has to be a set way of doing something until we build up enough evidence to justify changing it. This applies especially to aviation.

[bsieker]

And yet, the Ueberlingen accident would not have happened, if neither aircraft had been equipped with TCAS.

Not possible I'm afraid. We all have to have it.

I understand that it is mandatory. Whether or not that is actually a Good Thing is another matter.

Having two collision avoidance systems (radar/ATC/ground-based, and ground-independent aircraft-transponder-based) that may contradict each other is a real problem. Particularly and glaringly so if ATC instructs in one direction, and TCAS instructs in the opposite direction immediately afterwards. Do you reverse your manoeuvre to cross your conflict's original trajectory again?

ATC usually has a lot more information, than TCAS, and may instruct differently from TCAS to avoid further conflicts. The premise that ATC is already "out" when TCAS issues an RA does not hold.

Would you advocate doing away with the ground-based system and rely exclusively on TCAS? The trend towards Free Flight will go in that direction, but we're not there yet.

Therefore we need a common standard in how we respond to it (which is where I came in on this discussion).

Common standards are a wonderful thing, if we can agree on what they should be for sound reasons. I don't see a silver-bullet solution that will cure all problems. It is clear that making following the RA compulsory in all cases is not it. (If it were, TCAS would be coupled to the autopilot to perform the avoidance manoeuvre automatically; I don't see anyone actively advocating that.)


Bernd

[PBL]

Where does the idea of an "unknown conflict" come from?

If you cannot see it in the Überlingen report, just take it as posited in my decision problem.

What does the TCAS have to say about it? Follow the commands to resolve the conflict. Done.


As I said in my last post, we all understand by now that that is your position.


To reverse the question, why would I not follow the RA?

Because following the RA would put you into immediate conflict with traffic which you are not painting but which ATC is.

I'm continuing to partake in this discussion because I don't want any impressionable pilots reading this thread and thinking that it's a good idea to start tinkering with standard responses to RAs

Thank you for your concern for "impressionable pilots". Speaking of impressionable pilots, I wish you could understand and resolve the decision problem which I keep asking you about, rather than simply asserting that TCAS solves it (which it obviously - to others - does not).

PBL

[joernstu]

I'm all for academic arguments and theories as that is the way we progress science but in many cases there has to be a set way of doing something until we build up enough evidence to justify changing it.

I don't think that this is an "academic argument" any more. Perhaps it was an "academic argument" when the impact of TCAS on RVSM was analysed using airtraffic data for 2 days.

There are very real problems, in which TCAS has plays a role, as two mid-air collisions since 2002 demonstrate.

If you look for more evidence you will find complains of ATC on a device that manipulate the outcome of their instructions, leaving them without a coherent image of the situation in the air.

Another problem with TCAS is that the manufacturer (and following him many other organisations) present TCAS as a technical solution in a conflict situation where every other means to avoid a collision have failed. In the Ueberlingen mid-air TCAS gave an RA when both a/c were at 7.11 NM seperation. The limit for seperation in this airspace at this night was 7 NM, usually 5 NM. This demonstrates that TCAS can act at points in time, where other means have not yet failed, rebuting the claim that TCAS only acts when everything else has already failed.

More theoretical arguments are multiple aircraft situations (for which it can be proved that TCAS cannot give advice solving the collision threat) and aircraft without transponders.

Based on these problem (some of them real, some of them more theoretical) I get the picture, that TCAS is not the silver-bullet some want it to be. Of course TCAS is better than noting (at least I hope that this is the case as it was certified for use in aviation), but I also get the image, that it is still far from optimal. The problem is, that some of the problems with TCAS can only be solved by spending lots of money and/or replacing existing TCAS units against newer models.

[joernstu]

The (sadly last) communication from the Tu-154 to Zurich was: "'Ja', we have traffic at your... 2 o’clock now at 360". Where does the idea of an "unknown conflict" come from? :confused:

This communication was not from TU154M to ATC Zurich, but from the controller in Zurich to the TU154M crew.

This means that the TU154M crew was warned of traffic from 2 o'clock (by ATC) and from 10 o'clock (by TCAS and visually confirmed). This leads to the asumption (not only by PBL, but by the accident investigators at BFU), that the TU154M crew was occupied with the search for the traffic at 2 o'clock. The report says in chapter 2.6.3.1 under "Distribution of tasks" (on page 98 in the english version, on page 103 in the german version)
It is probable that he (PNF, TU154M) at least monitored the PF as the descent was initiated, but he then trained his attention on the visual search. He did not advise the PF that they were approaching teir cleared level of FL350. At this time the PNF's attention was concentrated on the visual search, and was probably centered in the wrong sector (emphasis added).

[PBL]

It has been worrying me that some people don't appear to understand the decision problem I have posed or why certain solutions are unsatisfactory. It occurred to me this morning while looking for reasons not to get out of bed at 06.00 that there is another way to present the dilemma.

[Begin Exercise Statement]

Scenario 1: You are flying along, level at FL 360. ATC says descend immediately to FL 350 due to conflict with traffic at 2 o'clock.

What do you do? Why?

Scenario 2: You are flying along, level at FL 360. You have another aircraft in sight at your 10 o'clock. It's night, so you have little range information, just direction. ATC says descend immediately to FL 350 due to conflict with traffic at 2 o'clock.

What do you do? Why?

Scenario 3: You are flying along, level at FL 360. You have another aircraft in sight at your 10 o'clock. It's night, so you have little range information, just direction. ATC says descend immediately to FL 350 due to conflict with traffic at 2 o'clock. At the same time, you get a climb RA for your visual target at 10 o'clock.

What do you do? Why?

[End Exercise Statement]

As far as I understand, for example, FullWings's answers, they would be:

1. Descend. Reason: conflict avoidance with unseen traffic.
2. Descend. Reason: conflict avoidance with unseen traffic.
3. Climb. Reason: RA advises to do so.

The difference between scenarios 2 and 3 is, however, minimal. Yet the avoidance manoeuvre is exactly opposite.
And the unseen conflicting traffic does not go away.

PBL

[FullWings]

PBL,

Replying to your latest post first:

As far as I understand, for example, FullWings's answers, they would be:

1. Descend. Reason: conflict avoidance with unseen traffic.
2. Descend. Reason: conflict avoidance with unseen traffic.
3. Climb. Reason: RA advises to do so.

The difference between scenarios 2 and 3 is, however, minimal. Yet the avoidance manoeuvre is exactly opposite. (My bolding)
And the unseen conflicting traffic does not go away.
Yes. Nothing very surprising here. Does the absolute direction of the avoiding manoeuvre matter if it resolves the conflict? In your scenario No. 3 you're assuming that the traffic you can see is the one that has generated the RA... That may not be the case, especially as "It's night, so you have little range information, just direction." The information presented to you also depends on the type of TCAS installation and what modes you have selected; TCAS is much less accurate in displaying bearings than distances, so the actual traffic could be in a significantly different direction. Also, if you can't see your 2 o'clock traffic and it isn't on TCAS then it's probably far enough away not to be an immediate risk, unlike the traffic that is causing the RA.

[PBL]

FullWings,

Yes. ... Does the absolute direction of the avoiding manoeuvre matter if it resolves the conflict?


The manoeuvre in Scenario 3 (which you affirm) obviously does *not* resolve the conflict with the unseen 2 o'clock traffic, and might well exacerbate it. It also conflicts with the principle you proposed in your [email protected], currently #41, that

The danger in aviation comes mostly from the aircraft you can't/haven't seen or aren't aware of


In contrast, the manoeuvre in Scenario 2 resolves the conflict with the unseen 2 o'clock traffic.


In your scenario No. 3 you're assuming that the traffic you can see is the one that has generated the RA...


I am not assuming anything at all.


That may not be the case .... The information presented to you also depends on the type of TCAS installation and what modes you have selected; TCAS is much less accurate in displaying bearings than distances, so the actual traffic could be in a significantly different direction


No. TCAS is said to be accurate to 15 degrees. The difference between 10 o'clock and 2 o'clock is 120 degrees, more than 30 degrees (= 2 x 15 degrees) which is the resolution necessary to resolve which aircraft is causing the RA.


Also, if you can't see your 2 o'clock traffic and it isn't on TCAS then it's probably far enough away not to be an immediate risk

Or it is a military aircraft, or a NORDO, or an older bizjet, or a Legacy whose pilots just kicked off the transponder and ATC is giving you a primary return assuming (appropriately) that he is maintaining altitude.

I am much less convinced than I was about your understanding of TCAS scenarios.

PBL

[bsieker]

you're assuming that the traffic you can see is the one that has generated the RA... That may not be the case, especially as "It's night, so you have little range information, just direction." The information presented to you also depends on the type of TCAS installation and what modes you have selected; TCAS is much less accurate in displaying bearings than distances, so the actual traffic could be in a significantly different direction.

As much as 120 degrees away? Is TCAS bearing accuracy reallky that bad (even in the worst case)?

Further, you are then assuming that you have traffic, that you can see, which is not on a collision course (could be), and traffic that is causing an RA, which you cannot see.

Also, if you can't see your 2 o'clock traffic and it isn't on TCAS then it's probably far enough away not to be an immediate risk, unlike the traffic that is causing the RA.

Probably not.

If ATC gives you an "expedite!" instruction to avoid it, it is probably quite close.

Further, in contradiction to your first quote, here you assert that traffic you cannot see, is probably far enough away.

So by your own logic, whether or not you can see your traffic, is not an indicator of proximity.


Bernd

[FullWings]

joernstu,

If you look for more evidence you will find complains of ATC on a device that manipulate the outcome of their instructions, leaving them without a coherent image of the situation in the air.
If I'd set two aircraft on a collision course in error I'd want something to manipulate the outcome of my instructions! Never heard/seen any complaints myself, especially from people who understand how the systems work.

Another problem with TCAS is that the manufacturer (and following him many other organisations) present TCAS as a technical solution in a conflict situation where every other means to avoid a collision have failed. In the Ueberlingen mid-air TCAS gave an RA when both a/c were at 7.11 NM seperation. The limit for seperation in this airspace at this night was 7 NM, usually 5 NM. This demonstrates that TCAS can act at points in time, where other means have not yet failed, rebuting the claim that TCAS only acts when everything else has already failed.
OK, RA given at 7.11NM, limit 7NM. Time to close 0.11NM was less than a second, so loss of separation was assured as no realistic course change could be made in that timeframe.

TCAS only knows what is happening, not "intent" (maybe in the future with mode-s, etc.) It has to assume there will be no outside intervention and I think 20-30s before collision is not unreasonable. In the event under discussion, primary means of separation had or were shortly going to fail so the secondary systems started to activate. There is inevitably going to be an overlap between the two but in the example above it's very small.

More theoretical arguments are multiple aircraft situations (for which it can be proved that TCAS cannot give advice solving the collision threat) and aircraft without transponders.
And these cases form what percentage of the threat population in an ATC radar environment?

Of course TCAS is better than nothing (at least I hope that this is the case as it was certified for use in aviation), but I also get the image, that it is still far from optimal.
At least we agree on something. :)

[bsieker]

In the event under discussion, primary means of separation had or were shortly going to fail [...]

Evidently not. ATC was about to issue instructions to separate traffic.

It was neither failed, nor was it going to fail shortly. If TCAS "thinks" late ATC intervention is a failure of ATC, then maybe its requirements are flawed.


Bernd

[FullWings]

In contrast, the manoeuvre in Scenario 2 resolves the conflict with the unseen 2 o'clock traffic.
Because you haven't come close enough to the other traffic to trigger an RA yet. That's why your scenarios are different - the aircraft are in different positions...


Originally Posted by FullWings
In your scenario No. 3 you're assuming that the traffic you can see is the one that has generated the RA...
I am not assuming anything at all.
No? Then I'm having problems with the meaning of this sentence:

..you get a climb RA for your visual target at 10 o'clock.
As opposed to: "...you get a climb RA"?

No. TCAS is said to be accurate to 15 degrees. The difference between 10 o'clock and 2 o'clock is 120 degrees, more than 30 degrees (= 2 x 15 degrees) which is the resolution necessary to resolve which aircraft is causing the RA.
OK, the aircraft that's going to hit you is at 9 o'clock. You haven't spotted this but you have seen the one at 10 o'clock. You ignore the RA because "you've seen it". Crunch. :ouch:

Or it is a military aircraft, or a NORDO, or an older bizjet, or a Legacy whose pilots just kicked off the transponder and ATC is giving you a primary return assuming (appropriately) that he is maintaining altitude.
Or it's a cruise missile, a weather balloon, a flying saucer, a flock of birds, etc. Very easy to find edge cases where the system may not work but how realistic are they in a positive radar environment? If you stray into the path of a UFO during an RA it's just not your day is it? (BTW, NORDO stands for "non-radio"; doesn't mean they're not squawking...)

I am much less convinced than I was about your understanding of TCAS scenarios.
Out of interest (and I'm not trying to "one-up" you or anything), how many real or simulated (as in sat at the controls) TCAS encounters have you experienced?

[FullWings]

Evidently not. ATC was about to issue instructions to separate traffic. It was neither failed, nor was it going to fail shortly. If TCAS "thinks" late ATC intervention is a failure of ATC, then maybe its requirements are flawed.
Yes but those instructions came too late to stop the aircraft coming within 7NM of each other at the same level, on a collision course. That, by definition, is a "failure of separation". "Late ATC intervention" is a "failure of separation", involving much form filling, suspension of the individuals involved, official reports, etc. I'm coming to think your definition of "separation" is "not actually hitting each other", which is not an industry standard!

[bsieker]

Yes but those instructions came too late to stop the aircraft coming within 7NM of each other at the same level, on a collision course. That, by definition, is a "failure of separation". "Late ATC intervention" is a "failure of separation", involving much form filling, suspension of the individuals involved, official reports, etc. I'm coming to think your definition of "separation" is "not actually hitting each other", which is not an industry standard!

Following your argument through would mean that if ATC fails to separate traffic before it comes inside the 7nm limit, the controller should shut up, and leave it to TCAS to do the job (or not). Better to take the risks of TCAS failure, than to violate an industry standard!

I don't think you're advertising that course of action.

But what, then, is your point?


Bernd

[FullWings]

Following your argument through would mean that if ATC fails to separate traffic before it comes inside the 7nm limit, the controller should shut up, and leave it to TCAS to do the job (or not).
No, the controller carries on trying to separate the traffic. If, in extremis, there comes a point where there are conflicting instructions from ATC and TCAS, TCAS is followed.

The order things happen is something like this:
1. ATC do their job and for virtually all the time get it right; but they are as human as the pilots so...
2. The two groups between themselves may, every now-and-then, trigger a STCA. This may lead to a change of plan or giving of further instructions.
3. A TCAS TA might be issued so that we can get the hosties off our laps.
4. If it gets to the point of an RA, then TCAS "controls the vertical" and ATC are informed ASAP of a deviation. Climb or descent instructions from ATC are ignored as their information is not as up-to-date as that possessed by the TCAS computers. (Amongst other reasons.)
5. When we've all missed each other, we initiate a return to our cleared levels and await further instructions from ATC...

But what, then, is your point?
My point is that TCAS is here, the installation and use of it is mandated and because of this, the way it is used has also been mandated. Until there is some real evidence that the way it is being used is wrong/dangerous, then this state of affairs will continue.

An open question: Can you point me in the direction of any hard evidence in support of the 'danger' hypothesis? Links, etc.?

[PBL]

In contrast, the manoeuvre in Scenario 2 resolves the conflict with the unseen 2 o'clock traffic.


Because you haven't come close enough to the other traffic to trigger an RA yet.


Exercise 1: Modify the scenario to make your reply inappropriate.


In your scenario No. 3 you're assuming that the traffic you can see is the one that has generated the RA...


I am not assuming anything at all.


No? Then I'm having problems with the meaning of this sentence:


..you get a climb RA for your visual target at 10 o'clock.


Exercise 2: Explain the difference between an assumption and a statement.


OK, the aircraft that's going to hit you is at 9 o'clock. You haven't spotted this but you have seen the one at 10 o'clock. You ignore the RA because "you've seen it". Crunch.


The TCAS screen apparently was painted two targets at 9 o'clock and at 10 o'clock, and the 9 o'clock was lit up red. That was not a scenario I set.


Or it is a military aircraft, or a NORDO, or an older bizjet, or a Legacy whose pilots just kicked off the transponder ...


Or it's a cruise missile, a weather balloon, a flying saucer, a flock of birds, etc. Very easy to find edge cases where the system may not work but how realistic are they in a positive radar environment?


According to Flight International, bizjets are quite realistic in the positive radar environment round London, indeed are said by the airlines to be causing a disproportionate increase in traffic. I would imagine in other places as well.

I don't think there is much point in continuing discussion unless you are going to take it seriously. Either you see the point of my scenarios, or you don't. If you do, it would be nice to get the answers. If you don't, it would be nice to have you say so loud and clear.

PBL

[FullWings]

The TCAS screen apparently was painted two targets at 9 o'clock and at 10 o'clock, and the 9 o'clock was lit up red. That was not a scenario I set.
No, I'm saying there's only one target on the TCAS display - it's been misidentified as the aircraft you can see (which is not in conflict at present).

I think I might be starting to understand why we seem to be having a bit of an irreconcilable argument. You seem to be looking at your "scenarios" from the point of an experimental set-up, where you have a "God's eye" view and have set the pieces in place and started the clock running. You know what the optimal response should be because you know everything about this imaginary universe.

I'm coming from the real world and thinking I'm sat there with only a limited amount of information to go on. I know there's conflicting traffic somewhere and I also know there has been a breakdown of ATC. I can't be sure of where these aircraft are or indeed how many are involved. I *can* see something out of the window but hey, that's not unusual in itself. I get an RA: I know I'm not in possession of all the facts so I have very little choice but to follow it - I have very little reason to disbelieve the proffered escape advice; even if I did, do I think that with my limited view out and lack of a complete picture that I could do any better than TCAS? Not really.

There are known unknowns and unknown unknowns... ;)

[bsieker]

FullWings,

I'm coming from the real world and thinking I'm sat there with only a limited amount of information to go on. I know there's conflicting traffic somewhere and I also know there has been a breakdown of ATC.

Industry standard or no, silence from ATC up until the point of the RA does not mean that you know ATC has broken down.

I get an RA: I know I'm not in possession of all the facts so I have very little choice but to follow it - I have very little reason to disbelieve the proffered escape advice; even if I did, do I think that with my limited view out and lack of a complete picture that I could do any better than TCAS? Not really.

I don't think anybody is debating that scenario.

And I would dare say that all agree that if there is no word from ATC, by all means, follow the RA. It's the best information you have, and in all likelihood will avoid the traffic.

The problem is when you get conflicting advice from ATC and TCAS, particularly if in the order:

- ATC advice
- starting following ATC
- TCAS RA

Which I do not see you considering at all. You seem to be stuck in the scenario where ATC has in fact failed. But the premise that that has always happened at the time TCAS issues an RA does not hold.

We know late ATC intervention happens, and just because it is not "industry standard", you cannot simply ignore it when looking at the entire socio-technical system, which includes two flight crews, two aircraft with TCAS, and ATC with primary and secondary radar.


Bernd

[FullWings]

The problem is when you get conflicting advice from ATC and TCAS, particularly if in the order:

- ATC advice
- starting following ATC
- TCAS RA

Which I do not see you considering at all. You seem to be stuck in the scenario where ATC has in fact failed. But the premise that that has always happened at the time TCAS issues an RA does not hold.
From a pilot's POV I have to assume that there has been a failure of ATC when in receipt of an RA and act appropriately. Why? If I assume that ATC has not failed and carry on with their advice (possibly against an RA) and I am wrong in that assumption, I could be part of an accident. If I follow the RA, it will keep me clear of traffic, whether ATC are giving valid commands or not...

I one of my previous posts I said:

No, the controller carries on trying to separate the traffic. If, in extremis, there comes a point where there are conflicting instructions from ATC and TCAS, TCAS is followed.
and

If it gets to the point of an RA, then TCAS "controls the vertical" and ATC are informed ASAP of a deviation. Climb or descent instructions from ATC are ignored as their information is not as up-to-date as that possessed by the TCAS computers. (Amongst other reasons.)
I think that covers the conflict case?

Off flying across the Atlantic now with my trusty TCAS, EGPWS and sextant.

[robin]

Sorry

This bit of the thread is baffling me.

I assume that most of those posting are talking about the situation in Class A and B airspace.

In my limited experience, mainly outside of this type of airspace, taking TCAS over ATC is quite dangerous. In Class G, for example, there are many a/c either not squawking or using Mode A only. For them to ignore an ATC contact and only work off their TCAS seems to be perverse.

[PBL]

robin,

whose "Class A, B, G airspace" are you talking about?

PBL

[robin]

The UK's airspace

[ATC Watcher]

I do not want to interfere too much in this fascinating debate but there are a couple of misconceptions and assumptions ( most coming from Fullwings ), that I’d like to correct :
Fullwings :
Yes but those instructions came too late to stop the aircraft coming within 7NM of each other at the same level, on a collision course. That, by definition, is a "failure of separation". "Late ATC intervention" is a "failure of separation", involving much form filling, suspension of the individuals involved, official reports, etc. I'm coming to think your definition of "separation" is "not actually hitting each other", which is not an industry standard!
Not quite. In real life, late ATC intervention is rather common for various reasons, not all due to ATC errors, and 99,9999 % of the cases it solves the problem. There is no consequence. If a controller had to be replaced every time he use the word “expedite” there will be little staff left in my center !
ATC works on establishing separation ( typically 1000 ft and 5 NM )
TCAS was not designed to re-establish separation ..It was designed to prevent metal from touching each other.
(In most cases TCAS missed distance is unlikely to be more that 300 feet.)
Mixing the 2 leads to misconceptions on how both work in real life.
The misconception that once you get below “separation “ a controller should not issue any instruction is totally wrong and dangerous. ATC is responsible to provide separation , and anti collision avoidance at all times , unless the PIC in command has informed him that the crew is following an ACAS RA. From this moment on , pilot takes over responsibility of anti collision from ATC., but before that moment, the controller is responsible, above or below 5NM .
The problem we encountered in Ueberlingen ( and in many other less dramatic incidents ) is that ACAS intervenes BEFORE separation is lost , and pilots do not inform ATC that they are following an RA, enforcing the controller’s mind that everything is still under HIS control, and might even continue to issue instructions well after TCAS has interfered.
Note : As a controller you do not know if an aircraft has a functioning TCAS or not (e.g. : one can fly 10 days with a U/S unit) and the same service is given regardless of having TCAS or not.
Fullwings :
From a pilot's POV I have to assume that there has been a failure of ATC when in receipt of an RA and act appropriately.
From my ATC experience and recorded data, until very recently, most RAs were not due to ATC but caused by excessive vertical rates by crew. Therefore the ICAO note to reduce vertical rate before leveling off.
There are many non-ATC other cases , such as ghosts targets, or level busts for instance , that are causing RAs.
The notion that an RA only occur when ATC has failed is wrong.
Lastly :
Fullwings again :
Or it's a cruise missile, a weather balloon, a flying saucer, a flock of birds, etc. Very easy to find edge cases where the system may not work but how realistic are they in a positive radar environment? If you stray into the path of a UFO during an RA it's just not your day is it?
Not a UFO, but a military a/c ( quite a few where I come from ) or an out of tolerance SSR that will not be accepted by TCAS., but visible on radar.
In my ATC system, an SSR failure will be compensated and tracked on primary , that one too will be invisible on TCAS . Remember that I have legally to separate aircraft from what I know.
If ATC instructions differ from what TCAS is telling you , there you have a dilemna. Do I have , as a controller, the right picture, with every player, moving as my radar tells me, or does TCAS has the correct solution ? . You will never know until you try , but either way, it is not a 100% guarantee that what you chose is the correct action.
Finaly :
Bsieker :
The problem is when you get conflicting advice from ATC and TCAS, particularly if in the order:
- ATC advice
- starting following ATC
- TCAS RA
This is indeed one of the real issues .Seeing the data I see , all post 2002, leads me to believe that we have not solved the problem yet and that another accident is probable.

[PantLoad]

At my company, the SOP is to ALWAYS FOLLOW THE TCAS RA, except in cases of windshear, terrain warnings, and stall warnings. We are to follow the RA, and as soon as practical, inform ATC of the RA maneuver.

THIS INCLUDES FLIGHT IN VMC WHERE THE OFFENDING TRAFFIC IS IN SIGHT. (Is what you're looking at really the conflicting traffic, or is there another aircraft out there that you don't see, that's about to hit you!!!!)

The logic is this: The RA is a coordination between two aircraft with TCAS. The two TCAS computers have identified a conflict and have devised a resolution. Failure of one aircraft to follow the plan...will screw things up...since nobody tells the TCAS computer of aircraft #2 that the pilot(s) of aircraft #1 has decided to do his own thing.

At my company, several years ago, there was a major $%%^&. One of our flights was departing a Florida airport...climbing out...received a TCAS RA. The flight was in IMC. They followed the RA...as per SOP...but, immediately afterward, got a second RA. They followed it, as per SOP, as well.

At first thought, one would assume that the second RA was due to the following of the first RA. After the BIG investigation, it was determined that the controller, that morning, suffered from a major brain fXrt...and the two conflicts were real...and the second one was not due to the crew following the RA of the first conflict...it was the real thing, too.

Now, the crew figured they were having a bad day, too. What's up with getting two RAs...one right after the other...in IMC!!!!!

Thank God, these guys followed the SOP! Otherwise, we'd be doing a 'coulda, shoulda, woulda' analysis on PPrune!


PantLoad

[alf5071h]

FullWings, pursues the more practical and simpler route (#60 onwards), which often the regulatory segments of the industry choose to follow, albeit hidden in the paperwork. This practical approach has been most valuable in persuading the industry of the relevance of new equipment in the threat environment, and the need for standardised (world wide) procedures.
ACAS, like EGPWS and windshear warning systems, has enabled the industry to visualise a particular threat to safe operation, which previously many people did not believe existed (numerically), or did not whish to believe posed a significant risk. We now have growing evidence of the magnitude of the threat and a means (amongst others) of counteracting it.

I suspect that ACAS will continue to evolve (refinable) as did GPWS. I further suspect that any significant ‘failures’ in the overall ACAS safety system will originate from human error at the point of operation and not at the point of design, again compare this with EGPWS and the many incidents of incorrect crew action (again a suspicion, not provable).
The greater problem for the industry is how to get pilots to follow the ‘instructions on the tin’; something which applies to many systems and is fundamental to human behaviour.
ACAS may not assist our quest for correct behaviour in its use of terminology, e.g. using ‘advisory’ and ‘resolution’ (legal/engineering bias) when describing situations which warrant much higher or urgent levels of alerting and warning. These terms may mislead non English speaking pilots or the regulators (often with government/ legal structures) who may apply ICAO recommendations from their perspective and not from that of the crew. But such is the outcome of the worldwide adoption of a system based on a single national culture, perhaps without the necessary depth of understanding and debate on its design philosophy and operation.

ATC Watcher identifies a significant issue with the ATC/ACAS system, perhaps the issue; communications. The ATC based anti collision avoidance depends on communications at all times. Communications are fallible; even the alleviating action of the aircraft informing ATC of an ACAS manoeuvre depends on communication. Until this problem is resolved then there is a role for ACAS.
From the descriptions above, I see ATC anti collision avoidance as the strategic activity, generally proactive, but reactive when necessary and if the conditions (time) allow. ACAS is the tactical, reactive system; the last line of defence and thus at the time of warning it must have precedence.

There are similarities here with CFIT. ATC can provide early defences with safe altitudes and alerts, but still suffer problems of communication in the reactive sense, e.g. delays in transmitting MSAW warnings. EGPWS again provides the tactical, last line of defence.
For both ACAS and EGPWS, the success of these systems depends entirely on the crew following the instructions.

Peter Re PhD; x,y,z, = the 3 aircraft, but lets use your coordinate system. Am I correct in assuming that the 3D solutions to the quartic equation are inside the ACAS alerting area, and thus are realistic solutions for the risk of collision and not just imaginary space-time zones?

[joernstu]

If I'd set two aircraft on a collision course in error I'd want something to manipulate the outcome of my instructions!
You assume that the instructions inevitably would have lead to a collision.

Never heard/seen any complaints myself, especially from people who understand how the systems work.
Controlling the airspace by radio communications takes time. A controller telling one aircraft to descend, the other to climb or maintain altitude can make these instructions only sequentially. In this scenario the controller's instructions would have prevented a collision.

If the TCAS system recognises this collision threat and if the second instruction was made not timely enough, TCAS issues an RA, which can be inverted to the controllers instructions.

Now, the controller cannot judge the sitiuation and actions of the aircraft under his control. He can only pray, that they solve the conflict on their own. Hardly a mental state I want to be in.:bored:

[PBL]

robin,

The UK is unusual in offering some level of IFR separation through ATC in its Class G airspace. Most countries, such as the US, only offer separation in Class G airspace through flight planning (that is, ATM not ATC).

Most TCAS-equipped kit will be flying most of the time in Class A or B airspace, and (in most countries) ATC is not going to be providing separation under IFR for Class G airspace, so TCAS-ATC interactions in Class G airspace would not be an issue in most countries. However, I have no experience with IFR in Class G airspace in the UK, so I can't usefully comment on that.

PBL

[PBL]

alf,

I don't necessarily agree that *the* issue with ACAS/ATC interaction is communication. The communication issue would theoretically be solved by having secondary radar interpret and display the Mode S interactions between two TCAS-equipped aircraft undergoing an RA sequence.
Under a TCAS RA manoeuvre, the aircraft involved can well depart cleared airspace. That means that the controller has suddenly to replan, and that may neither be desirable nor, in some circumstances, possible, perfect communication or not.

And since TCAS may (and does) propose manoeuvres when aircraft are still legally separated, (and in the case of "zoom climbs" the crews likely will not bust their clearance) that means this replanning problem may be artificially introduced.

I must say that it does surprise me that, even at this late date, many line aircrew seem to be unaware of the problems of TCAS/ATC interaction. (For example, FullWings suggested he wasn't aware of problems; ATC Watcher implicitly pointed out that he was aware of lots.) I could have understood it, say, a decade ago, but I don't understand it five years after Überlingen.

I think some of it may be due to Eurocontrol's and others' attempts to impose uniform RA behavior on aircrew. If you are trying to get all pilots everywhere to follow an RA, it works against that message to publicise all the incidents in which TCAS might prompt inadvisable or unnecessary manoeuvres. So the incident narratives are suppressed (to which phenomenon I can attest, having tried to get data on TCAS incidents).

I originally said that I am not taking a stand on whether this is a "good" or a "bad" thing; that I was simply noting a phenomenon. That is not quite true; I do take a stand. I am for transparency of information on all safety-relevant systems in which there is a public interest. Just as the UK has an Airprox Board that reports publically on all designated near misses, I think data on TCAS manoeuvres should be publically collected and publically analysed and displayed.

Then maybe we would not see such travesties as Eurocontrol's declaration in their EUR-RVSM Safety Case that they had not identified any anomalous ACAS-RVSM interactions and that therefore they did not need to address such potential interactions in the Safety Case (even after I pointed such out to them, they maintained their "view").

PBL

[PBL]

The logic is this: The RA is a coordination between two aircraft with TCAS

Exactly. And one of the questions is, if you have been following this thread, what does it do with *three* aircraft?

Answer: no one actually knows, but many people seem to assume it will just be OK.

PBL

[PantLoad]

PBL,

Yes, good question!!!!

In the case of my company's incident that occurred several years ago, I honestly don't know the version of TCAS that was installed on our aircraft at that particular time. But, the TCAS system worked for the two conflicts. The second conflict occurred almost immediately after the first.

But, who knows what would happen if two or more conflicts occurred at exactly the same time...from two or more different directions...two or more different altitudes!!!!!

I guess, if that happens to you, you can definitely classify that as a 'bad day'!!!! :rolleyes:


PantLoad

[FullWings]

Wow, seems to be a lot of interest in this subject... Shame I can't post from the jet (yet!).

ATC Watcher,

ATC is responsible to provide separation , and anti collision avoidance at all times
Absolutely. I have stated that quite positively.

The notion that an RA only occur when ATC has failed is wrong.
Not what I said. What I am saying is that the penalty for assuming that TCAS has got it wrong (and being wrong in that assumption) is much higher than for assuming ATC has got it wrong (and being wrong in that assumption). Apply a bit of logic to that and you can see why I'd follow an RA. It's a sort of Pascal's Wager for aeroplanes...

If ATC instructions differ from what TCAS is telling you , there you have a dilemna. Do I have , as a controller, the right picture, with every player, moving as my radar tells me, or does TCAS has the correct solution ? . You will never know until you try , but either way, it is not a 100% guarantee that what you chose is the correct action.
At the end of the day it's down to probabilities, isn't it? In order to hit an aircraft during an RA (assuming you're following the guidance correctly) there would have to be another aircraft in addition to the one(s) that gave you the RA that a) Doesn't have a working TCAS or transponder and b) just happens to be in exactly the right place at the right time. I would suggest (although I haven't published a paper on it yet) that you are much more at risk from the *known* target(s) you are avoiding because of a *definite* risk of collision than the *unknown* ones that *might* be there (or not)?

alf5071h,

The voice of reason, as always.

joernstu,

Originally Posted by FullWings
If I'd set two aircraft on a collision course in error I'd want something to manipulate the outcome of my instructions!
You assume that the instructions inevitably would have lead to a collision.
If you (as a controller) have sent two aircraft directly at each other and there is no further intervention (from you, pilots, TCAS, etc.) then they're going to collide. ATC in Africa have plenty of practice at setting this up!

Controlling the airspace by radio communications takes time. A controller telling one aircraft to descend, the other to climb or maintain altitude can make these instructions only sequentially. In this scenario the controller's instructions would have prevented a collision.
We don't know. ATC voice instructions have a very high potential latency compared with TCAS interactions. If there are other aircraft on the frequency and one transmits for, say, 20 seconds then there is no chance to issue any avoiding instructions until too late. The situation becomes worse because if a potentially serious conflict is noticed by pilots, one of the first things that often happens is they check with ATC about the situation, thereby blocking information flow from ATC to the pilot(s); Standard VHF R/T is only half-duplex.

[FullWings]

PBL,

Taking some comments out-of-order...

...I do take a stand. I am for transparency of information on all safety-relevant systems in which there is a public interest. Just as the UK has an Airprox Board that reports publically on all designated near misses, I think data on TCAS manoeuvres should be publically collected and publically analysed and displayed.
I can't disagree with that, although "public" should really mean interested academic groups, IMHO, unless you want it all appearing in the Daily Mail under "Killer Deathjets in Head-on Plunge!!"

If you are trying to get all pilots everywhere to follow an RA, it works against that message to publicise all the incidents in which TCAS might prompt inadvisable or unnecessary manoeuvres.
Without wishing to be rude, I think you've got it slightly backwards. What we really don't need is people going around responding to RAs with Manoeuvre($Random). I'm sure you can see that?

So the incident narratives are suppressed (to which phenomenon I can attest, having tried to get data on TCAS incidents).
I'll take your word for it. I suppose you might have to cross proprietary and DPA boundaries to get this information as an individual, although I'm sure that agencies such as the AAIB and CAA would have as much access as they desired?

Maybe what needs to be done is to run a series of simulated encounters, no, let's make that a *lot* of encounters, using the actual software involved and analyse the results, possibly making changes to the software... Hmm I wonder if anyone has ever done that? (Sorry, been up for 36hrs after a delayed nightflight so am a bit sarky. ;) )

[joernstu]

If you (as a controller) have sent two aircraft directly at each other and there is no further intervention (from you, pilots, TCAS, etc.) then they're going to collide.

If this really is the case, how do you explain, that not every loss of separation situation inevitably lead to a mid-air?

I understand that you - and many company's SOPs - instruct the crew to follow the TCAS RA. This will be the solution for 2 aircraft situations, which are the most probable to occur. But this threat is not only about what TCAS can do, but also about what it cannot do - and the effects the TCAS design produce.

ATC in Africa have plenty of practice at setting this up!I imagine, there is a reason for stagger and right-of-way rules in poorly monitored areas.:)
But if the african ATC is so capable in producing dangerous situations, and the probablity of aircraft flying without operational TCAS is highest in poor regions like Africa, why don't we hear about more mid-air collisions over Africa in the news?

We don't know.So, there is a probablity, that ATC will not solve a critical solution. But why does it sound to me, that you weight the probability of shortcomings in the TCAS system more lightly, than you weight an ATC instruction not solving a conflict situation. (Note that the scenario was not about latency in communications, preventing the crew to get the instruction.)

ATC voice instructions have a very high potential latency compared with TCAS interactions. If there are other aircraft on the frequency and one transmits for, say, 20 seconds then there is no chance to issue any avoiding instructions until too late. The situation becomes worse because if a potentially serious conflict is noticed by pilots, one of the first things that often happens is they check with ATC about the situation, thereby blocking information flow from ATC to the pilot(s); Standard VHF R/T is only half-duplex.Latency and problems with half-duplex communication are well known problems, but what would solve this? I don't think that one can rely on technical conflict solution as much, as would be necessary for freeflight, e.g..
Perhaps the introduction of full-duplex digital communications and transmission of digitized instruction, with person-to-person communication being an exception for uncommon situations could help, but their impact on aviation safety is still unknown.

[bsieker]

If you (as a controller) have sent two aircraft directly at each other and there is no further intervention (from you, pilots, TCAS, etc.) then they're going to collide.

Only if
- they're going directly head-on
- both are flying very accurately.

The latter is usually true these days with high-precision navigation, although some oscillations around the exact altitude are normal for every feedback-control loop. (I don't know about other types, but Airbus FBW aircraft's autopilots have a "soft-altitude" mode for cruise that allows up to 50ft deviation to reduce thrust changes for comfort and economy.)

As to the first point, of the two recent Mid-Airs, only in one were they going in directly opposite directions (GOL/ExelAire), in the other (Bashkirian/DHL)they were at around 90 degrees, and for a collision in that setup you have to get it exactly right, otherwise they'll miss.

Not what I said. What I am saying is that the penalty for assuming that TCAS has got it wrong (and being wrong in that assumption) is much higher than for assuming ATC has got it wrong (and being wrong in that assumption). Apply a bit of logic to that and you can see why I'd follow an RA. It's a sort of Pascal's Wager for aeroplanes...
I'm curious what statistic this assertion is based on.

The penalty for both might be a Mid-Air. The DHL crew in the Ueberlingen accident followed the RA to the letter. And were rewarded with a collision.

Consider the Ueberlingen scenario with a slight modification: The Tuplolev has, as allowed for dispatch, an unserviceable TCAS system, but fully working transponder.

Thus, the DHL Boeing gets a resolution advisoary, but the Tupolev gets none. The Tupolev does get, however, a manoevering instruction from ATC ("descend FL350 expedite", iirc). The difference here being between not following an RA versus not getting it. Educate me about TCAS: would the DHL crew have known about the Baskirian's TCAS being inoperable?


Bernd

[FullWings]

...how do you explain, that not every loss of separation situation inevitably lead to a mid-air?
OK. "Separation" in an ATC sense means the horizontal, vertical and time buffers between aircraft. If any of these becomes less that the proscribed value for the type of airspace/aeroplane/service, etc. then there has been a "loss of separation". This does not mean that there has been/will be a collision, simply that the "protected area" around an aircraft has been infringed. If the limit in a particular scenario is 5nm and the aircraft come within 3nm of each other, then that is a "loss of separation"; On the NATS 10 minute separation is applied with 1,000' vertically and 60nm laterally (well, a bit less as you are allowed to offset) - if you end up 8mins apart, action is taken to increase the separation again.

why don't we hear about more mid-air collisions over Africa in the news?
We fly offsets and make position reports to each other on a common frequency. Conflicts are often discovered and managed by the pilots and may not involve ATC at all...

Perhaps the introduction of full-duplex digital communications and transmission of digitized instruction, with person-to-person communication being an exception for uncommon situations could help, but their impact on aviation safety is still unknown.
This sort of thing has been slowly coming for quite a while. We use ADS & CPDLC in remote areas quite a bit now, and are trialling them in the Maastrict FIR. I think we also did a FANS flight from the Southern USA on a direct track to the holding fix in the UK. It's getting there.

[FullWings]

Consider the Ueberlingen scenario with a slight modification: The Tuplolev has, as allowed for dispatch, an unserviceable TCAS system, but fully working transponder.
Yes...

the DHL Boeing gets a resolution advisory, but the Tupolev gets none. The Tupolev does get, however, a manoevering instruction from ATC ("descend FL350 expedite", iirc). The difference here being between not following an RA versus not getting it. Educate me about TCAS: would the DHL crew have known about the Baskirian's TCAS being inoperable?

Would the DHL crew know about the other aircraft's TCAS? No, unless they'd been told about it.

Would the DHL aircraft's TCAS know about this? Yes. It would treat the Tu-154 as a "dumb" target and expect to be doing all the manoeuvring whilst tracking it.

If two or more TCAS equipped aircraft come into conflict with each other there may be an element of coordination between the TCAS units to decide who does what; e.g. "You go up, I'll go down". This is what makes manoeuvring against an RA potentially serious as other aircraft may have begun to manoeuvre in expectation of a certain manoeuvre from you. It's a bit like indicating left on a busy road then suddenly turning right.

[bsieker]

FullWings,

thanks for the reply,

I have no intimate knowledge of the TCAS algorithms, but my point was to see if this was a credible scenario:

- Two airliners, both with transponders, one with serviceable TCAS, the other unserviceable.

- On conflicting trajectories ("collision course")

- TCAS-aircraft gets RA ("Descend, Descend!"), complete with red area, possibly larger than in the case whin which the conflict is expected to climb, but I'm on shaky ground on this.

- Non-TCAS aircraft gets ATC-instruction to descend, but, significantly, does not get an RA. (With the instruction to descend to the next FL below, separation is supposedly restored, from an ATC point of view.)

This scenario is facilitated by:

- Lack of knowledge by ATC of RA
- Lack of knowledge by TCAS-aircraft flight crew of lack of TCAS in conflict aircraft
- Rules of dispatch for U/S TCAS unit (10 days, is it?).

If this isn't a clear hazard (read: danger) created by following an RA I don't know what is.

And for this scenario we don't even need to suppose one flight crew disregarding an RA, rules may well be for all operators to always follow the RA!

I'd be grateful for anyone to point out possible factual errors in this one, since I'm not familiar with TCAS internals and the exact conditions under which what kind of RA will be issued by TCAS on encountering a "dumb" target.


Bernd

[alf5071h]

Bernd, when setting such a scenario it should be to qualified with the probability of it occurring.
Your example assumes one TCAS inoperative, that the strategic defence has already failed (ATC plan), and that the Controller’s command occurs after the TCAS warning. The latter involving a critical time span of approx 30sec.
Then there is the assumption that the two aircraft will manoeuvre precisely as required to result in a collision.

TCAS will command a minimum descent (1500ft/min ? (check spec values)), the manoeuvre assumes a standard crew response time and acceleration (2sec, 1.25g).
The aircraft responding to an urgent ATC command is not bounded by time or acceleration, but experience suggest that it will not be greater than the TCAS manoeuvre.
Thus there is a vast range of variables in both the start and action conditions that will not result in a collision.
However, given that the initial manoeuvres meet the collision criteria, then TCAS will continue to evaluate the situation and can increase the decent rate attempting to avoid the collision.
If you wish to debate the next level of what-if … ATC demands a higher rate of descent for the non TCAS aircraft, then the whole gambit of time, reaction, acceleration, and TCAS counter-counter response starts again. The practical number of what-ifs depends on the total time span, which provides a limit to the scenario.

No doubt in theory a collision scenario can be generated, but to a simplistic pilot this seems most unlikely and thus we are required to trust the overall controller / ACAS system; ergo training / discipline.
The safety certification should show that the probability of the collision scenario is sufficiently extreme to discount it; thence it meets an acceptable level of safety. The principle of aviation safety is that it is not absolute; it accepts that rare events can occur, but the probability of them resulting in death has to be extremely remote (10-9).
Your approach appears to take the limit case as ‘the’ hazard without qualifying (bounding) it with the probability of its occurrence. Yes it is a real hazard, but if by following TCAS it only occurs in one manoeuvre in 10 million, then the industry accepts that following TCAS is the safer option.

[bsieker]

alf5071h,

It is extremely hard, to quantify the probabilities of very rare events.

But here is the reason why I think that it is not too far-fetched, and is a realistic, if rare, scenario to be considered when advocating "Always follow TCAS!".

It is essentially the same scenario as happened at Ueberlingen, the only difference being that at Ueberlingen, the Tupolev did get an RA, but chose to follow ATC instead. And that scenario did result in a collision. In that case, too, the Boeing's TCAS would have monitored the Tuploev's course, and it issued an increase-descend RA.

It has been debated why the Boeing's TCAS did not issue a reversal RA ("CLIMB NOW! CLIMB!") in this instance, as would seem appropriate.


While it may be true that this particular accident could have been avoided, if both pilots had followed their respective TCAS RAs, the Bashkirian crew's action were rational in light of the perceived three-aircraft-conflict.

But let's look at the decisions of the individuals involved in the modified setup:

- TCAS-aircraft crew:
Get TCAS RA (possibly after a TA)
SOP: follow ATC, except when getting in RA, in which case follow RA
Rational choices available: follow RA
Rational choice taken: follow RA.

- non-TCAS-aircraft crew:
Get ATC instruction
SOP: follow ATC, except when getting in RA, in which case follow RA
Rational choices available: follow ATC
Rational choice taken: follow ATC (in the absence of an RA)

- ATC:
notice conflicting traffic at the same FL
SOP: separate traffic
Rational choices available: issue climb or descend instructions to either of the involved aircraft
Rational choice taken: issue descend order to non-TCAS aircraft

So, ATC could take any of four different choices, but without knowing the circumstances, neither is favoured over the others (perhaps descend instructions are favoured, because it is unclear if the aircraft can climb quickly enough, or if it is even at its operational ceiling). Other factours may make the descend instruction to the non-TCAS aircraft the preferred one.

Are there data availabel on the availability of TCAS? What, thus, are the chances of two aircraft with inop TCAS "meet" each other?


Bernd

[PBL]

alf,

The safety certification should show that the probability of the collision scenario is sufficiently extreme to discount it; thence it meets an acceptable level of safety.

"Should" but rarely "does".

The quality of safety certification is a big issue. I have my problems with some of it (see my paper on the EUR-RVSM safety case). The big three issues concerning the U.S. and U.K. principals on certification of dependable-system SW are
* Explicit and accurate statements of dependability claims
* The provision of evidence sufficient to demonstrate those claims
* Transparency: public disclosure of the claims and evidence so that they can (one hopes) be checked through peer review

I just ran a panel session on this at SAFECOMP. It is a problem not just in aviation.

Let me add a bit of personal history for those who like gossip. Back in the days when I was a more assertive debater than the softie I have become :) , I got into a discussion on the Bluecoat list with an ex-MD avionics engineer name of Ray Hudson, who claimed he had seen enough in-flight evidence of the reliability of his systems to justify the usual 10^-9 claim. I said no, Ray, that is not possible and here are the scientific papers that show it definitively. He called me the usual non-pilot, non-avionics names as well as commented frequently on my ancestry; I said that whether or not he had evidence for his claims really didn't depend on how many legs my mother had. Result was that I was made a "participation offer" by Bill Bulfer that I couldn't accept, and departed.

Many years later, this theme has been the subject of a PhD dissertation at Cornell (John Downer) after I suggested it to Trevor Pinch; a main theme in the U.S. National Academy of Sciences Committee on Certifiably Dependable Software Systems recent report on just that; a major public concern of the U.K. principals in BSI oversight of IEC 61508 (the non-aviation international standard on functional safety of programmable electronic systems), and subject of concern in major non-aviation sectors (such as the automobile industry, which, in contrast to aviation, really does have kit that goes 10^9 operational hours - and sometimes more). As well as my panel at SAFECOMP. And Bluecoat appears moribund. Bad call by BB :p

PBL

[joernstu]

@bsieker

I think, the chances of your scenario leading to a mid-air collision will be rather low. As my knowledge on TCAS algorithms is mostly based on the Ueberlingen accident report, it is an educated guess, though.

This is how I think the system will work:

aircraft 1 (TCAS) get's RA descend.
aircraft 2 (ATC) get's instruction descend.

Both aircraft will descend following SOPs (1.25g, reaching approximately 1500ft/min descend rate).

TCAS onboard aircraft1 will monitor conflict situation. If aircraft 2 is lower than aircraft 1, TCAS will issue a Level off or perhaps a Reversal RA, if aircraft 1 is lower than aircraft 2, it will issue an Increase RA. In case the altitude of aircraft 1 and aircraft 2 are identical, I think TCAS would vote for an increase RA, as this would probably be more comfortable for the passengers and the course of action would be more straight forward (but this is only a guess).

In the Level off/Reversal RA scenario, aircraft 2 will continue its descend with 1500ft/min, aircraft 1 will reduce its descend rate. As aircraft 1 was above aircraft 2, the chances are, that both aircraft will avoid a mid-air.

In the Increase RA scenario, aircraft 1 will increase its decend rate following SOPs (1.25g, reaching approximately 2500ft/min). As aircraft 2 will continue its descend with 1500ft/min and aircraft 1 was lower than aircraft 2, chances are, that both aircraft will miss each other.

Both scenarios are based on the assumption, that ATC does not give further instructions to aircraft 2 after the first "descend" order.

If ATC was monitoring the situation, the controller could have noticed, that aircraft 1 was descending. In this case, the heshe could have issued a "level off" order to aircraft 2.

This case only has an impact on the "Level off/Reversal" Scenario above, where it would reduce the chances of both aircraft avoiding a mid-air.

But the premises for this last scenario would be rather high, as conflict recognition systems at ATC most probably will identify the conflict situation between aircraft 1 and 2, influencing the controller's actions.

[Bus14]

Peter,

Totally off-thread, but I'd rather follow a moribund thread on Bluecoat than a liveley one in this place. Isn't there enough water under the bridge for you and Rainman to come back, collectively or individually, to liven the old place up?

[bsieker]

Joern,

I agree with you that what you describe is a likely outcome. I don't think, particularly in crossing (as opposed to opposite) traffic, that the chances of a collision are very high, particularly if both aircraft manoeuvre.

But "rather low", as you put it, is not a level of probability I'd be comfortable with if it describes mid-air collisions. I'd prefer "extremely remote". ;)

But we know that at Ueberlingen they did collide, and what makes my hypothetical setup sufficiently different from Ueberlingen that everyone seems to be confident it would not be a problem?

Yes, a lot of factors have to come together, such as land-communications failure, ground based collision warning system failure, high ATC workload, ...

But it does happen in real life. And who can put a number on its probability?

Or even give a sound argument why it is low enough to discount in the face of an actual occurrence?


Bernd

[PBL]

Bus14,

Ray lost the argument some years before he began it, so that's not an issue :)

The main points about Bluecoat and my participation are
* I spend much of my time thinking about and discussing accidents. That is a no-no on bluecoat.
* BB's participation philosophy includes careful control of discussion, and that was my real problem. My experience with technical on-line discussion groups, which extends over some 25 years now, is that they are fragile things whose worth is equally shared between the contributions of its "top" members and the occasional decisive intervention by chatterers. So I am not much into control of discussion because one loses the second of those factors.
* I am a bit out of touch now with people and developments in avionics (John Rushby, Paul Miner, Kevin Driscoll and co) and more in touch with rail and automobile people. The problems commercial aviation had, mildly, with digital avionics two decades ago are now beginning in spades with cars, but they are not the same issues. It turns out to be hard to specialise in both.

But I'll think about it.

PBL

[FullWings]

While it may be true that this particular accident could have been avoided, if both pilots had followed their respective TCAS RAs
Right. This is the reason we're having this discussion. Technically I agree with the "may" bit too as it covers probabilities down to "unlikely, even during the total lifespan of the universe". What Alf & I have been saying is that the aviation industry and regulators, using available data, are satisfied that TCAS provides the best protection in a very short-term conflict, if the commands are followed.

...the Bashkirian crew's action were rational in light of the perceived three-aircraft-conflict.
Great play is made of the "third aircraft" at Überlingen. After a quick skim through the report and voice transcripts, all I can find in terms of direct evidence is a call from the controller to the Tu-154: "...we have traffic at your... 2 o’clock", whereas radar replays show the traffic to have been at 10 o'clock. The Tu-154 would have been roughly at 2 o'clock, viewed from the 757, so from someone under stress, watching two blips on his radar merging into one, it's understandable that he got it the wrong way round. The report shows that both parties had been seen by each other some time before the collision, so the "third aircraft" conjecture was moot by then.

[joernstu]

After a quick skim through the report and voice transcripts, all I can find in terms of direct evidence is a call from the controller to the Tu-154: "...we have traffic at your... 2 o’clock", whereas radar replays show the traffic to have been at 10 o'clock.

If you look at the hard evidence, this radio call is the only indication for the three aircraft situation in the heads of the TU154 crew. But even the BFU investigating the accident assumed, that the TU154 crew was searching for traffic in the wrong sector.

[bsieker]

FullWings,

I notice that you still haven't addressed my main point:

Why is my modified scenario so different from Ueberlingen that it can be discounted as "extremely unlikely"?


The only reason I can imagine for your reluctance to address it is that you feel Ueberlingen was a fluke, it is considered that low "acceptable risk", and nothing needs to be done about it.

Is that it?


Bernd

[ATC Watcher]

Alf5071h :
The safety certification should show that the probability of the collision scenario is sufficiently extreme to discount it; thence it meets an acceptable level of safety. The principle of aviation safety is that it is not absolute; it accepts that rare events can occur, but the probability of them resulting in death has to be extremely remote (10-9).

Your approach appears to take the limit case as ‘the’ hazard without qualifying (bounding) it with the probability of its occurrence. Yes it is a real hazard, but if by following TCAS it only occurs in one manoeuvre in 10 million, then the industry accepts that following TCAS is the safer option.
I remember clearly the FAA initial “ marketing” speeches when introducing TCAS in the very early 80s: No safety case as we know today was made, it was a political decision that mandated TCAS in the USA , not a rational one.
An initial independent paper showed that in 100 encounters, TCAS will solve X number of cases , would not make any difference in X number but in 4 cases it “ could induce a collision “
But the system was judged extremely beneficial and it was mandated.

If you look carefully at the official TCAS II training manual you will find in a general text mentioning something like : “in rare cases the system might induce collisions “ ( I am overseas at the moment , with no access to my archives for correct text /references )
4% is far from 1 in 10 million , but to be fair the percentages were calculated with version 5.0, since then we had 6.0, 6.04, 6.04A and now 7.0, so the percentage should be (much) lower hopefully by now, but I do not know of any scientific study with new figures on the recent versions.

Bsieker :
The only reason I can imagine for your reluctance to address it is that you feel Ueberlingen was a fluke, it is considered that low "acceptable risk", and nothing needs to be done about it
I think you are correct in your assumption.Fullwings is unfortunately not the only one who believe this.
Whereas Ueberlingen ended in a real collision , there are many more Ueberlingen –type incidents on record , both before and after 2002.
Some were heavily mediatized ( like the JAL/JAL case in Japan ) most others were not.
A very recent one is a carbon copy but with Climb RA +ATC instruction to climb to teh other. Aircraft missed mostly because one of the aircraft was powerful and outclimbed the other one . As it has been said here already, it is extremely difficult to make 2 aircraft collide , even if you intend to . (Ask any Military interception controller about it ) It is not because there is actually no accident that we can disregard the issues. Believing that the problem dramatically illustrated in Uberlingen is unique is wishful thinking.

[alf5071h]

Bernd, I still believe that there are weaknesses in the development of your scenario (#101).
In particular the omission of the overall time frame, and I believe, a biased rational for the controller’s choice of action. Yes it can happen but is it realistic - probabilities?

Re your “But we know that at Ueberlingen they did collide, and what makes my hypothetical setup sufficiently different from Ueberlingen that everyone seems to be confident it would not be a problem?”
I don’t think that anyone is saying that they are confident it is not a problem just that it appears to be sufficiently improbable. The analogy is that aircraft are certificated as being safe to fly, but some crash, more often with human involvement. ATC Watcher, asked if Ueberlingen could happen again (#1), I replied yes.

I sense that I could become the focus of a pincer move between you and ATC Watcher; thus having identified conflicting traffic (in an uncontrolled environment), I now deviate from my flight path! :)

Peter, ATC Watcher, my description of safety certification was somewhat simplistic and is more related to aircraft certification and installed systems. In these instances the certification ‘does’ show the appropriate probability. However, for a combined ATC/TCAS system I would struggle to find any such rigorous analysis; and perhaps it is this aspect which is causing concern. Therefore in this discussion I believe the ‘safer’ option is for me to withdraw all numerical examples.
However, for Bernd’s scenario and Peter’s 3 aircraft problem, perhaps looking at such things as a probability density function for the risk of collision might identify non-perfect solutions but ones with acceptable risk?

Being sceptical of independent papers and manuals, I would ask if the rare case actually induced a collision or a only a flight path conducive to a collision, i.e. a change in the level of risk (relative vs absolute). Similarly, the ‘rare case’ would in my way of thinking require an associated probability.

The increasing concern may be heightened by ‘near miss’ reports and crew error in TCAS operation. Near misses are interesting, particularly if viewed from different areas. For ATC, anything less than 5nm and 1000ft is a cause for concern, whereas a pilot, 1nm of 500ft may appear safe, and more recently anything that does not result in a TA. So ATC quite rightly ask questions about the overall system, and pilots are currently happy with TCAS's anti collision qualities. ( I caution myself to be aware of “ a threat to safe operation, which … people did not believe existed (numerically), or did not whish to believe posed a significant risk” (#87))
I am not writing this problem off, nor would I be surprised to learn that the ATC/TCAS safety case was accepted directly from N America without proof, and perhaps it is only now that we find weaknesses or at least difficulties in providing data (probabilities) for proving a level of safety.
Of course the problem might actually be in the process of determining the safety level, what is the required value and how is such a system, with its many human contributions, to be certificated? ... Automation?

[CDN_ATC]

Is there a specific reason the rate of descent for an RA is 1500'/min?

Reason I ask, many commercial operators in regular arrival descents from cruise descend at 2500-3500 feet per minute depending, and I've seen 737NG's give 5500-6000'/min when asked to increase rate of descent to clear traffic.

So my obvious question since an RA means two airplanes are too close to each other, why not move in the appropriate direction as fast as safely possible, I mean passenger comfort is one thing, but I think they'd prefer a little upset stomach to the possible alternative?

Also scenario (Assume structured airspace):

Airplane 1 Proceeding AAA-BBB at FL360
Airplane 2 Proceeding AAA-BBB at FL350
Airplane 3 Proceeding AAA-BBB at FL340
(All three airplanes are vertically stacked on top of each other)

Airplane 4 Proceeding BBB-AAA at FL350

How would TCAS re-act to this scenario? Also what if the offending airplane has an older version of TCAS...

[bsieker]

alf,

I still believe that there are weaknesses in the development of your scenario (#101).

I was hoping for someone to point me to them.

In particular the omission of the overall time frame, and I believe, a biased rational for the controller’s choice of action. Yes it can happen but is it realistic - probabilities?

Well, I assume the same timing as at Ueberlingen.

As to the controllers choices of action: I guess he could have chosen lateral manoeuvering as well. Is that likely if the FL below the collision course is clear, also considering that a turn takes longer to initiate than a dive or a climb?

Is my notion about a preference for descend over climb correct? I figured, particularly at high altitudes, descending was quicker and more easily achieved for some aircraft than climbing.

I sense that I could become the focus of a pincer move between you and ATC Watcher; thus having identified conflicting traffic (in an uncontrolled environment), I now deviate from my flight path! :)

I'm glad you put the smiley there. I don't think it's likely to happen. I was looking forward to your answer, as you appear interested in a meaningful discussion and are not just defending a position of "you must always follow TCAS!". As I have indicated I lack inside information about TCAS algorithms. I rephrased my call for comments on my hypothetical(?) setup in an increasingly aggressive tone to provoke a reaction. I thank you, Joern and ATC Watcher for finally taking the bait :)

I think without data mining through huge data spaces (and I'm talking a lot more than the 2-day data set that Eurocontrol used; more like a year or two) we will not be able confidently to put a figure on the probability.


(and now for something completely different ...)

Is there a specific reason the rate of descent for an RA is 1500'/min?

It is probably a compromise between (a) safely clearing your conflicting traffic by a comfortable margin on the one hand, and (b1) avoiding the creation of additional conflicts plus (b2) maintaining passenger comfort on the other hand.

A descent rate of, say, 6000ft/min would bust one flight level every 10 seconds! Sounds like an ATC nightmare in dense traffic.


Bernd

[CDN_ATC]

:hmm:Normally yes, but if I ask for "A good rate of descent" and get 6000'/min, it's not my ears and guts that are bothering me...

[punkalouver]

Originally posted by CDN_ATC
So my obvious question since an RA means two airplanes are too close to each other, why not move in the appropriate direction as fast as safely possible, I mean passenger comfort is one thing, but I think they'd prefer a little upset stomach to the possible alternative?

Because it is not necessary. Enough time is given that you don't need to make abrupt manouvers. That is why on an initial RA you have 5 seconds just to start you evasive action at only 1/4 g added or subtracted to your load factor. Abrupt pullups at cruise altitude can be dangerous.
Going against the RA was not rational and a lot of people died because of it.

[PBL]

Going against the RA was not rational

Given that there is a good argument in the public domain as to why it was rational, would you care to share both your reasons for thinking it was not, as well as show us the mistake in the argument?

Can you solve the decision problem that I posed?

PBL

[punkalouver]

We have already been through this. Please read or re-read post #50 from myself.
I find it amazing that a specialist on aircraft safety is telling us that if we end up in a similar scenerio as the one over Germany that night that we should ignore the RA and in fact go against it.

Please read more about sometimes trying to out-think the TCAS as you advocate.

http://www.asasi.org/papers/2005/Hiroaki%20Tomita%20-%20near%20collision%20in%20Japan.pdf

[PBL]

We have already been through this. Please read or re-read post #50 from myself.


I read it when you wrote it and decided that you were confused and had not understood the problem.


I find it amazing that a specialist on aircraft safety is telling us that if we end up in a similar scenerio as the one over Germany that night that we should ignore the RA and in fact go against it.

Which specialist would that be? I don't know of one.

PBL

[bsieker]

I find it amazing that a specialist on aircraft safety is telling us that if we end up in a similar scenerio as the one over Germany that night that we should ignore the RA and in fact go against it.

I find it amazing that you make this statement.

Would you mind showing us the post where someone said that, when confronted with the Ueberlingen scenario, one should ignore the RA and go against it?

What I (and others) said was that the choice taken by the Bashkirian crew was (one of several) rational choices in the given situation. Namely assuming two intruders, one they can see, and one they cannot see, and being given ATC instructions before receiving the RA.

Rational choice means it can be defended as being thought through: Looking at the individual pieces of information presented to them, evaluating them and making an informed decision on the course of action.

That the choice to follow ATC instead of TCAS was a rational choice does not mean:

- that it was the only choice or
- that it was the only rational choice or
- that the crew actually decided rationally

- it doesn't even mean that it was necessarily (in hindsight) the best choice


I second PBL's call to show us exactly why you think the choice made by the Tupolev crew was not rational.

-----
(And now for something completely different.)

Disclaimer: I do not have a degree in psychology or similar training, so I'm really asking this question out of curiosity.

As to the chances of avoiding traffic visually:

There is a phenomenon, well-known to Rally-drivers, that, if you're a proficient driver, you tend to steer towards the object that you look at. It seems that if you look at a tree, you're more likely to hit it than if you deliberately look beside it. Driving becomes completely skill-based, and without thinking you follow your eyes.

The effect can be observed even in a rally (or other racing) simulation on a computer.

I actually have no idea how much of this is applicable to aviation. Just a random thought.


Bernd

[ATC Watcher]

Bsieker :As to the chances of avoiding traffic visually:
There is a phenomenon, [...] you tend to steer towards the object that you look at.

Indeed , one can even try this on a bicycle .And this remark is very relevant in the case of Ueberlingen. I always thought that should both a/c had been IMC they most probably had missed. Looking at the last seconds on the FDR of the TU154 could confirm this. But I speculate and this is outside the TCAS debate.

But being also a glider pilot, I can confirm that visual avoidance do work in the air most of the time ! ( even last minute visual avoidance, mostly by turning sharply away from intruder ).
But we are far slower and far more manoeuvrable than a passenger jet.

[punkalouver]

O.K., here is my rational response based on this accident report. It is nightime, I am cruising along at high altitude in airspace where all aircraft are required to have transponders and most that do have transponders have two of them in case one fails. It is fairly quiet on the radio. My aircraft is equipped with an updated version of TCAS and I have been trained in how it works.
Therefore I know that they communicate with each other and in the event of a RA in my aircraft, the conflicting aircraft's TCAS will not give instructions to do the same evasive manouver. Because I follow aircraft incidents on PPrune and other forums, I am well aware that manouvering to follow an ATC instruction against the RA nearly caused the world's worst aviation disaster over Japan 18 months previously(link provided in my last post).

All of a sudden I get a TA with the TCAS display showing traffic at my altitude and converging from the left. Moments later ATC says traffic at two o'clock, descend immediately and nearly simultaneously, I get a RA saying to climb. There is no other traffic displayed on my TCAS display. I do the rational thing and follow the RA and everyone survives this conflict.

I know.......Maybe there is some military flight out there that has an emergency which just happens to be affecting his transponder at the very exact time that I am getting an RA. I'm sure there are all kinds of obscure scenarios that can be thought of. Conspiracy theorists do it all the time.
By the way, if in this scenario there actually was an intruder at my two o'clock with no altitude given as pointed out by ATC(perhaps due to mode C not working), I would not go opposite to the RA.

P.S. Sorry for calling you an aviation safety specialist. That was an error on my part.

[joernstu]

All of a sudden I get a TA with the TCAS display showing traffic at my altitude and converging from the left. Moments later ATC says traffic at two o'clock, descend immediately and nearly simultaneously, I get a RA saying to climb. There is no other traffic displayed on my TCAS display. I do the rational thing and follow the RA and everyone survives this conflict.

This rational action did not prevent the DHL Boeing from colliding.

For the Tupolev Crew: You forget, that they communicated with ATC for several seconds before they got the TCAS RA. They initiated the descend before TCAS issued its RA.

Your whole argumentation on rational action is biased by your western training. The Tupolev crew wasn't so "furtunate". In Russia the controlling of traffic is handled differently if you are an international flight that if you are a national flight. For national flights the controller has the authority to command the crew to take a certain action.

I know.......Maybe there is some military flight out there that has an emergency which just happens to be affecting his transponder at the very exact time that I am getting an RA. I'm sure there are all kinds of obscure scenarios that can be thought of. Conspiracy theorists do it all the time.

If you have followed this threat, you will see that a Ueberligen-like mid-air is still possible today, it sounds extremely simplicistic to present insufficiencies in the TCAS system as "obscure scenarios" of "conspiracy theorists".

[CDN_ATC]

What about this:


747 off of JFK full of fuel taking off into the NAT tracks, operating at maximum available cruising altitude for the moment at FL360, ATC asks if they're able FL 370 for traffic earlier, they are not able that level.

Now my question IF TCAS asked them to climb for an RA, is the airplane going to get up there? Also does TCAS take that into account or is it completely separate from those systems?

Since airplanes flying long haul are often operating near or at their maximum cruising altitude for their current weight, what if the RA tells them to climb and they cannot? Then what?

[PBL]

All of a sudden I get a TA with the TCAS display showing traffic at my altitude and converging from the left. Moments later ATC says traffic at two o'clock, descend immediately and nearly simultaneously, I get a RA saying to climb. There is no other traffic displayed on my TCAS display. I do the rational thing and follow the RA .........
By the way, if in this scenario there actually was an intruder at my two o'clock with no altitude given as pointed out by ATC(perhaps due to mode C not working), I would not go opposite to the RA.

This is wonderful. You give us two different courses of action depending on whether there isn't an airplane where ATC says it is or whether there is.

Need I say it: if you have this trustworthy information, then you don't need either TCAS or ATC.

Let me show you a different way of thinking (I emphasise the word "thinking" :) ).

You paint an intruder at 10 o'clock. ATC gives you a descent for traffic at 2 o'clock. You don't descend (which would be against the RA).

BAM! In the few seconds of consciousness available to you, you might like to consider where your reasoning went wrong. Maybe it was because the likelihood is rather high that there is actually an airplane where ATC implies there is an airplane, even if your on-board kit doesn't see it; or maybe .... sorry, that's all folks.

I'll offer to write your epitaph. First proposal: "I've proved I'd do what I said I'd do".

PBL

[PBL]

what if the RA tells them to climb and they cannot? Then what?
Then they don't climb, and their "dance partner" is issued a Reverse RA.

At least, that is the thinking behind Reverse RAs. That it doesn't always work that way was proved at Überlingen.

PBL

[punkalouver]

Originally posted by joernstu
This rational action did not prevent the DHL Boeing from colliding.

This rational action(FOLLOWING THE RA) would have prevented the collision as the report obviously states.

Originally posted by PBL
This is wonderful. You give us two different courses of action depending on whether there isn't an airplane where ATC says it is or whether there is.

I don't believe so. Read my previous post completely and carefully this time. In both scenarios I am not going to go against the RA's(as I have continuously stated on this thread) whether there is traffic at two o'clock or not. That is what I meant and that is what I said

You already have 71 epitaphs to write. No need to write my epitaph or anyone else's in this scenario because I would have FOLLOWED THE RA and ignored ATC once it was contradicted by the RA and I will do so in the future.

Why, because I have done my "thinking" before the flight even started and realized that Following the RA is the safest thing to do in a scenario like this, not trying to figure out the situation with little time available.

It is very true that the Russian system of training was at least partially at fault in this accident as has been mentioned in the report, however I was asked basically what I would do in this situation. Or maybe the captain listened to a couple of guys on an web site somewhere questioning the prudence of FOLLOWING THE RA.

More example of people who don't follow RA's or who try to out think a situation.

http://www.arinc.com/tcas/acasll_bulletins.html

Originally posted by CDN_ATC
Since airplanes flying long haul are often operating near or at their maximum cruising altitude for their current weight, what if the RA tells them to climb and they cannot? Then what?


There are situations where an aircraft is unable to comply fully with an RA due to performance limitations. The TCAS can be configured(not by the pilots) so that no climb RA will be given above a certain altitude for your aircraft which may or may not be your maximum certified altitude. When at a max altitude due to weight or in a low energy state or an non-normal configuration(gear/flaps that can't be retracted, etc) reacting fully to an RA can lead to stall warning. Use great care in such a situation to prevent an aircraft upset.

[CDN_ATC]

Thanks for those replies on my questions guys, really appreciate it.

I still find TCAS somewhat an imperfect system, if it can issue RA's when there's not an airplane near them, then I wonder if it will always issue the "right" direction.

I don't think the procedures should change, but something that must be taken into effect (that it's far from perfect)

[bsieker]

[...] I was asked basically what I would do in this situation. Or maybe the captain listened to a couple of guys on an web site somewhere questioning the prudence of following an RA.

We already knew that you would always follow an RA.

You were asked to show the error in the argument that the Baskirian crew's actions were rational, and how you arrived at your conclusion that it was not rational.


Bernd

[punkalouver]

Originally posted by bsieker
You were asked to show the error in the argument that the Baskirian crew's actions were rational, and how you arrived at your conclusion that it was not rational.

Aside from the fact that they are all dead? Well, if you are flying in the mountains and ATC says descend immediately for traffic and moments later your GPWS goes off, which takes priority? If you are climbing slowly because you are heavy and ATC direct an immediate increase in climb due to conflicting traffic and moments later the stick shaker sounds, which takes priority? If you are in the situation over Germany that night and you have the conflicting messages that this crew had, which takes priority? There is only one rational answer to each, and in my opinion and none of them involve ATC taking priority.

But.....a previous post said that my action is biased by western training and that in Russia...."For national flights the controller has the authority to command the crew to take a certain action." It is a regulation to follow ATC instructions where I fly as well. Does that mean I am going to fly into the ground or stall the aircraft to follow an ATC instuction for immediate traffic separation? Do the Russian crews need to be told to ignore their stall warning or continue descending into the mountain peaks against their TAWS warning because of an ATC instruction?

P.S. I forgot to mention to the ATC poster that it is common to turn our transponder(TCAS) to TA only after an engine failure due to performance limitations. Also on parallel approaches to runways with less than 2500 foot spacing.

[joernstu]

"This rational action did not prevent the DHL Boeing from colliding."

This rational action(following the RA) would have prevented the collision as the report obviously states.

What would have been and what was are two completely different things. Fact is that the Boeing crew did behave just like you said you would. You said, that this would be the rational action. This must mean: following the rational course of action did not prevent the collision for the Boeing crew in the Ueberlingen case. The mid-air itself proves this.

Arguing about what-if is a completely different matter.

Had both aircraft followed the RA, the mid-air would have been prevented.
Had there only been the resolution issued by the Controller, the mid-air would have been prevented.
Had TCAS issued a reversal RA for the Boeing and Tupolev, the mid-air might have been prevented.
Had the STCA been available for the controller at ACC Zurich, the mid-air would have been prevented.

I could go on like this for pages ...

[punkalouver]

Originally posted by joernstu
What would have been and what was are two completely different things. Fact is that the Boeing crew did behave just like you said you would. You said, that this would be the rational action. This must mean: following the rational course of action did not prevent the collision for the Boeing crew in the Ueberlingen case. The mid-air itself proves this. I could go on like this for pages ...

It would appear that you are going on. In the end, the vast majority of us follow the rational route and accidents are prevented. A few don't follow the rational route either intentionally or by mistake and only luck or chance prevents an accident as in the JAL case which I provided a link to and I'm sure you read. Most properly functioning systems and societies depend on the vast majority of us acting responsibly. Of course accidents and deaths occur frequently because of those who break the rules and occasionally to those who don't.

I hardly think that because the Boeing crew doing the proper procedure and dying because of it should mean that as a result we should all just stop following RA's and do something different on the off chance that the other guy may be not doing what should be done. After all that would be irrational(or lacking in LOGIC). As I said earlier it is an odds game with a much better chance of survival happening if you FOLLOW THE RA.

And fortunately people who behave like this(ignoring the RA) are few and far between, although there appear to be three on this thread. Strangely(according to their profiles), they all appear to be from Germany, two from near Bielefeld(the third an unknown German location) and two are aircraft accident analysts. One even has logic as an interest. What are the odds of that?

[PBL]

punkalouver,

bsieker and joernstu and I work together. They have put a considerable effort into analysing the Überlingen collision, amongst other accidents, so you are discussing with people who understand these situations very well.

Your recent postings support my earlier suggestion that you have not understood the decision problem. Here is the reason.

I suggested a consequence of your point of view, namely that you could be involved in a collision. You replied in a way that suggested that you think it is not possible under the conditions described (at least, that is what you say).

Well, such a view is obviously mistaken. It is obviously possible that you could collide in such a situation, even to people to whom TCAS is the best thing since sliced bread.

Let me attribute to you some insight, and assume you agree that it is possible that you might collide. Then you must think it is extremely unlikely. So in that case I would ask you to show your reasoning for this unlikelihood.

What you have said so far shows that you think it unlikely that you would ever come in such a situation. OK, but you were asked to judge, not the absolute probability of being in that situation, but the conditional probability *given the situation described*, for that is the judgement on which a rational decision is based. And the conditional likelihood can be a very different quantity.

For example, the absolute probability that I will be hit head-on by a car at a closing speed of over 30 kph while riding my bicycle is (I hope!) low. However, the conditional probability that I will be hit head-on by a car at a closing speed of 30 kph given the situation that a drunken driver has veered to my side of the road while going at 50 kph and I am 20 m from his front fender is rather high.

I think the very best you could do, if you wish to persuade people about the rationality of always following the RA, is to engage this and other decision problems that might be presented to you, and persuade those who are sceptical (such as I am, and bsieker and joernstu are) that following the RA is the best solution in each and every one of those situations. Personally, I doubt you can do that. But it may be well worth a try, to see where it succeeds and where it fails.

PBL

[punkalouver]

Unfortunately I don't have time to reply for the next few days. I don't think there is much more that I can say than has already been posted by myself already. I suggest that anyone reading this thread, read our posts for the last few days and decide for themselves what is best and why we are trained the way we are(and what the legalities are)instead of this thread going on
forever, basically saying with us all saying the same thing in a different way.

Perhaps I have misunderstood your whole arguement. I believe I did say that there is a risk of collision in the scenario that actually happened that night but that it is an odds game which I believe you agreed with. Following the RA reduces your chance of collision especially at high altitude on a dark night where VFR traffic is a non-issue and all aircraft are or should be under ATC control.

I have mentioned a disaster(over Europe), a near world's worst disaster(over Japan) and linked to a Eurocontrol newsletter that gives multiple examples of closer than necessary near misses due to not following the RA. That is in reality all I can do to prove my point. Perhaps you could prove yours by showing me some articles where lives were saved because the pilots ignored their RA. I suppose you will point out that the DHL pilots would be alive today if they had ignored the RA. But I don't think that is enough evidence for us to realistically consider doing this as a regular procedure or consider it at all.

All I can do is ask that everyone out there FOLLOW THE RA.

Thank YOU

[PBL]

I don't think there is much more that I can say than has already been posted by myself already

OK, noted. Thanks for your contribution.

PBL

[joernstu]

And fortunately people who behave like this(ignoring the RA) are few and far between, although there appear to be three on this thread. I can't remember advising anyone not to follow a RA, but surely you will prove me wrong? Following an RA you get may well be the best action for you, but as TCAS cannot solve every situation, you cannot be sure, that your action will save your life (and your passengers).

I cannot decide, what kind of action would induce the lowest risk - following an RA, not following an RA or flying without TCAS at all as the basis for my risk evaluation would be episodes. I think the same goes for you as your information basis is still only episodes.

[ATC Watcher]

Punkalouver :
it is common to turn our transponder(TCAS) to TA only after an engine failure due to performance limitations.

This is new to me, and a bit surprising coming from you, as you referred to the Eurocontrol ACAS bulletins that has as first item in their do and don't list the following :
To maximise the safety benefits and operational compatibility with ATC,ten fundamental dos and don’ts must be observed:
1- TCAS II must be operated in RA mode to provide full safety benefits


That aside , I find it a pity that you do not want to engage in the debate. The point many of us have ( and I am not working for or with PBL) is that the system is still far from perfect and even if you follow the rules , it will not protect you at all times. The GOL and DHL crew if they were still alive would sadly confirm this.

The sense reversal logic currently does not work and could induce a collision while trying to prevent one.

The interaction between human-ATC and automation -TCAS is still unclear for many , especially when ATC acts before the RA, and following the RA will mean acting against the current ATC clearance.

For those 2 reasons alone, following the RA is no guarantee that a collision will be prevented, because it needs 2 willing partners and you do not know what the other is doing.

Following your argumentation , would you be for the coupling of TCAS to the auto-pilot ? That would solve many problems wouldn't it ?

[punkalouver]

Concerning turning the transponder to TA after an engine failure...

Originally posted by ATC Watcher
This is new to me, and a bit surprising coming from you

Why are you surprised. This is our SOP and the SOP of many airlines if not most. Perhaps your training department felt it was not necessary to inform you of this.

Originally posted by ATC Watcher
That aside , I find it a pity that you do not want to engage in the debate. The point many of us have ( and I am not working for or with PBL) is that the system is still far from perfect and even if you follow the rules , it will not protect you at all times. The GOL and DHL crew if they were still alive would sadly confirm this.
The sense reversal logic currently does not work and could induce a collision while trying to prevent one.
The interaction between human-ATC and automation -TCAS is still unclear for many , especially when ATC acts before the RA, and following the RA will mean acting against the current ATC clearance.
For those 2 reasons alone, following the RA is no guarantee that a collision will be prevented, because it needs 2 willing partners and you do not know what the other is doing.
Following your argumentation , would you be for the coupling of TCAS to the auto-pilot ? That would solve many problems wouldn't it ?

Hmmm, I do see that you are from Germany as well. Anyways, I don't see the point of debating any more because I really have nothing more to add to what I have already said several times(Same with the others in my opinion). The thread is is just going on and on with myself and PBL and his coworkers(or aliases) repeating the same thing in different words. We have made our points.

You are absolutely right that you are not protected at all times, TCAS is not perfect and could induce a collision and I have never denied that. However the odds are in favour of you to FOLLOW THE RA.(See how I am repeating myself again).

The Gol accident involved non-functioning equipment on one the aircraft. I don't think it proved anything in terms of the whole TCAS design except what we already know; that won't provide any protection from an aircraft with no operating transponder whether a Piper Cub or a high level jet.

No opinion on autopilot coupling as I haven't heard arguements for and against this idea.

Originally posted by joernstu
I cannot decide, what kind of action would induce the lowest risk - following an RA, not following an RA or flying without TCAS at all as the basis for my risk evaluation would be episodes

Fortunately for us, you are not flying an airliner and I suspect you would be demoted or worse if you followed through on this opinion in such a scenario.

Originally posted by joernstu
I can't remember advising anyone not to follow a RA, but surely you will prove me wrong?

bsieker(one of your like minded coworkers) in effect said so in my opinion in post #101 when he said the Tupolev crew's decision was rational which started this whole debate on the last two pages. And you strongly hint at it right after you ask me to prove you wrong by saying...

I cannot decide, what kind of action would induce the lowest risk - following an RA, not following an RA

[CDN_ATC]

Is there a website which currently lists all TCAS versions in use, their abilities (TA or RA, Climb only or turns) etc?

[ATC Watcher]

There are today only 2 TCAS in operations I and II.

TCAS I ,is a receiver that only gives TAs . description and users manual here : http://www.seaerospace.com/bfg/tcas791pg.pdf

TCAS II : the mandated system that issues TA and RAs in the vertical sense .
There are currently 2 software versions in use : 6.04A and 7.0 .
main differences > 7.0 is RVSM compliant and reduces the threat /sensitivity levels (margins ) to eliminate false alarms.
7.0 is meeting ICAO ACAS SARPS, 6.04A does not.
For a description and user manaul see here : https://www.bendixking.com/servlet/com.honeywell.aes.utility.PDFDownLoadServlet?FileName=/TechPubs/repository/006-08499-0000_7.pdf

The horizontal RA s ( i.e. turns ) was planned with a so called TCAS III but R&D on it was abandoned long ago, and it is very doubtful that they will restart.

[ATC Watcher]

Punkalouver :
cconcerning turning the transponder to TA after an engine failure you said...This is new to me, and a bit surprising coming from you

Why are you surprised. This is our SOP and the SOP of many airlines if not most. Perhaps your training department felt it was not necessary to inform you of this.

I doubt this is the SOP of many airlines . Are you going to tell us that if you have an engine failure near Max Alt in dense continental airspace , and have to perform an emergency descent you are going to degrade your TCAS to TA only during he descent ?

Hmmm, I do see that you are from Germany as well.

I am not from Germany, but that does not constitute a burden is it ?

Most contributors here are from Germany perhaps because the critical knowledge is there : Lake Constance and the German BFU are located there and the University of Bielefeld has a chair on automation that is unique ( and they have for instance overturned many stones in the Airbus garden in the past).

[bsieker]

I can't remember advising anyone not to follow a RA, but surely you will prove me wrong?

bsieker(one of your like minded coworkers) in effect said so in my opinion in post #101 when he said the Tupolev crew's decision was rational

I would have let it rest, but since you seem to have misread my post, I will explain it again.

Describing a course of action as a rational choice does not mean that it is the only choice, nor necessarily the only rational choice, nor necessarily the best choice, nor even that the actor(s) involved acted rationally when choosing that option.

The scenario which I described in more detail, in which following ATC was the only rational choice, is the scenario in which that crew did not receive an RA at all, because of inop TCAS.

The point being that all parties following their respective rational (in this case, even best) choices, one of which is following the RA, the other, in the absence of an RA, following ATC, (courses of action, I take it, you would approve) and yet they end up in a collision.

The problem is not so much following an RA, but the poorly-understood and unregulated (non-)interaction between TCAS and ATC.

This, as we have seen, may lead to collisions, although and because one crew follows its RA.

You say that you always follow the RA because the risk is lower than following ATC, where both are in conflict, but without giving even the hint of an argument for it, in the face of strong arguments (i. e. Ueberlingen) to the contrary. The Boeing pilot could not know if the Tupolev's TCAS was inop, or if they were just ignoring its RA.

If you misconstrue this as me generally recommending not following RAs, I cannot help it.

I am merely pointing to what I think is a big problem with collision avoidance in the larger system "air traffic", not looking at TCAS as an isolated system, but at the larger socio-technical system, comprising (at least) of two or more TCAS systems, two or more flight crews, several air traffic controllers, primary and secondary radar systems, ATC software, ...


Bernd

[punkalouver]

Concerning going to TA after an engine failure....

Originally posted by ATC Watcher
This is new to me, and a bit surprising coming from you, as you referred to the Eurocontrol ACAS bulletins.

Yes I did refer to the ACAS Bulletins. They need to be read and understood completely though. Here is a link and quote from ACAS Bulletin 5, page 3, in the yellow box on the left side. The use of the TA-only mode is now limited to aircraft specific procedures, e.g. engine failure.

http://www.scribd.com/doc/351323/ACAS-Bulletins-5

Originally posted by ATC Watcher
I doubt this is the SOP of many airlines . Are you going to tell us that if you have an engine failure near Max Alt in dense continental airspace , and have to perform an emergency descent you are going to degrade your TCAS to TA only during he descent?

If I have an engine failure near max altitude in "dense continental airspace" it will be more likely a driftdown not an emergency descent. And yes, as per SOP I will place the TCAs in TA mode when we get to it in the checklist just like we always do in the simulator. Just like I believe most other airliners will do.

Originally posted by ATC Watcher
I am not from Germany, but that does not constitute a burden is it?

No burden. I just foud it strange that all the people who seemed to disagree with me were from a country that has as a guess by me, perhaps 5% of posters on this forum. Of course I discovered that they all are from the same office. Just curious that's all. Here is why I thought you were from the same country and I quote.....Based in Europe( Germany) I have to go in Australia shortly and wants to take the familly with me... from this thread.

http://www.pprune.org/forums/showthread.php?t=281165

Sorry for the mistake on where you are from.

Originally posted by bsieker
You say that you always follow the RA because the risk is lower than following ATC, where both are in conflict, but without giving even the hint of an argument for it, in the face of strong arguments (i. e. Ueberlingen) to the contrary. The Boeing pilot could not know if the Tupolev's TCAS was inop, or if they were just ignoring its RA.

Some of the hints of arguement that I have given several times are the collision in Germany and near collision in Japan due to not following the RA.
Seeing as you and your colleagues, perhaps legitimately feel that there are occasional scenarios that TCAS can't resolve safely, perhaps you could give some interim guidance for pilots on what they should do if a similar situation is encountered as the Tupolev crew. My advice is to Follow the RA. What is your advice.

[joernstu]

Fortunately for us, you are not flying an airliner and I suspect you would be demoted or worse if you followed through on this opinion in such a scenario.

You are still unable to give a plausible argument, supporting your assumption, that following TCAS induces the lowest risk. If this would be the case - ask your superiors to copple TCAS directly to your autopilot system.

I can't remember advising anyone not to follow a RA, but surely you will prove me wrong?

bsieker(one of your like minded coworkers) ...Thanks for proving, that your previous post was generalising and inacurate.

I want to remind you, that the argument in the lastest past was initated by you saying, that the Tu154 crews action were unrational. You have done little to support this allegation.

[punkalouver]

Originally posted by joernstu
You are still unable to give a plausible argument, supporting your assumption, that following TCAS induces the lowest risk. If this would be the case - ask your superiors to couple TCAS directly to your autopilot system.

As if I'm going to walk into my head office and ask for a system to be modified(however much that costs) when there almost certainly isn't any approved modifications available on the market for this. There is a big difference between theory and reality.

Originally posted by joernstu
I want to remind you, that the argument in the lastest past was initated by you saying, that the Tu154 crews action were unrational. You have done little to support this allegation.

I'm not going to respond to this as there is nothing to add to what I have already said. Instead I will leave your theoretical world of obscure possibilities and make a real world statement to all who fly with TCAS and could very well end up in the same situation as the Tupolev or JAL crew. Both had their reasons for ignoring the RA. One crew killed many while the other crew almost did. So please do as you have been trained. Do what Eurocontrol and the FAA and all regulatory authorities consider to be a RATIONAL reaction in such a situation. That is FOLLOW THE RA in spite of what ATC says.

[PBL]

punkalouver,

I thought you were bowing out of discussion because you had said everything you had to say? At least, that is what you said.

Having done that, I think it quite appropriate for others to summarise the issues you have addressed and the issues you have not addressed.

I also think it appropriate for others to correct your misrepresentations of their positions.

I see two different goals in this discussion.
Yours: you wish to derive advice to pilots from your understanding of TCAS
Mine (and that of, if I may anticipate, joernstu, bsieker and ATC Watcher): I wish to understand the technical workings and weaknesses of the TCAS system. And by that I mean not only the kit, which is nothing but an information device, but the entire collision-avoidance system, which can be considered to include pilots and airspace, regulations and controllers.

Somehow, you seem to think that I (and others) are attempting to derive advice to pilots. I, on the other hand, thought you were attempting to understand the technical workings of the system. That is, each thought the other shared a goal with him. I no longer think that is the case.

PBL

[punkalouver]

Originally posted by PBL
Somehow, you seem to think that I (and others) are attempting to derive advice to pilots. I, on the other hand, thought you were attempting to understand the technical workings of the system. That is, each thought the other shared a goal with him. I no longer think that is the case.

I agree. However, when one of your group(who's profile says accident analyst) goes on a widely read pilot forum and says that doing what the Tupolev crew did was a rational choice, it could easily be interpreted by some readers as advice even if it wasn't meant that way.

Perhaps you should say that you have "discovered or are aware of certain very rare occasions where following a TCAS RA could create a secondary conflict, however, in the vast majority of cases, all pilots following the RA including the 154/757 situation will prevent a collision."

Meanwhile I think some learning of the whole system has been accomplished on this thread about procedures such as when TA is selected and links to safety reports and newsletters and questions asked by others so this has been a useful thread. Glad to have been of help.

[bsieker]

[...] and says that doing what the Tupolev crew did was a rational choice, it could easily be interpreted by some readers as advice even if it wasn't meant that way.

Which is why I went to some length to explain what I meant by rational choice.

And I also modified the setup so that the "Tupolev" crew did not receive an RA, but only ATC instructions, to which scenario you failed to make any statement whatsoever. I assume because you then would have to realise that all pilots following their, even in hindsight, best (as opposed to merely one of several rational) choice, could induce a collision.

[...] in the vast majority of cases, all pilots following the RA including the 154/757 situation will prevent a collision.

And how do we know that it is indeed "the vast majority"? As long as we don't know that, we would not make a statement like that. Indeed, without the extensive data mining I mentioned before of air traffic data of a significant time period we cannot know.


Bernd

[punkalouver]

No more replies to obscure theory. You have the last word.

[songbird29]

ATC Watcher wrote
down-linking the RA message to ATC only ( Advanced studies made by Eurocontrol , called FARADS ( Feasibility RA down Link) and RADE (RA Downlink simulations in Bretigny)
Consequences of D/L RA to ATC are complex and many think ( inlc. me ) that it may induce more problems that it will solve.

Can you please expand on those problems induced by downlinking TCAS RA.

[ATC Watcher]

Songbird 29 :
The problems are numerous and complex and would vary depending on the medium used to down link the RA.
But as many believe that Mode S is the the only cost effective way to downlink RAs,. so let's take Mode S :

A recent QinetiQ study show that the average delay for controllers to be aware of an RA via Mode S is about 9 seconds and to get a verbal report from a pilot after an RA is currently about 29 seconds.
The main problem is what happens during those 20 seconds.

According to the current ICAO documentation the controller is still responsible and could/should issue instructions as long as the aircraft /pilot do not manoeuvre. The chances to have controllers intervene in those 20 seconds is higher than without RA downlink.
Next is the problem of intermittent short false RAs transmitted on mode S ( and that are down linked today ) but that are not resulting in an RA in the cockpit.

Lastly , should we decide to downlink one day, there is a strong debate about displaying or not to controllers the sense of the RA. Seen the mode S delay to display the information, a risk exits that the displayed information will be different of the actual RA ( especially in case of sense reversal ) This could induce again more unwanted and potentially dangerous controller intervention, especially in the time frame when he is still responsible,( i.e. before he gets a pilot R/T confirmation that the aircrfat is following the RA.)

RA Down link will show that the system has issued an RA at one point of time but not that the pilot is following it.
According the latest SOFREAVIA study on the survey of 4 major European airlines , still 10% of the RAs are ignored by pilots , or are acted against .
.

[Intruder]

RA Down link will show that the system has issued an RA at one point of time but not that the pilot is following it.
The downlink itself may not show if the pilot is following an RA, but the altitude readout on the ATC screen will be able to confirm it within a few seconds -- or earlier if the downlink is delayed.
As for switching to TA with an engine failure, Eurocontrol ACAS Bulletin 9 (Jul 07) addresses it specifically:
TCAS II provides maximum benefits when operated in RA mode and when all RAs are followed promptly and accurately. However, when the aircraft performance is degraded (e.g. an engine failure, an emergency descent, etc.), it might not be possible to comply with a Climb”RA. Therefore,airlines must define clear procedures to address degraded aircraft performance situations, having in mind that:
•When operated in RA mode, non-compliance with an RA will adversely affect the efficiency of the coordinated RA triggered on-board the other aircraft.
•When operated in TA-only mode, collision avoidance is still maintained by the TCAS II of the other aircraft
The use of the TA-only mode is described in the following note that will be included in the next version of the ICAO PANS-OPS, Doc 8168, due in November 2007: “Note 2. -The normal operating mode of ACAS is TA/RA. The TA-only mode of operation is used in certain aircraft performance limiting conditions caused by in-flight failures or as otherwise promulgated by the appropriate authority.”
And for those who doubt whether a TCAS RA must be followed when there are contravening ATC instructions, the same Eurocontrol bulletin makes it very clear that following an RA IS MANDATORY despite such ATC instructions:
Notwithstanding ATC clearances, TCAS II triggers an RA when it predicts that a risk of collision exists if the aircraft continue on the same trajectories. Therefore, although it might appear as a nuisance in hindsight, an RA is always necessary at the time when it is generated.
...as well as in ACAS Bulletin 8 (Jun 06):
Prompt and accurate pilot response to all RAs is key to achieve maximum safety benefits in all airspace, including European RVSM airspace.
This will also minimise the risk of a domino effect with a third aircraft at an adjacent RVSM flight level.

[ATC Watcher]

Intruder :
The downlink itself may not show if the pilot is following an RA, but the altitude readout on the ATC screen will be able to confirm it within a few seconds -- or earlier if the downlink is delayed.

Agreed, but it did not work in Ueberlingen . Radar return was 12 seconds there, but on most long range en route radars it is 10 seconds . If the first return is garbled or filtered out by the system ( as it often is the case for sudden aircraft mode C jumps ) , one has to wait until next return to confirm movement and direction.

In any case the main point is that responsibility for anti collision still remains with the controller until he gets confirmation that the aircraft/pilot is manoeuvring in accordance with an RA. So some believe (strongly I might add ) that down linking RAs will make controllers intervention during a TCAS sequence more likely and induce a new kind of problems.

The FRADS/RADE tests conducted by Eurocontrol in Bretigny last years , show controller acceptance of the technical display shown , but did not address the responsibility/ intervention issue.
It will not be an easy one to solve , because waiting 10 or 20 seconds doing nothing when aircraft are on a collision course is not what controllers were and are trained to do.

[Intruder]

It will not be an easy one to solve , because waiting 10 or 20 seconds doing nothing when aircraft are on a collision course is not what controllers were and are trained to do.
I am not advocating "doing nothing." However, there are other things an ATC can do RIGHT NOW without ANY change in TCAS logic:

Call a turn.

Ask the Pilot if he is monitoring TCAS. TELL the Pilot to monitor and follow TCAS.

Either of those would have prevented the mishap at Ueberlingen. Why are we so focused on automated intervention in the vertical plane only, when in a controlled environment ATC also has the option of using the horizontal plane? In many cases the pilots are not aware of the other airplane's actual or projected heading/course, so they cannot independently turn. However, ATC has a clearer picture of the collision intercept, and is better able to assess the best turn geometry.

[ATC Watcher]

100% agreeing with you Intruder .

In fact if you look at the very first post on this thread ( the one I started ) this was already my conclusion :
The conclusion of this all :
Do not philosophy about TCAS : It is too complex a system. My advice : if you are a pilot : follow the RA, and if you are a controller and you have to give anti collision instructions , give a turn as well, in case TCAS comes in , it will complement your instruction instead of nullify it.

[Tarq57]

Also 100% agreement. This edition of The Controller (http://www.the-controller.net/index.php?name=Issues&op=contents&pubid=11) recommended the same (or very similar) course of action, IIRCC.
Probably the average controller thinks of restoring separation (or preventing a loss) in the vertical plane first, because it is so much quicker to achieve than a turn. As one of my radar instructors used to say, vertical will prevent a loss of separation, faster. But if you chuck in a turn, you might still have a loss of separation, but at least they won't b(@@dy well hit each other!
Quite possibly the environment a lot of us currently work in has got us psyched to prevent a loss of separation at all costs, because of the perceived consequences of same. Be interested to know what a human factors guru would make of that.

[blueplume]

Hi all,


maybe I missed it in this thread but don't think so. I'm looking for a link to sensible explanations about how TCAS really works (I understand mode A/mode C basics) but want to know more about how transponders talk to each other. Had an airprox recently and have to write a report, would like to be better informed about how the transponders and TCAS interact.

Thanks.

[joernstu]

I'm looking for a link to sensible explanations about how TCAS really works (I understand mode A/mode C basics) but want to know more about how transponders talk to each other.

ACAS / ACAS II are specified in ICAO Annex 10 Chapter 4. I don't know if the algorithms of TCAS 6 or 7 (Implementations of ACAS) were published. If they can be found anywhere, I would be very interested too.

[joernstu]

I am not advocating "doing nothing." However, there are other things an ATC can do RIGHT NOW without ANY change in TCAS logic:

Call a turn.

I agree with you, a well advised turn can probably solve many loss-of-separation situations where TCAS and ATC are involved.

Problem is, that TCAS can directly influence the performance of crews, even upto the point that they argue on following ATC advises.

ACAS Bulletin vol6 describes on page 4 a situation, where ATC instructed a 747 to make a turn to the left for avioding conflicting traffic. The 747 pilot identified the traffic on his traffic display and interpreted, that the turn would point him directly into the traffic's path. So he waited longer than normally necessary which led to a loss-of-separation situation.

[Intruder]

Again, proper training (in this case, limitations of the resolution of the TCAS display) and oversight would help eliminate this type of problem.

OTOH, as in the personal incident I described earlier, ATC can make bad calls (reversal of turn) as well. So, ANY time there is a TA that looks to the Pilot to be an imminent RA, the Pilot MUST:

Prepare to respond to an RA
Query ATC as to whether they have the traffic and are aware of the conflict. For example: "Center, ABC123, I have a TCAS TA for co-altitude traffic 20 miles ahead. Do you have them?"

[ATC Watcher]

Intruder :
the Pilot MUST:[...]
Query ATC as to whether they have the traffic and are aware of the conflict. For example: "Center, ABC123, I have a TCAS TA for co-altitude traffic 20 miles ahead. Do you have them?"

If you want to get yourself in trouble, it is a good advice. :hmm:You are going to upset a lot of people down there .

I would also avoid to use the word " must" in here . There are existing ACAS procedures , and so far those MUST be followed. The rest is only our humble opinion, mine included.

[carpediem86]

I apologise in advance for not having read all relevant posts yet, but i meant to ask in the approach of a humble law student dealing with a moot court case identical to that of the ueberlingen collision only dated 2 years later, is there any ICAO or Eurocontrol document that clearly specifies that one should follow the TCAS-RA over the ATC instructions?

[FlightDetent]

Indeed. Search for ICAO Doc 8168 PANS-OPS.

[carpediem86]

thank you! Is that the only document which supports that claim or are there more, do you know?

Also i meant to ask, i have a document titled CAP413, do you know if this is accepted by either ICAO or EUROCONTROL as binding to all contracting states or is it a UK only material?

[airmen]

You may read this:

http://en.wikipedia.org/wiki/Traffic_Collision_Avoidance_System
http://www.caasd.org/work/project_details.cfm?item_id=153
http://www.aerowinx.de/html/tcas.html

for those who want facts and please remember the conclusion:

http://www.eurocontrol.int/msa/gallery/content/public/documents/Safety/ACAS_Bulletins-BUL6-D-1.1_21Apr2005.pdf

I have been installing and testing TCAS on several aircrafts and have to say that position accuracy is really not precise on the display, therefore follow the ATC instructions first and if there is a RA command then follow the TCAS vertical manoeuver!
Never use your TCAS display as a RADAR, it is not designed for that purpose!
The only way for TCAS to be fonctioning correctly is that everybody have to follow the same rules and procedures...if not there will be other cases to discuss...I am affraid

[ATC Watcher]

Carpediem: I apologise in advance for not having read all relevant posts yet
I always like those who have no time to read but want immediate answers,and now it includes future lawyers ! :E

The answer to your question is simple and was already given to you : ICAO PANS-OPS Doc 8168 : this is the bible that set the world standard and its current version says :
" Pilots shall follow the RA even if there is a conflict between the RA and an ATC instruction to manoeuver "
Now all the rest of the world regulations ( like the UK CAP, which , you are correct only applies in the UK ) and all airlines SOPs should reflect that standard. They might use other words , translations, etc,.. but that is the standard.
Please stay away from Wikipedia and other source of info, we are not at school making a project .

Airmen :
I have been installing and testing TCAS on several aircrafts and have to say that position accuracy is really not precise on the display, therefore follow the ATC instructions first and if there is a RA command then follow the TCAS vertical manoeuver!

Caution not to mix up ACAS displays azimuth resolutions and RAs. That was not the question asked and your answer can confuse the non-initiated. Once an RA has started , the position Display is useless , Pilot concentraton should be on the VSI , not on the position display.
Before an RA , ( and that includes during a TA , pilots should indeed follow ATC instructions , but once the RA has started, pilots should follow the RA, even if it contradicts the latest ATC instruction.

That partially explains why still today only 70% of the RAs are correctly followed , 20% are ignored and worse, in 10% of the cases pilots still choose to manoeuver opposite to the RA.

The other part of the explanation as to why this is still the case were/are discussed here.

[carpediem86]

I always like those who have no time to read but want immediate answers,and now it includes future lawyers ! :E



I will take that to be driven by the long-established hatred for my future proffession :p I had indeed read almost all posts yet i couldnt find what i was looking for.So i figured i'd post first before i finished my reading of all 9 pages of the thred ;)
And one more random comment: you have no idea how difficult it is to follow your posts with all these details and terminology, it sometimes feels as if i am reading chinese hehe.


The answer to your question is simple and was already given to you : ICAO PANS-OPS Doc 8168


Yep i did find the document and it is indeed what i was looking for. Am i correct that this document was not ammended as such before Ueberlingen? Hence if it indeed existed as such, fault could technically be attributed to the pilot who did not follow the RA, correct? Also, both the pilots of the aircraft, didn't they have to inform the ATC that there was a TCAS-descend for the one and a conflicting order for the other? I am not sure that i read something that clearly states so in the posts of the thread.

And one last question to ATC Watcher. I read in a few of your posts that when TCAS II came out, the manufacturing company for obvious liability reasons used the name advisory instead of command for the RA. Is there any document or article etc of the time that i can find this written on, or is it just a rumour/common knowledge in the world of the air?

[ATC Watcher]

Carpediem :
I will take that to be driven by the long-established hatred for my future proffession
Not really, but after having been scr..wed a few times , relying on the ethics of your future profession could be assimilated to masochism...:E

Back to the subject : Am i correct that this document was not ammended as such before Ueberlingen?
Correct > the old version was not clear and explicit about this situation. It was changed after Ueberlingen.
The Russian Pilots were not at fault technically as they followed ATC before the RA started. After that, according the rules published at the time , it was left to the pilot to judge to follow the RA or not .

Also, both the pilots of the aircraft, didn't they have to inform the ATC that there was a TCAS-descend for the one and a conflicting order for the other?
Yes they had to : the B757 pilot did it, but very late ( after 23 seconds ) and that message was blocked out by another transmission, so the controller did not get it .The Tu154 pilot did not advise ATC, (fact) probably because they never followed the RA.( my supposition)

I read in a few of your posts that when TCAS II came out, the manufacturing company for obvious liability reasons used the name advisory instead of command for the RA. Is there any document or article etc of the time that i can find this written on, or is it just a rumour/common knowledge in the world of the air?

Not common knowledge but a fact stated at the beginning, but on whose insistence I do not know. The company that built the TCAS software is the MITRE Corporation , on contract from the FAA. whether it is the Manufactures, the FAA or the MITRE lawyers that insisted on the change I cannot say. The first manufacturers were both Allied Bendix and Sperry/Dalmo Victor. They might also have influenced the change of terminology . Check their web sites and ask questions if you want to pursue that question .
I would be personally very interested to have the answer if you find it. . ( you can PM me or post it here )

Good luck .

[carpediem86]

I would be personally very interested to have the answer if you find it


I just emailed them about it so they will reply soon i hope. Thank you very much for your help and i will get back at you if i have something :)

[joernstu]

Yes they had to : the B757 pilot did it, but very late ( after 23 seconds ) and that message was blocked out by another transmission, so the controller did not get it.

The B757 reported the RA to ACC Zurich, but the controller was working with another aircraft on another workstation, where he could not hear the B757 (or the TU154M).

[ATC Watcher]

but the controller was working with another aircraft on another workstation, where he could not hear the B757 (or the TU154M).

You are Correct on the other workstation ( and the different frequencies ) I should have said "covered" instead of "Blocked". But the end result was the same.

For info to others , the other workstition was only 2,5m away and the controller could follow both transmissions and reply to them, but as the frequencies could not be collapsed together he had to go from one workstation to teh other to transmit. . The problem was that at the time the B757 called, the A320 also called and his call covered the 757 call.

One can find this in the report of the controller and on the communication/time chart published with the BFU report.

[songbird29]

ATC watcher, it was still on my to-do-list to react to your statements about TCAS downlinking to ATC.
A recent QinetiQ study show that the average delay for controllers to be aware of an RA via Mode S is about 9 seconds and to get a verbal report from a pilot after an RA is currently about 29 seconds.
The main problem is what happens during those 20 seconds.According to the current ICAO documentation the controller is still responsible and could/should issue instructions as long as the aircraft /pilot do not manoeuvre. The chances to have controllers intervene in those 20 seconds is higher than without RA downlink.
First, there may be an average of 29 seconds before a pilot informs ATC. But that does not mean that ATC is unable to become aware much earlier, when an aircraft deviates from its assigned level. At the most 9 seconds after the pilot's following the RA instruction, the climb or descent will become visible in the data block on the controller's screen. Visible, but the controller will only observe height readout if he happens to look to that particular spot on his radar screen. And in the case of a TCAS RA, chances are high that ATC has missed to see the development of a close encounter and consequently will not look to the spot where collision risk exists (because he is too busy elsewhere or because it's an unknown military or GA), unless a simple message from the TCAS RA downlink mechanism through Mode S draws his attention to it.

Second, in case of doubt wether the pilot follows the RA, ATC might ask. Might ask, because I am not sure the question is welcome at the flightdeck during those hectic and critical 29 nine seconds before the average pilot comes to informing ATC himself. But, to all intents and purposes, a simple question is much better than a contravening instruction from ATC which cannot be other then confusing.

Third, either ATC trust that pilots act in accordance with their SOP's and adhere to the RA, or we take it for granted that pilots ignore a potentially life-saving warning. Shouldn't we just agree that it is bad airmanship when pilots play around a TCAS RA. Ok, we know it happens, 10 % according to the study you quote. That means that there is still work to do at this front. But bad controllership also exists and I maintain that it is bad controllership if ATC prefer to ignore knowledge of RA's which affect the progress of a flight. Politique de l'autruche, the ostrich putting its head in the sand.

should we decide to downlink one day, there is a strong debate about displaying or not to controllers the sense of the RA. Seen the mode S delay to display the information, a risk exits that the displayed information will be different of the actual RA ( especially in case of sense reversal )
I'd think this debate is academic and by extension false argumentation. In the case of an RA, the provision of separation by ATC has failed (in the neutral sense of the word) and is replaced by the pilot taking ultimate responsibility for the safety of flight. The only action to be taken by controllers is... to refrain from any action, to shut up except for advising other aircraft in the vicinity which might be affected by the evasive action.

Finally, I must say that I have never understood the lax attitude of the controllers' international organisation IFATCA vis-a-vis the possibility of getting technical life-saving improvements, which could to a high degree prevent a collision from happening.

In the aftermath of the Uberlingen accident technical improvements remained limited to some improvent of procedures and airborne TCAS upgrades. This was necessary and useful, but we are still waiting to see ATC improvements that preclude ambiguity with RA instructions through downlinking of the RA. ATC administrations and airlines who have to pay the bill were wary of the financial implications. Uberlingen was quickly put back in their minds in favour of the rise in traffic figures, resultant capacity problems and cost reduction.

But the controllers had to get through the assassination of one of their colleagues. His recent acquittal by a Swiss court could not bring him back. Every TCAS RA event happening in their sectors reminds them of the shortcomings of the system. Yet, their representative body IFATCA, instead of putting pressure on their administrations to provide them with the best possible technical information about TCAS RA's, kept pressing on legalistic argumentation about responsibilities and accountabilities.

[PBL]

Carpediem,

you have no idea how difficult it is to follow your posts with all these details and terminology


I work with real lawyers on real aviation cases, amongst other things trying to keep real people out of jail. But my day job is with a university.

I think I can appreciate how difficult it is for people not in the field to understand complex technology. I have been in that position. My inclination is to do what it takes to understand all I have to know in order to make correct informed judgements (I am known for this, and people pay me for it occasionally). There is another way of dealing with things, followed by most experienced lawyers, which is to employ the right experts (that is where your judgement comes in) and believe (and have them transcribe for the court) what they say.

In a real court case, you would be offering ATC Watcher lots of money to say what he has to say. In a moot court, you are unfortunately faced with the first option: no matter how hard it is, you have to get your mind round the details.

Hence if it indeed existed as such, fault could technically be attributed to the pilot who did not follow the RA, correct

That is, if I may say so, far too naive a stance for an putative aviation lawyer.

Obvious question, which should be the first one you ask as a lawyer: who has jurisdiction?

Answer: obviously Germany, since the airspace was German.

So you had better look up applicable German law. If you don't read German, you are lucky in this case in that the applicable German law is translated in the accident report from the BFU, which I may presume that you have read, as a lawyer.

You will therefore have observed that the advice about following RAs is not strictly identical with the advice in PANS-OPS 8168.

But Germany is an ICAO signatory. So which advice applies and why?

You will also observe that there is another applicable German law (which is more or less universal), which says that the airplane on the right (i.e., the Tu-154) has right of way. According to that regulation, your suggestion that
fault could technically be attributed to the pilot who did not follow the RA, correct?
is not correct.

You're the lawyer. You sort it out.

Whoever suggested you work on this either had it in for you or thought you were a legal genius. It is certainly worth trying to sort out, but don't expect anyone here (or anywhere else) to have packaged answers for you.

PBL

[joernstu]

First, there may be an average of 29 seconds before a pilot informs ATC. But that does not mean that ATC is unable to become aware much earlier, when an aircraft deviates from its assigned level.

Even without TCAS RA aircraft are allowed to deviate from their assigned flightlevel. Only if the deviation exeeds 200ft ATC can become aware of the deviation.

As the radarsystems updates the displayed information in intervals, the timespan between begin of the deviation and recognition by ATC can be several seconds.

Even with all these factors for delay, technically there are two institutions trying to control during a conflict situation (ATC and TCAS). These two obviously need to be coordinated in some way.

In the case of an RA, the provision of separation by ATC has failed

This need not be true in all cases. In the case of Ueberlingen, the TCAS RA was issued before the aircraft underrun the 7NM separation limit.

[ATC Watcher]

In a real court case, you would be offering ATC Watcher lots of money to say what he has to say.
Thanks Peter, I am indeed too naive, Capediem was also asking me in PM a lot more questions ( which I gave him ) .
I realize now that I could perhaps make a living out of this !:O


Now answering songbird :

A lot of things to comment upon in your post.
The easy ones :
First : the 29 seconds is indeed an average. That means some are sooner , but some are also taking longer or missed altogether.
On the sooner ones, you say after 9 seconds max for a radar return, this is not universal . The Zurich radar return was 12 seconds in Ueberlingen for instance .
Also many controllers always wait the second Mode C update to verify trend , as many small deviations are just garbling or aircraft altitude hold variations.

Second : In doubt why not ask the pilot if he is following an RA. : Why not indeed . Just that in case of imminent collision, as a controller, you are more likely to want to issue avoiding instructions instead of loosing precious seconds asking the pilot if he has a functioning TCAS and if he is following an RA.

Thrid : I maintain that it is bad controllership if ATC prefer to ignore knowledge of RA's which affect the progress of a flight. Politique de l'autruche, the ostrich putting its head in the sand.

ATC at the moment prefers to refrain from Downlinking RAs , not to ignore RA knowledge. On the contrary ATC urges pilots to report to ATC immediately if they are getting and following and RA. ( In Ueberlingen should the 757 crew had done so earlier , the outcome might ( emphasis on might ) have been different )

A little more info ( to confuse you even more ) on RA downlink to justify IFATCA ( and my own ) position on RA downlink:

A very recent Bretigny Survey of a single Mode S radar messages show the following picture :
90% of RA downlink messages are false RAs not displayed in the cockpit, but are broadcasted. . Half of those were identified as coming from a special transponder manufacturer on certain Boeing 737s ( 51 airframes identified so far , mostly on the UK register ) and a solution for those is expected.
The other half are short false RAs whose reason is not yet understood.
Of the 10% that remains, a further study indicates the following percentages : in 20% the pilots actions are inappropriate, in 10% pilots are acting in the opposite direction of the RA, and only 70% are more or less followed correctly. this confirms other larger studies both in Europe and in the USA.

Now to answer you final questions :
Finally, I must say that I have never understood the lax attitude of the controllers' international organisation IFATCA vis-a-vis the possibility of getting technical life-saving improvements, which could to a high degree prevent a collision from happening.

No, IFATCA on the contrary has the correct approach to this. They believe that hastily introducing RA downlink for the wrong reasons ( The Japanese pushed this in ICAO after their JAL/JAL encounter) while there are still so many issues unsoved will create more additional problems than it solves.

I for one believe that downlinking RAs Mode S messages at the moment will increase controller involvement and contribute to more confusion , or Ueberlingen type scenario.


IFATCA, instead of putting pressure on their administrations to provide them with the best possible technical information about TCAS RA's, kept pressing on legalistic argumentation about responsibilities and accountabilities.

This is a fundamental issue : valid for TCAS, but also for ADS-B and ASAS : Who is responsible for anti collision and when.

Nobody has resolved this issue yet and will resolve it soon I think.

For the moment I am responsible for anti collision TCAS is only a safety net that i should not take into consideration when I work. ( I am even not sure/ aware who has a functioning one ).
If the pilot is coming on the frequency and say "Moving , TCAS RA , " I Know my responsibility ends and he takes over . ..,I do not think any regulator will change this on account of RA downlink. So this is why IFATCA wants the legal bit solved before getting the Downlinks on their radar scopes. Also, based on the current surveys , Mode S RA messages are full or garbage anyway.

We are not putting our heads on the sand. We have been burned already by a hasty and immature introduction of version 6.04 ,often without training , in life traffic. We have had Ueberlingen, so time for the Regulators to take their responsibilities I would say. I support IFATCA 100% on this one .

The ball is in EASA and the FAA camp.

[songbird29]

Joernstu wrote :
Even without TCAS RA aircraft are allowed to deviate from their assigned flightlevel. Only if the deviation exeeds 200ft ATC can become aware of the deviation.
True for the 200 feet deviation, it's actually 299 feet. But a TCAS originated FL deviation will very soon get beyond 300 feet. I agree this has some impact on the time lapse before a controller could see the FL deviation without a TCAS RA downlink. But remember the context of my point. Because there is no attention getter related to the FL deviation, it is not very likely that the controller will see it because his attention is drawn somewhere else. What the controller needs is the attention getter in order that he can take the right measures, which normally will be to refrain from action vis-a-vis the RA affected aircraft.
As the radarsystems updates the displayed information in intervals, the timespan between begin of the deviation and recognition by ATC can be several seconds.Yes, several seconds, but always better than the average 29. In busy areas, with multiple ground radar antennes integrating the signals for display on the controller's radar screen, the delay is only two or three seconds.
Even with all these factors for delay, technically there are two institutions trying to control during a conflict situation (ATC and TCAS). These two obviously need to be coordinated in some way.
Very well put, this is the heart of the matter. And it requires a good solution, see my further points to conclude that a good solution is achievable and is better than a do-nothing attitude, waiting for the best solution.
-
ATC Watcher wrote:
First : the 29 seconds is indeed an average. That means some are sooner , but some are also taking longer or missed altogether. On the sooner ones, you say after 9 seconds max for a radar return, this is not universal . The Zurich radar return was 12 seconds in Ueberlingen for instance .
You put the 29 seconds and the 9 seconds in one basket but they apply to different magnitudes. The 29 seconds gives the average time lapse before pilots inform ATC on the radio, whereas the 9 seconds is, at least according to the Qinetiq study, the average radar return. My point was that on average there is 20 seconds to gain for getting the information where it is needed. Now on the detail, in Zurich 2002 the radar return was 12 seconds. However, the Swiss violated here the Eurocontrol standard of 8 seconds. I think this is mentioned in the investigation report but I haven't checked up yet.
Second : In doubt why not ask the pilot if he is following an RA. : Why not indeed .
As you will have read, I have my doubts myself, but it's better, or shall we say less harmful, to ask a question than a contravening and confusing instruction against TCAS.
Just that in case of imminent collision, as a controller, you are more likely to want to issue avoiding instructions instead of loosing precious seconds asking the pilot if he has a functioning TCAS and if he is following an RA.
Exactly, the instinct of a controller is to do as you describe. But the difficulty is that in the case of a TCAS RA, he should not follow his instincts any more. TCAS has taken over and the controller should act accordingly which in this case is to refrain from action, other than removing third party traffic. You will have seen my own hesitation to put up this possibility of asking the pilot. The main point is that the controller should, at the earliest possible time, be stopped from doing the wrong thing, that is to issue a contravening instruction which is so much confusing in the hectic situation of an RA.
90% of RA downlink messages are false RAs not displayed in the cockpit, but are broadcasted. .
So, connect the downlink to those RA's which are displayed in the cockpit. Problem solved.
Of the 10% that remains, a further study indicates the following percentages : in 20% the pilots actions are inappropriate, in 10% pilots are acting in the opposite direction of the RA, and only 70% are more or less followed correctly.
A remaining problem indeed. As I said before, a lot of work remains to be done. On the detail again, it is not clear to me wether the '70 % more or less correctly' is of the total population, or only of the 10% that remains. If the latter is the case, then, logically, the real problem is only for 3 % of the TCAS total?
I for one believe that downlinking RAs Mode S messages at the moment will increase controller involvement and contribute to more confusion , or Ueberlingen type scenario.
I don't think your statement is supported by the downlink simulations which were organised in Bretigny. Furthermore, how can knowledge, acquired by a downlink, be confusing. It is lack of knowledge and bad decisions taken based on a lack of knowledge, which is confusing. Not only in ATC but anywhere else. In this context but nothing to do with TCAS the Amsterdam crash of the El Al a/c flying into an appartment building springs to mind. The crew couldn't see what had happened with the engines which had fallen off, by this lack of knowledge they decided to turn in the wrong direction which caused the dramatic going down of the aircraft.
For the moment I am responsible for anti collision TCAS is only a safety net that i should not take into consideration when I work. ( I am even not sure/ aware who has a functioning one ).
STCA (short term conflict alert, the ATC safety net) is only a safety net. I'm sure all area controllers take it into consideration when they work (admittedly, provided the number of false alerts is reasonable, but if this is the case management and technical staff should urgently repair the tool). Why should TCAS, as you say equally 'only a safety net', be treated differently when this is made possible? Why wait the average 29 seconds before it is taken into consideration anyway? I am pleading for improving the controller's situational awareness, rather than impeding. Improved situational awareness will improve his decision making, and not confuse. In my previous message I called this good controllership.
immature introduction of version 6.04 ,often without training , in life traffic.
True. The first TCAS versions were introduced, pushed through even, without safety cases as to the impact on ATC. However, that's spilt milk, ATC has to live up to the situation and should embrace the improvement at hand, which brings the individual controller in a better position to apply his judgement when he gets involved in a horrifying RA situation.
The ball is in EASA and the FAA ca
I have noticed with satisfaction that you have put in bold 'for the moment'. That would seem to imply that you, and dare I say through you IFATCA, provide an opening to accept the downlinking of TCAS RA. You are right of course that the legal stuff should be solved. But the statement that the ball is elsewhere (what has the FAA to do with Europe, but that's another matter, I would think worldwide ICAO is the Ageny to address), alludes to a passive attitude from the side of IFATCA. Laxism may have been a too strong expression, but I would expect IFATCA to be active on this front. If it's only a few sentences in some documents to be changed : don't wait but go for it.

[joernstu]

Furthermore, how can knowledge, acquired by a downlink, be confusing. It is lack of knowledge and bad decisions taken based on a lack of knowledge, which is confusing.

One scenario I can think of and where this would be problematic is a situation between one aircraft with transponder and TCAS and another without with ATC providing separation.

If aircraft 1 (the one with TCAS) for some reason deviates from its flightpath towards the flightlevel of aircraft 2 and almost simultaneously its TCAS transmits a bogous RA to ATC, the controller will not advice aircraft 1 to return to its flightpath as heshe would see the TCAS RA indication on the screen and could miss to recognise that aircraft 2 cannot be the cause for the RA.

So perhaps for downlinking TCAS RAs in addition to transmitting only real RAs (those issued to the crews), ATC would also need a database on all aircraft listing their transponder state. This database itself could introduce other failure scenarios.

The indication of TCAS RAs to the controller has to be correct 100% when it is made - or the indication should not be made at all.
Reason for this is, that no indication made at all will not change the current situation where ATC and TCAS simultaneous try to control the aircraft - not changing the current level of safety. TCAS RA indications to ATC which aren't 100% correct could lead to controllers acting as though they were 100% correct but in those cases where they aren't lead to dangerous or fatal situations - in extreme lowering the level of safety instead of improving it.

[ATC Watcher]

Songbird , here we go again :
A bit more in depth this time.

On the delay : Yes, several seconds, but always better than the average 29
.
Problem is that ATC using radar always look at the past. whether it is, 4, 9 or 12 seconds. TCAS is a very dynamic system, which can issue corrective RAs and whose version 7 can even issue corrective reversal RAs. Those, in order to be effective have to be followed by pilots within 2.5 seconds according the manufacturer manuals. We agree so far right ?
My point :
This will only happen in some particular situations, I agree, but in a mode S download situation what will be displayed to the controller on his screen will be totally different of what is happening in real time, hence increasing the possibility /likelihood of controller intervention.

On the technical downlink medium :So, connect the downlink to those RA's which are displayed in the cockpit. Problem solved.

This will mean re-encoding the displayed RAs and re-broadcasting them .I am not an engineer, but this will be complex and possibly expensive , and adding an avionic box is not the idea.
Also in Eurocontrol they have already decided that should we go for RA downlink , it ill be using the mode S broadcast. The cost of using the other (better) method of using the 1090 Extended squitter was judged prohibitive for that function only.

Replying to your question :
it is not clear to me wether the '70 % more or less correctly' is of the total population, or only of the 10% that remains. If the latter is the case, then, logically, the real problem is only for 3 % of the TCAS total?

The 70% is within the initial 10% which are valid correct RAs. The rest is raw data , you cannot start to make percentages values on unvalidated data !

Increasing Controller involvement 2 :
I don't think your statement is supported by the downlink simulations which were organized in Bretigny.
Be careful in analyzing the outcome RADE simulations. the rather large RADE 1 was only a simulation on the HMI ( i.e the display) of RAs, and indeed most controllers there found the HMI adequate.
The 3 subsequent low-scale RADE simulations start to show another picture, and the 4th one ( RADE2T, only with 4 controllers from 2 APPs ) show the real limitations. The number of RAs shown to controller during those simulation was so low that I would refrain from drawing firm conclusions one way or another.
What I myself found interesting (and worrying),in those simulations is that despite this simulated environment, and the low numbers, on 2 occasions controllers intervened after an RA ! Training is still a major issue, also for controllers..

On the safety nets :
STCA (short term conflict alert, the ATC safety net) is only a safety net. I'm sure all area controllers take it into consideration when they work (admittedly, provided the number of false alerts is reasonable, but if this is the case management and technical staff should urgently repair the tool). Why should TCAS, as you say equally 'only a safety net', be treated differently when this is made possible?

Ah, the old discussion between a safety tool and a safety net !
We could debate this for hours,. Short version : NO controllers should not use STCA and TCAS as safety tools. ( and they are trained and told not to )

Finally :
(what has the FAA to do with Europe, but that's another matter, I would think worldwide ICAO is the Ageny to address),

The FAA owns the TCAS system . Changes to its software and procedures have to be discussed with them trough RTCA. It is not a European system. ICAO do only issue SARPs for ACAS . They have no influence on TCAS.( and frankly no expertise either )

As an aside, I do not represent or talk for IFATCA here, but it happens that I agree with what they say on this particular point , which also happens to be very close to what the IFALPA ATS committee says by the way.

In a nutshell : No against Downlinking of RAs per se , but not now. Solve the problems first to make sure we will not add more problems than we want to solve, and increase, not decrease the likelihood of controller intervention .

Absolutely against introducing it now here and there to " see what happens" and " gain experience " etc...

[The Blu Riband]

Do all operators use TA/RA all the time?
Do any operators or countries allow for selecting TA only on , say, parallel runway approaches.

[FlightDetent]

As far as I understand it is an FAA recommendation to consider TA only. Original Airbus manuals state TA or TA/RA AS RQRD with a note on the FAA stance.

My employer does not go any further and the accepted practice is always TA+RA. Never had any problems with parallel runway operations (not always approaches) in FRA, MAD, BCN, BRU, AMS, LHR, FCO, LED, SVO, OTP, HEL, MAN, OSL, CDG ... However, the runway spacing is not very tight and there is little or none GA/VFR traffic, which I suppose may be the main reason to select TA only above 800 ft on approach.

FD (the un-real)

[The Blu Riband]

I have had 2 RAs at DEN. Out of 5/6 visits this year.
On 1 occasion we were 4 across - to the 4 parallel rwys.
We had a/c on both sides, very close, gear down, and if we all reacted, or did full go-arounds, we would certainly lose sight of each other. And could I trust the software to cope with 3 or 4 simultaneous reactions?

[punkalouver]

As a followup......This quote was made by Joernstu earlier in this thread.

Originally posted by joernstu
I cannot decide, what kind of action would induce the lowest risk - following an RA, not following an RA or flying without TCAS at all as the basis for my risk evaluation would be episodes. I think the same goes for you as your information basis is still only episodes

http://www.eurocontrol.int/msa/gallery/content/public/documents/ACAS_Bulletin_9_Jul-07.pdf

I direct you to the Eurocontrol ACAS Bulletin #9 and quote...

"Five. The factor of collision risk reduction thanks to TCAS II in the operational world, taking into account some non TCAS II equipped aircraft, inaccurate pilot responses and lack of responses, altitude reporting inaccuracies, etc."

There have been some irresponsible statements made much earlier in this thread. It is rational to Follow The RA even if it contradicts an ATC instruction.

[bsieker]

I direct you to the Eurocontrol ACAS Bulletin #9 and quote...

" 5. The factor of collision risk reduction thanks to TCAS II in the operational world, taking into account some non TCAS II equipped aircraft, inaccurate pilot responses and lack of responses, altitude reporting inaccuracies, etc."

Reading your quote I thought this was point "5" from an enumeration of various points. The meaning, however, is that TCAS has reduced the risk of a mid-air collision by a factor of FIVE.

And I'm sure they arrived at this figure by finding out that there were 80% fewer mid-air collisions since the mandatory introduction of ACAS-II per billion flight hours, compared to before. Or did they?

Otherwise, this figure is completely bogus. (I'm certain it is bogus anyway, since luckily the number of mid-air collisions is too low to derive any statistical significance from it.)

There have been some irresponsible statements made much earlier in this thread. It is rational to Follow The RA even if it contradicts an ATC instruction.

Trying not to re-iterate the entire argument: we did not doubt that following the RA was a rational choice. The point was that it was neither the only rational choice, nor necessarily the best (as the DHL reaction clearly shows: they followed the RA, and had a mid-air collision. QED).


Bernd

[punkalouver]

Originally posted by bsieker
And I'm sure they arrived at this figure by finding out that there were 80% fewer mid-air collisions since the mandatory introduction of ACAS-II per billion flight hours, compared to before. Or did they?

You would be sure of that if you believed risk reduction is the same as fewer actual midair collisions.

I don't have the details of how they came to their conclusion or Joernstu came to his conclusion, but I am left with a choice of believing an organization called Eurocontrol and their statements and excellent newsletter or the supposed risk evaluation of some guy on a website called Joernstu.
Choice made. I believe the collision risk is less with TCAS installed.

Interesting that they underlined "operational world" in their statement. Maybe they are reading this thread. Perhaps this "risk evaluation of would be episodes" should be sent to them and they can reply with their "bogus" data as you call it. After all, safety is our common goal.

[joernstu]

It is rational to Follow The RA even if it contradicts an ATC instruction.

Of course it is rational to follow the RA and blend everything else out. This will well be the action that comes nearest to the mathematical "rationale".
Problem is, as soon as you introduce humans into your system, the question of rationality becomes a lot wider. A course taken by a human will be rational, if he acts acording to the limited information he has. If he chooses, the best way for him to solve his problem will be course A, course A will be a rational choice. You cannot decide on the rationality of a humans actions by looking at the outcome alone - the pilot's cristal ball isn't near as good enough for this ;-)

[punkalouver]

Well thank you, I'm glad you agree with me. After all it was a statement(way back in the thread) that contradicting the RA was a rational choice that got me involved in this thread.

[joernstu]

Well thank you, I'm glad you agree with me. After all it was a statement(way back in the thread) that contradicting the RA was a rational choice that got me involved in this thread.

Well, it can be, as I described before.

[PBL]

After all it was a statement(way back in the thread) that contradicting the RA was a rational choice that got me involved in this thread.

Yes, but you should be aware that until you can adequately solve the decision problem posed to you then, few TCAS experts will take your interventions seriously.

PBL

[punkalouver]

I'd say Eurocontrol and I are on the same wavelength about how to proceed in such a situation. The BFU as well. I take them seriously and don't refer to statements from them or one of them as bogus. It is not for the TCAS experts to take me seriously. It is me taking them seriously which I do.

Why don't you send this problem or situation to the appropriate Eurocontrol department(we already know the BFU's opinion) and we can see what their reply is. I will start holding my breath now.:ooh:

[PBL]

I'd say Eurocontrol and I are on the same wavelength about how to proceed in such a situation. The BFU as well.

Well, my friend, this is where your youth and lack of experience are showing through again, as also my inclinations as a professional educator.

As far as I know, the colleagues at Eurocontrol were reading this thread when there was technical discussion (I doubt they are bothering any more, and I can't be bothered to ask).

The BFU has nobody expert in safety-critical digital systems,
and why should they. They don't design them, build them or analyse them.

PBL

[punkalouver]

Hmm, so now I am youthful and inexperienced by reading Eurocontrol newsletters that say to FOLLOW THE RA, ageeing with them and feeling we are on the same wavelength. As well, the BFU in their final report recommends "never manouver in the opposite sense to an RA(directly from the report that deals with your scenario). I am youthfully on the same wavelength with them despite my inexperience.

I thought about your scenario the other day as I was flying along at a high altitude. There was 180° opposite traffic almost on our track that we knew was several thousand feet below us. We saw their contrail well before we crossed paths. For much of the time their contrail appeared above us. It was definitely above the horizon until it got much closer and became obvious that it was much below us. Sort of like the cloud tops that often appear to be at our level initially the pass by down below. Your scenario has the option to visually manouver to avoid the visually aquired traffic which is opposite to the RA that has alerted you to a strong collision potential. As I have previously said, visual illusions, especially at night can make it extremely difficult to accurately assess the trajectory of another aircraft in this situation.

In your scenario, which is the accident scenario, it is safer to climb. Why, because we know how extremely dangerous it is to descend as proven by the European collision and other near collisions and it is unlikely that the TCAS is wrong. The other guy will likely be descending as well because he is well trained. Manouvering visually may make things worse in many cases (especially at high speeds, high altitude and night). How easy is it to manouver visually around a rotating beacon and a set of nav lights closing in at 500 miles per hour, possibly against a backgound of many ground lights). Is this your actual experience and non-youthfulness speaking on how easy it would have been for the 154 to descend and visually manouver around the 757).

How dangerous is it to climb or maintain level flight(assuming that there really is an aircraft out there that you can't see)? I don't know, but I say less dangerous. Why? The big sky theory. The chances of actually hitting that aircraft are still extremely small. ATC is going to calling for an immediate climb or descent if your aircaft will be within 5 miles of each other(or whatever the minimum separation is). TCAS is calling for a manouver when the collision potential is much higher. It is nice to have hours upon end in an office to analyze various scenarios. When you have few seconds to make your decision, certain procedures should already be clear in your mind. Sure, there is always the obscure scenario where acting contrary to the SOP saves the day. But that is a rare day. and it wasn't the Day(or night) over Germany.


That the BFU has no experts on TCAS is not surprising. Safety boards typically bring in outside experts during an investigation.


It is unfortunate to hear that you are close enough to Eurocontrol to be
aware that they were reading this thread yet you "can't be bothered" to attempt to discuss with them your scenario and their recommendations and reasons for their recommendations for your scenario.

In case you happen to find the time, their address, fax and phone number are on their newsletter.

[joernstu]

punkalouver, in all your argumentation, you continously overlook, that the topic of this threat is the discussion on TCAS philosophies. This cannot be limited to the teaching material published by any organisation.

I wonder - if you really have read the Eurocontrol bulletins as you said you have, why hasn't it occured to you, that all the cases published there have one causal factor in common: TCAS. Some of the problems described are related to training, but some of them seem to be very hard to mitigate, e.g. the interception of a A340 by a military jet transmitting altitude information described in Bulletin 9. Apart from changing the practice by the airforce there seems to be little, that can prevent this kind of incident from happening. What strikes me in this incident is, that only the disregard of the A340 crew to follow the TCAS RA when reaching critical altitude led to a reversal RA. TCAS could have noticed that the intruder (airforce) was not acting according to the TCAS logic and could have issued a reversal earlier - but this will be solved in TCAS 7.1 as Eurocontrol promised.

Interestingly enough, you quote the BFU report but seem not to recognise, that the first point stated by the BFU report under "systematic causes" (page 119 in the german version) is the insufficient integration of TCAS into the aviation system.

[punkalouver]

Originally posted by joernstu
punkalouver, in all your argumentation, you continously overlook, that the topic of this threat is the discussion on TCAS philosophies. This cannot be limited to the teaching material published by any organisation.

I haven't overlooked that at all. I have just responded and perhaps added to 2 very, in my opinion, irresponsible statements.
1) that the actions of the TU-154 crew were rational
2)The statement "I cannot decide, what kind of action would induce the lowest risk - following an RA, not following an RA or flying without TCAS at all as the basis for my risk evaluation would be episodes. I think the same goes for you as your information basis is still only episodes."

There are occasional pilots out there who will read or hear about about some obscure or one-off scenario(not necessarily TCAS related) and as a result decide that they have a better plan of action than the SOP. Perhaps it is how to deal with a certain type of fire or ditching procedure. I have flown with these types. Statements like your coming from a supposed accident analysis panel or whatever it is called I believe increases the likelyhood of some people disregarding or going against an RA.

Originally posted by joernstu
I wonder - if you really have read the Eurocontrol bulletins as you said you have, why hasn't it occured to you, that all the cases published there have one causal factor in common: TCAS.

I think, perhaps it is you who should read the bulletins. The are all kinds of examples where TCAS worked as programmed and saved the day including two in Bulletin 9(one an altitude bust,the other is an ATC error). The A-340 incident was caused by proper procedures not being followed which can cause an incident in many ways in aviation.

Originally posted by joernstu
Interestingly enough, you quote the BFU report but seem not to recognise, that the first point stated by the BFU report under "systematic causes" (page 119 in the german version) is the insufficient integration of TCAS into the aviation system.

I read this statement which is located a second time earlier in the report. I will print here the next sentence in the same paragraph. "The regulations concerning ACAS/TCAS published by ICAO and as a result the regulationsof national aviation authorities, operational and procedural instructions of the TCAS manufacturer and the operators were not standardized, incomplete and partially contradictory."
So the training and regulations were not proper or were contradictory. This could lead to problems in many areas. I believe the BFU is correct. This is a regulatory issue which I believe has been corrected.

[PBL]

Young man, this is getting really tedious. Let me try to put another perspective on things.

I have just responded and perhaps added to 2 very, in my opinion, irresponsible statements.
1) that the actions of the TU-154 crew were rational

You introduce the notion of responsibility. This is not a foreign notion to engineers, but part of most of the codes of conduct of most professional engineering societies.

The engineering facts which you choose not to like, inter alia that, given the decision presented to the Tu-154 crew, it would have been rational for them to choose to go against the TCAS RA, were discovered, as far as I can tell, by me, and have been published in an engineering organ after peer review by two published TCAS experts, one a Fellow of the Royal Aeronautical Society who liked the paper, and one by an engineer for a major ANSP, who did not like what he read but was unable to construct any valid counterarguments.

It is in the code of conduct of many professional engineering societies that if one becomes aware of significant safety issues with engineered systems, one is obliged to make these issues known at least to the profession and to users of these systems. This obligation falls under the concept of "professional responsibility".
I did that, and continue to do so.

Instead, you suggest this very action is "irresponsible". That is a perjorative term. Were you to be a professional engineer and a member of a professional engineering organisation, you would be censured by that organisation for saying such things publically. For most societies censure the public defaming of work of fellow engineering professionals.

Defaming is or course different from technical criticism, which is generally encouraged. But you have been repeatedly asked over almost six months for arguments contravening the engineering fact that you do not like, and you have repeatedly evaded the question. While repeating your defamation.

If you pretend to be a professional, the only way you can get away with that behavior without censure is that this is an anonymous forum and nobody knows who you are. I think that it in general a good thing, because there are all sorts of facts that people don't like and which they attempt to suppress, and it is good to have a place in which they can be brought up and discussed without political consequences. But by the same token I don't think the forum should be used to propagate abuse.

So I'll just ask you to behave like a professional. If you are not sure what that involves, let me suggest you check out a few codes of conduct of engineering societies which are available on the WWW.

PBL

[punkalouver]

All I can say in response to the last post is to suggest that anyone(preferably people not associated with myself or PBL) who is interested in this back and forth posting, to read what I have printed and post what they think I have said here is unprofessional.

I don't disagree or necessarily know what what engineering facts you have discovered. There are no facts on this thread or any of your studies that I have ever said I don't like. I assume they are correct.

I strongly disagree with the two posts that I previously mentioned and stand by them. I don't have technical studies to throw into the forum to back up my arguements. All I have is real world results to look at. Publications giving example after example of saves by TCAS, near collisions caused by improper procedures regarding TCAS and near and actual collisions caused by ignoring TCAS. Then of course there is the occasional TCAS related incident.
As for the not responding for six months, I have no technical engineering or studies to give mathematical probabilities of how much safer it is to follow the RA in the midair collision incident. Just real world actions and opinions.

Perhaps you could respond on how to visually avoid that 757(probably seen only as dim lights) closing in at 1000 km/h or more, probably descending as well, and quite possibly against a background of lights. There are no mathematical formulas for this and theory and statistics don't work in a split decision moment like this. Perhaps blind luck does.

Now new terms have been brought in.

Defamation. Which rightly or wrongly makes me think lawsuit in my part of the world and makes me glad for anonymity.
abuse. Please, contact the moderator and ask him to review this thread and give an opinion on what abuse I have given out on this thread and please publish the response. There has been no abuse here. I know there will be no follow up on this one.

Maybe its time to lighten up with this enlightening link.:D

http://www.successunlimited.co.uk/humour/engineer.htm

[ATC Watcher]

Punkalouver:For the record : I am not associated with PBL (nor yourself.)
I am also not an engineer, just a simple Controller with some extra knowledge.

I however think that you should open your mind beyond what you read in the Eurocontrol ACAS bulletins. They are meant for the education of the public users , (i.e pilots and controllers) , and provide standard basic solutions to selected illustrative incidents . They are not the TCAS Bible .They do not cover the philosopy of the system , which is what we try to discuss here.
You said :
actual collisions caused by ignoring TCAS
We only have one in Europe :so let's take it as an example ( because we all know this one quite well by now) I asume you wrote this because you mean the TU154 pilot ignored TCAS ?
Now this is how I see it :
The collision was not caused ONLY because the 154 ignored TCAS.
To occur this collison needed more :
1) To have both aircraft fitted with TCAS ( because with no TCAS there would not have been a collision ) and
2) for the other aircrfat to follow his RA .

The 757 followed his own RAs more or less as the book said ,and still collided.
(so TCAS did not help/protect him at all )
Finally on this point : based on the regulations in force in July 2002 , it was not irrational for the Tu154 crew to follow the ATC instruction.

Next :
I have no technical engineering or studies to give mathematical probabilities of how much safer it is to follow the RA in the midair collision incident.
Nobody has. Therefore when I see statements like it is 4 times safer to follow RAs , I smile a bit behind my PC. In any case the numbers are far too small to deduct any useful statistics.

Remaining in Europe ,and only looking at facts : since 1970, there were only 2 high altitude collisions before Ueberlingen.( Nantes in 1973 and Zagred in 1976 ) Those 2 collisions would most probably have been prevented if TCAS would have been available then. However in the period 1976-1994 . No collision and No TCAS either. For the period 1994 -2000 a large portion of aircraft at High altitude in Europe were fitted with TCAS , but not all, especially not those of the former Soviet Union States .Again no collision during that period.
But since mandatory carriage of TCAS in Europe ( 2000) 2 years later one collision partlially caused by TCAS.
So, allow me to question the philosophical statement that " TCAS is good for you "
So when you say that ( quoting you ) :
Publications giving example after example of saves by TCAS,
I also smile a bit. We did not have a collision in 26 years despite many , many millions of flights in that period, and suddendly, we now have a system that would have saved dozens of collisons every year ?
TCAS was introduced in the USA without a proper safety case , for political reasons. It was later introduced in Europe for legal reasons (it would not have been sustainable legally for a State to have a collision in its airspace that could have been prevented by a system that was technically available then ).

Now it is its technical ( in abroad sense ) shortcomings that are being discussed.

[john_tullamarine]

The moderator thinks that folks should have the occasional ten deep breaths prior to getting too excited and agitated over a discussion ...

[punkalouver]

Originally posted by ATC Watcher
Now it is its technical ( in abroad sense ) shortcomings that are being discussed.

Thanks for your reply. I fully agree that there are probably technical shortcomings and PBL is quite possibly aware of them. Have never said different.

Originally posted by ATC Watcher
TCAS was introduced in the USA without a proper safety case , for political reasons.

It has been over 21 years now since the midair collision over L.A. of a DC-9 and a mode-C equipped Piper. For most of the past 21 years in the U.S. all airliners have had TCAS and there have been no airliner midairs. In the 21 years before the L.A. collision there was:

1) 25 September 1978; Pacific Southwest 727-200 vs Cessna172; San Diego, CA-143 dead.
2) 6 June 1971; Hughes Airwest DC9-31 vs. F-4; Near Duarte, CA-50 dead.
3) 9 September 1969; Allegheny DC9-31 vs. Piper Cherokee; Near Fairland, IN-84 dead
4) 19 July 1967; Piedmont 727 vs.Cessna 310; Hendersonville, NC-82 dead.
5) 9 March 1967; TWA DC9-14 vs. Beech Baron; Near Urbana, OH-26 dead.

Keep in mind that there have been way more airline flights in the 21 since the L.A. collision than the 21 previous years. This reality is what makes me smile behind my PC.

Originally posted by ATC Watcher
based on the regulations in force in July 2002 , it was not irrational for the Tu154 crew to follow the ATC instruction.

From the final report in part:
The TU-154 AOM contains general recommendations regarding the use of TCAS. The AOM reference draws attention to the fact that manouvering opposite to TCAS can lead to a collision.
The TU-154 AOM stated that manouvering opposite to the RA is PROHIBITED.

One final thought about the rationality of manouvering visually to avoid that 757(against the RA) based on "engineering facts". Exactly what facts were used to determine that one can successfully visually manouver around this fast moving jet. Is it just assumed that it can be successfuly accomplished or is there actual data on how easily it is done(in daytime, at night, over brightly lit areas, in marginal visibility or with different cloud formations in the background and the illusions they can create with false horizons which I have also seen with northern lights, etc.). If so please publish this engineering data.

Like I said I have no engineering data. But I do have real world knowledge of a midair between two airliners caused by improper evasive action by one of them when none was required due to illusion.

http://www.prop-liners.com/midair.htm

And I know that according to a Flight Safety Foundation(perhaps the best known safety study group) publication, a JAL 747 captain figured the could go against the RA and visually avoid a DC-10. He was right. He missed by 135 meters and only injured 99 people on his plane and lodged a galley cart in the ceiling(following an RA is a one-quarter G manouver by the way).

http://www.flightsafety.org/fsd/fsd_mar04.pdf

And finally our much talked about TU-154 captain who attempted to visually manouver by a 757. Results catastophic.
I eagerly await these "engineering facts".

[PBL]

I fully agree that there are probably technical shortcomings and PBL is quite possibly aware of them. Have never said different
That statement is hard to reconcile with your statement that it is "irresponsible" for me to publish them on this forum.

You list five midair collisions in the US in 11 years to 1978, and then there was one in the next 13 years until TCAS was mandated. It is hard to tell what you want to make of these figures, such as they are. But you do not attempt to account for confounding factors, and that omission is going to invalidate any conclusions you may want to draw.

The most obvious confounding factors are that there were two major revisions of airspace in the U.S. in those thirteen years from 1978 to 1991. The introduction of TCAs and then "Class" airspace had far more effect of separating commercial fast jets from GA traffic than any technical collision avoidance system could have had. If you think the midair collision figures say anything at all, then they say at least that.

PBL

[joernstu]

It has been over 21 years now since the midair collision over L.A. of a DC-9 and a mode-C equipped Piper.

The Piper involved in the Aeromexico 498 collision (31.08.1986) was not equipped with a mode-c capable transponder.

N4891F was equipped with a NARCO Model AT-50A transponder without a mode C altitude encoder.

[joernstu]

From the final report in part:

The TU-154 AOM contains general recommendations regarding the use of TCAS. The AOM reference draws attention to the fact that manouvering opposite to TCAS can lead to a collision.
The TU-154 AOM stated that manouvering opposite to the RA is PROHIBITED.


Please provide me with the exact location of your quote. I searched the report and the only occurence of "AOM" does not nearly state anything you quoted.

The important section (which is diametrically to your "quote") can be found on page 53 in the english report, where the TU154M FOM is quoted (although in translation from russian):



TU154 Flight Operations Manual
[...]
(2) For the avoidance of in-flight collisions is the visual control of the situation in the airspace by the crew and the correct execution of all instructions issued by ATC to be viewed as the most important tool. TCAS is an additional instrument which ensures the timely determination of on-coming traffic, the classification of risk and, if necessary, planning of an advice for a vertical avoidance manoeuvre.

[joernstu]

And I know that according to a Flight Safety Foundation(perhaps the best known safety study group) publication, a JAL 747 captain figured the could go against the RA and visually avoid a DC-10. He was right. He missed by 135 meters and only injured 99 people on his plane and lodged a galley cart in the ceiling(following an RA is a one-quarter G manouver by the way).


To my experience, almost no accident can be traced to one single failure. There are always a large number of occurences, that interactively lead to (near-) disaster. Perhaps you should have a look at Reasons "Swiss Cheese Model". In the JAL-JALincident for example, an important factor was the mix-up of callsigns by ATC.

[punkalouver]

Originally posted by joernstu
The Piper involved in the Aeromexico 498 collision (31.08.1986) was not equipped with a mode-C capable transponder.

This is correct and the statement that the PA-28 had mode-C was an error on my part.

Originally posted by PBL
That statement is hard to reconcile with your statement that it is "irresponsible" for me to publish them on this forum.

I don't disagree with your technical shortcomings that you claim TCAS has(and the publishing of them on this forum). I assume that they are what you say they are. What I disagree with are two conclusions that you and a colleague have come to based on these shortcomings which are probably highly uncommon occurences. You know the two statements I am referring to.

No doubt airspace changes such as TCA's(or should I say class B airspace) and mode-C veils have reduced midair collision potential. Is this what a confounding factor is? While we have seen several fatal midairs over the years(outside the U.S.) such as...

30 July 1971; All Nippon Airways 727-200 vs jet fighter; northern Honshu, Japan, CA-155 dead.
5 March 1973; Iberia DC9-32 vs Convair 880; Nantes, France-68 dead
1979 Aug 11 - Tupolev Tu-134 vs.Tupolev Tu-134 Near Dniprodzerzhynsk, USSR -178 dead
12 November 1996; Saudia 747-100 vs Il-76; near New Delhi, India-349 dead
29 September 2006; Gol Linhas Aereas 737-800 vs ERJ-135; near Peixoto de Azevedo, Brazil-154 dead
Sep 13 1997 - Luftwaffe TU-154 vs USAF C141 over Namibia, Africa-33 dead

...where properly working TCAS in both aircraft(or TCAS in one and working transponder inthe other) would likely have prevented them, there has never been a midair collision event caused when TCAS was installed and properly used. And actually, only one fatal accident when improperly using TCAS. Improper use of many different types of equipment has caused many deaths in aviation. No engineering data on my part just real world knowledge.

Page 103 of the midair accident report states in part:

The TU-154 AOM contains general recommendations regarding the use of TCAS. The AOM reference draws attention to the fact that manouvering opposite to TCAS can lead to a collision.
The TU-154 AOM stated that manouvering opposite to the RA is PROHIBITED.

The actions taken by the Tu-154 captain that night were not rational , they were, to use non-engineering terminology... F:mad:ing Insane.

As for the JAL incident, In the end it was an attempt to visually avoid conflicting traffic against the command of the TCAS RA. The causes leading up to the RA are irrelevant and could very well be due to different errors in each situation. Our discussion is about how to react to an RA.

I still eagerly await any engineering facts on the ability to successfully manouver visually around TCAS conflicting traffic while climbing or descending toward that traffic.

[joernstu]

pukalover, thanks for mixing all posts in this threat as (assumingly) a reply to my remarks. This makes your post unreadable. Also your ambigous use of bold types doesn't aid the readability.

I still eagerly await any engineering facts on the ability to
successfully manouver visually around TCAS conflicting traffic while climbing or descending toward that traffic.

Perhaps you should ask the engineers in you company for these. The notion of avoiding traffic by visual reference was introduced by you, as far as I am aware.

[bsieker]

The notion of avoiding traffic by visual reference was introduced by you, as far as I am aware.

To be fair, it wasn't.

It was introduced by PBL, in post #20:

[...]
They were faced with an "intruder" at 10 o'clock which they saw, and an unknown conflict at 2 o'clock which they didn't see, and for which they had an advisory to descend (that is, he was at or above their altitude).

[...] Or avoid him, descend towards an aircraft that you do see, and hope to avoid him using visual means? (my emphasis)

However, this has nothing to do with "engineering facts", it is part of the argument why the Tupolev crew's actions were rational. A refutation has to offer more than "always follow the RA, because Eurocontrol says so in their marketing brochures, and I believe them, and it is also SOP."

[joernstu]

Page 103 of the midair accident report states in part:

The TU-154 AOM contains general recommendations regarding the use of TCAS. The AOM reference draws attention to the fact that manouvering opposite to TCAS can lead to a collision.
The TU-154 AOM stated that manouvering opposite to the RA is PROHIBITED.
Thanks, now I found it. This part can be found more detailed on pages 64, 65. If you read these, you will see, that the quoted section orders the crew with point 1 to get into contact with ATC prior to following the RA and, yes, it states that it is prohibited to initiate a manoeuvre contrary to the advisory issued by the system.Perhaps a legal loophole, but the TU154M first got the ATC order, than the TCAS RA and initiated the descend before the RA. In so far, the crew's action was not in contradiction with this part of the AOM.

The actions taken by the Tu-154 captain that night were not rational , they were, to use non-engineering terminology... F:mad:ing Insane.So you have a AOM / FOM which in one section states, that ATC has priority over TCAS and that TCAS is only a last-line barrier - and than that you are prohibited from acting contrary to the RA.

And a pilot making a decision based on these contradicting orders is in your opinon not only "not rational" but "[...] insane".:sad:

The causes leading up to the RA are irrelevant and could very well be due to different errors in each situation.In a prior post, you said that you go along with the findings of BFU accident investigators and now you say, that accident investigator's findings are irrelevant.:hmm:

Our discussion is about how to react to an RA.No. Our discussion is on "TCAS philosophies". This includes reactions to RA but is not limited to it. This discussion has also to be on how TCAS operates and where its limits are (althogh this part seems to be of no interest for you).

[punkalouver]

Originally posted by joernstu
In a prior post, you said that you go along with the findings of BFU accident investigators and now you say, that accident investigator's findings are irrelevant.........No. Our discussion is on "TCAS philosophies".

Afraid not. You just are not understanding what I am saying over and over.

1) FOLLOW THE RA
2) The actions of the TU-154 crew was not rational and neither was the JAL captains. They were extemely dangerous decisions.
3) your statement "I cannot decide, what kind of action would induce the lowest risk - following an RA, not following an RA or flying without TCAS at all as the basis for my risk evaluation would be episodes. I think the same goes for you as your information basis is still only episodes." is irresponsible.
My opinions only. I agree with what the Japanese report says. I never said their findings were irrelevant. I am giving you an example of how visual manouvering against the RA around traffic is not necessarily a safe thing to do(whether it is due to a conflict from ATC error, pilot error or in uncontrolled airspace).

Originally posted by joernstu
Perhaps a legal loophole, but the TU154M first got the ATC order, than the TCAS RA and initiated the descend before the RA. In so far, the crew's action was not in contradiction with this part of the AOM. So you have a AOM / FOM which in one section states, that ATC has priority over TCAS and that TCAS is only a last-line barrier - and than that you are prohibited from acting contrary to the RA. And a pilot making a decision based on these contradicting orders is in your opinon not only "not rational" but "[...] insane".

According to the accident report...The AOM reference draws attention to the fact that manouvering opposite to TCAS can lead to a collision.
The TU-154 AOM stated that manouvering opposite to the RA is PROHIBITED.

If you get or have much real world flying experience outside the office, you will find that or already know there are actions that are illegal yet safe to perform and actions that are legal but unsafe to perform(eg. engineering facts that say it is within limitations to land on a contaminated short or crosswind affected runway but you know better). Also actual contradictions of rules and regulations. We can't easily change these rules and procedures. That is when judgement comes into play and knowledge through experience, reading etc allow you to hopefully make a rational decision. As for the insanity statement, I'm sure it crossed the minds of the crews properly following their RA.

Finally concerning visual manouvering against the RA around conflicting traffic...


Originally posted by bsieker
However, this has nothing to do with "engineering facts", it is part of the argument why the Tupolev crew's actions were rational. A refutation has to offer more than "always follow the RA, because Eurocontrol says so in their marketing brochures, and I believe them, and it is also SOP.

After reading this statement and considering that PBL(your coworker) said "The engineering facts which you choose not to like, inter alia that, given the decision presented to the Tu-154 crew, it would have been rational for them to choose to go against the TCAS RA, were discovered, as far as I can tell, by me..."

I am starting to wonder if the so called rational decision to descend and visually manouver around conflicting TCAS RA traffic which is quite possibly attempting to manouver around you is based on an assumption. An assumption that this is a safer action, rather than being based on any study or engineering facts that this really is a safer action. I have no data to prove or disprove this. Only two unsuccesful examples.

[joernstu]

3) your statement "I cannot decide, what kind of action would induce the lowest risk - following an RA, not following an RA or flying without TCAS at all as the basis for my risk evaluation would be episodes. I think the same goes for you as your information basis is still only episodes." is irresponsible.

You have shown over and over and over again, that you draw conclusions without any statistical information. This may well work for you, but in the real world claim have to be supported by evidence.

I am not willing to make any risk assessment without the appropriate data to found the results. If this is irresponsible for you - I can do nothing about it. But I hope that I will never rely on your risk evaluation.

[punkalouver]

Well, that is fine. However, if you knew you had to go on one of two flights, both will get in similar situation as the TU-154(in terms of known and visually acquired RA traffic and ATC reported but unseen visually or on TCAS traffic) at night or day and you knew that one captain would follow the RA and the other would attempt to visually manouver against the RA.....who would you prefer to fly with.

I think that you will find that most pilots will follow their SOP's and Eurocontrol newsletters on how to react to an RA even if there is other traffic out the. This probably happens quite frequently when a conflict arises and there are lots of airplanes out there on the TCAS display some of which have no altitude displayed(usually light aircraft). They have made their risk evaluation.

[joernstu]

The one, where the Boeing's TCAS would have generated a reversal?
The one, which would be flying only under guidance of appropriate staffed ATC?
The one with a defect radio?

If the Ueberlingen case is boiled down to the last seconds before the collision, tweaking any of the variables only a little would have prevented it.

[bsieker]

punkalouver,

I'm starting to wonder if you have a serious problem understanding written words.

We have repeatedly stated, and you have repeatedly misquoted us or simply ignored that:

1) When one choice is "rational" that does neither mean that it is
- the only rational choice
nor
- the best choice
nor
- the safest option

2) Rationality has nothing to do with what hindsight tells you about the danger.

3) It can be argued just as soundly that the DHL crew's action were extremely dangerous. Had they not followed the RA, no collision would have occured.

4) More generally, without TCAS, Ueberlingen would not have been an accident, but merely a later-than-usual separation by ATC. A non-event.

ad 1)
Yet you say we had claimed it was safer to do as they did, instead of following the RA. You also repeatedly ignore that fact, that they received the ATC order, and started manoeuvering according to it, before they received the RA. Meaning they never initiated a manoeuvre opposite the RA.

The interaction between ATC and TCAS is one of the key points of discussion, and this accident, more clearly than anything, shows that the premise of TCAS specifications, that it is only active when ATC is out of the picture, does not hold. It is very serious and must be addressed. Simply ignoring it will not make it go away.

ad 4)
This, too, is a serious matter when discussing what TCAS can do, what it cannot do, what its shortcomings are, and that and how they should be made known. Simply ignoring them will not make them go away.

Repeating your mantra of "Follow the RA" will not make any of the problems go away, either.

Despite what you think and what EC bulletins tell you with pretty pictures, they are not always simply caused by reckless crews acting against RAs.

And in case it got lost again: We do not advise not to follow an RA, even if you seem to think so.


Bernd

[PBL]

Folks,

I had hoped that my explanation of and plea for professional behavior this morning might have changed the substance of the discussion.

Those of us who want to have a technical discussion here have a problem, or rather a series of them. Let me enumerate a few.

First, I told our abusive young friend this morning that if he was a member of a professional society his behavior would contravene the norms of that society. He claimed he wasn't doing that - and then did exactly that again a number of hours later.

First conclusion: he has no clue what to do when a professional clearly states he has a problem with certain exchanges, and why.

Second, our friend doesn't discuss. He contradicts, mimics, repeats himself, but doesn't enter anything any of us would regard as a serious attempt to sort out the issues.

Second conclusion: he doesn't know what a technical discussion is or how to engage in one.

Third, our friend cites data, but doesn't know what a confounding factor is.

Third conclusion: he has never taken an elementary course in data analysis.

Fourth, in response to a request to solve a decision problem,
he repeats "follow the RA!"

Fourth conclusion: he has no idea what a decision problem is, or what a solution to a decision problem looks like.

Fifth, TCAS is a procedural system, based on information and algorithms. The decision problem presented to the human operator by the TCAS information subsystem is an essential part of the TCAS system. Enumerating solutions to that decision problem is part of the technical analysis of TCAS. "Decision problem" here is a technical term with a well-defined meaning. Our friend claims that the possible solution to the decision problem which my analysis has brought to light is "irresponsible".

Fifth conclusion: our friend has little understanding of the analysis of algorithms, and little grasp of the appropiate concepts for analysing them. He is using a perjorative term to describe a technical fact. This is comparable with someone, say, calling the number 5 "perverse".

Sixth, he doesn't argue his case; he simply repeats it.

Sixth conclusion: if we continue to answer as we have been, this discussion will go nowhere.

It is this sixth conclusion to which I wish to draw particular attention.

PBL

[john_tullamarine]

So long as the thread doesn't become abusive etc., it can continue .. those who find it tedious are free to leave the discussion ..

[punkalouver]

It looks like the moderator has spoken just before I posted this. It is important not to let emotion cloud posts or analysis.

Regarding rational....

Originally posted by bsieker
I'm starting to wonder if you have a serious problem understanding written words.

I think we'll have to just agree to disagree on how we interpret this. No need for further discussion on it. Perhaps something to do with mother tongue language.

Originally posted by bsieker
It can be argued just as soundly that the DHL crew's action were extremely dangerous. Had they not followed the RA, no collision would have occured.

Sort of similar to the car that runs a red light at a busy intersection and smashes into traffic going through the green light. If only the cars going through the green light had not done so, there would have been no collision. They were behaving extremely dangerously. Traffic lights are dangerous.

Originally posted by bsieker
You also repeatedly ignore that fact, that they received the ATC order, and started manoeuvering according to it, before they received the RA. Meaning they never initiated a manoeuvre opposite the RA.

The TU-154 AOM stated that manouvering opposite to the RA is PROHIBITED.
That means initiating or while following ATC instruction. Are you sure it is me who has a serious problem understanding words?

Originally posted by bsieker
And in case it got lost again: We do not advise not to follow an RA, even if you seem to think so.

Good. But please be more specific(I know it is just placement of words) butthis is important. Do you advise pilots(excepting stall warnings, GPWS, etc) to ALWAYS FOLLOW THE RA? Please reply.

Finally PBL...

For much of his posts now he just accuses me of being abusive. Once again Mr. Moderator, please let me know if this is so in your opinion and I will apologize. (edited later-the moderator has spoken)

PBL is correct on some things. I have never taken a data analysis course and don't plan to. I don't know what a confounding factor is. I know almost nothing about algorithms but do know how to spell it( and I think they can be used to solve a Rubik's Cube). I'm not even sure what perjorative means. Most pilots that I fly with don't know much about algorithms either. I'll ask around to see if many have taken a data analysis course.

But I do have something that much more important to a pilot than any of these things. I have common sense. One of the most important things a pilot can have. I hope this is not still tedious to you.

I also know, well, I suspect that there is no technical data or engineering facts on the relative safety manouvering visually around TCAS RA traffic at night. Just blind assumptions and we will never see any engineering facts unless it is somehow hastily created. I was accused of not answering a question. How the tables have turned.

So seeing as I agree that this dicussion isn't really going anywhere, why don't we end it. Perhaps a lock on the thread after bsieker lets us know if he would be willing to say "GPWS and stall warnings exluded, we recommend that pilots always follow the RA"

[PBL]

Having decided the thread was moribund, and at the risk of starting another BS Fest, let me nevertheless point people to a worrying incident of which I was not aware, published by TwoOneFour on another thread:

http://www.pprune.org/forums/showpost.php?p=3758275&postcount=3

PBL

[punkalouver]

I have read of previous cases of an aircraft's TCAS giving itself an RA. Same thing, the RA traffic was colocated with the aircraft receiving the RA. Perhaps the equipment in the back, as the person posting suggests, was causing some sort of malfunction. There have been many reports over the years of electronic equipment causing interference with electronic equipment in aircraft especially claims by pilots that a cellphone left on interfered with nav equipment. Worrying indeed, however I would think the pilots still use their nav equipment for navigation and low instrument approaches as they were designed to be used.

The reality is that if the equipment is written up, it will either be released as ground checked serviceable, or perhaps replaced and checked as serviceable and put back in an aircraft.

Sorry if you have found this thread becoming tedious, I hope you have not instructed your two coworkers not to post here. I still would like to know if bsieker recommends pilots always follow an RA. I have given up on expecting any engineering facts on visual avoidance of aircraft but if you care to actually conduct an analysis, I am happy to give you some starting information.

This is from a Canadian accident report which references actual research on the subject.

http://www.tsb.gc.ca/en/reports/air/1995/a95h0008/a95h0008.asp

1.9.9 Pilot Avoidance Techniques
Assuming that a pilot will be capable of visually detecting another aircraft in flight and determining that the closing geometry represents a mid-air threat, then the final stage in the see-and-avoid sequence is to initiate an effective avoidance response. The aim of that response will be to increase the miss distance between the aircraft. The effectiveness of that response is dependent on a number of factors; in general, either pilot can alter the geometry of a collision by changing some combination of aircraft speed, altitude, and heading. Because each of these actions will affect the geometry differently, it is essential that the pilot choose an appropriate combination of actions that will merge to achieve a corresponding effect.<7>
The optimum avoidance response will differ depending on the time to impact. There is research evidence<8> to indicate that, outside 10 seconds to the point of closest approach, the pilot should use compatible manoeuvres combining speed, altitude, and heading change. However, once the aircraft are inside the range of approximately 10 seconds to impact, the pilot should employ an altitude change only. This conclusion is based on an argument that, when two aircraft are confined in close quarters, the essential action is to minimize the relative cross-sectional areas of each aircraft. Under these circumstances, it has been generally found that any application of bank will increase the relative cross- sectional area and thereby increase the probability of impact.<9>

As an example, a Piper Navajo aircraft similar to the one involved in this occurrence will have a vertical cross-section of approximately 13 feet when in level flight. At bank angles in the range of 45 to 60 degrees, the vertical cross-section will be in the range of 28 to 34 feet. The final value of the vertical cross-section will be dependent on the aircraft's wing span and on the applied bank angle.

An Australian study on midair collisions referenced this as well.

http://www.atsb.gov.au/publications/1991/pdf/limit_see_avoid.pdf

3.2 Evasive manoeuvre may increase collision risk
James Harris in his paper "Avoid, the unanalysed partner of see" focuses attention on the ‘avoid’ side of seeing and avoiding (Harris 1983). He stresses that an incorrect evasive manoeuvre may cause rather than prevent a collision. For example, in a head-on encounter, a bank may increase
the risk of a collision. Figure 17 illustrates this. In the top diagram, two (stylised) high-wing aircraft are approaching head-on with wings parallel. There is a limited number of ways in which the aircraft can collide if they maintain a wings-level attitude, and the area in which the
two aircraft can contact or the ‘collision cross-section’ is relatively small. However, if the pilots bank shortly before impact, as in the lower diagram, so that the aircraft approach each other with wings perpendicular, then there is a much larger collision cross section and consequently, a higher probability of a collision. This is not to suggest that banks are always inappropriate
evasive manoeuvres, but that in some cases, evasive action can be unsuccessful or even counterproductive. At least one foreign airline accident has been attributed to an unnecessary evasive manoeuvre (Civil Aeronautics Board 1966).

Yet the parts of James Harris study that I have seen do not even discuss the possibility of an evasive manouver by an aircraft being incorrect due to misjudged trajectories as would appear to have happened in the JAL and DHL cases. Perhaps you have considered this. If so please let us know.

If you need any more assistance, please don't hesitate to ask. I am happy to have been able to provide some information on this subject to those who are reading this thread.

[punkalouver]

Upon closer examination, I have discovered some very interesting information about the supposed rational decision made that night over Germany by the TU-154 captain to descend in accordance with ATC instruction but contrary to the TCAS RA.

According to PBL, Bseiker and Bernd this was a rational decision by the TU-154 captain because ATC had advised mistakenly of traffic at 2 o'clock while the crew saw lights and TCAS traffic at 10 O'clock. Better to descend visually around the known and seen traffic at 10 o'clock than risking a collision with the reported yet unseen traffic at 2 O'clock(which in fact did not exist) according to these three posters from the University at Bielfeld.

This was the statement made by PBL in post # 20...


Please read my analysis of the decision problem presented to the Bakshirian crew at Überlingen. They were faced with an "intruder" at 10 o'clock which they saw, and an unknown conflict at 2 o'clock which they didn't see, and for which they had an advisory to descend (that is, he was at or above their altitude).
What would you do? Climb towards another conflicting aircraft that you don't see but ATC does? Or avoid him, descend towards an aircraft that you do see, and hope to avoid him using visual means?
Please give some good reasons for your answer that will also be good reasons for any other pilot in this situation.

In fact, the decision to ignore the RA was made at least 10 seconds before they were even given this mistaken traffic location by ATC. When this erronous information was given to the Tupolev crew they were already descending at 2,000 feet per minute. So when the decision was made to ignore the RA, in the Tupolev crew's mind there was only one other aircraft out there. See this link(pg 3) as well as final report.

http://www.casa.gov.au/fsa/2004/aug/22-29.pdf


Interestingly, the Tupolev crew in the very last seconds realized the TCAS was correct and did finally follow his RA with control column fully back and thrust levers fully forward. Much too late.
So then the logic comes out that TCAS was at fault because if it wasn't installed the accident would not have happened instead of the logical thought process of...If the equipment was used as designed and regulations followed, this accident would not have happened.
Of course these posters are not responding to several pertinant questions I have asked. This really makes me wonder about the accuracy of all posts and studies done by this group.

[john_tullamarine]

I think we should all have a cup of tea and ten deep breaths ...

[punkalouver]

As an update to the idea that it would at all be rational to ignore an RA at night with a high speed intruder, I have provided a link to the limitations of the see and avoid principle study published by the Austalian accident investigation board. I have quoted a reference to it by the NTSB

http://www.atsb.gov.au/publications/2005/pdf/See_and_Avoid.pdf

The report also indicated that there was considerable data available that was against the reliance on see-and-avoid. Although see-and-avoid was often effective at low closing speeds, it usually failed to avert collisions at higher speeds. It was estimated that see-and-avoid prevents 97 percent of possible collisions at closing speeds of between 101 and 199 knots but only 47 percent when the closing speed is greater than 400 knots. In addition, the human visual system is better at detecting moving targets than stationary targets, yet in most cases, an aircraft on a collision course appears as a stationary target in the pilot's visual field.
An approaching aircraft, in many cases, presents a very small visual angle until a short time before impact. In addition, complex backgrounds such as ground features or clouds hamper the identification of aircraft via a visual effect known as 'contour interaction'. This occurs when background contours interact with the form of the aircraft, producing a less distinct image. The report continued by indicating that even when an approaching aircraft has been sighted, there is no guarantee that evasive action will be successful, as it takes a significant amount of time to recognize and respond to a collision threat.

You may notice that this detailed report has a section on page 22 titled Evasive manoeuvre may increase collision risk that references a crash that I posted about in post #48 of this thread which was later dismissed as a confused post.

I would say that for someone posing as an accident analyst promoting the idea that going against the RA and attempt to visually manouver around conflicting high speed traffic as a rational decision is dangerous. There is no study to back this up theory and as far as I can tell, is based on nothing except perhaps an unwillingness to admit that the original statement was in error.

There has never been a midair collision between two TCAS equipped and operating aircraft where the RA instructions were followed. Yet coordinated RA's occur frequently.

[bsieker]

The IEEE runs a series of blogs associated with Spectrum. Prof. Peter Ladkin (PBL) is a guest blogger on the Risk Factor blog (http://blogs.spectrum.ieee.org/riskfactor) written by Robert Charette.

TCAS has been in the news again recently. In August, David Kaminski-Morrow of Flight International reported on a recommendation by Eurocontrol to implement change proposals CP112E and CP115 ASAP. These changes were approved in Spring 2008 in DO-185B, the RTCA document which defines the TCAS standard.

Aimee Turner of Flight International reported recently that the European Commission, in answer to a question tabled by a Euro-MP, has expressed its intent to mandate the new standard through EASA quickly.

PBL has been asked by Bob Charette to comment on an article from the Wall Street Journal, by Andy Pasztor, about an increase in perceived traffic conflicts in the U.S. and Europe, saying that "safety experts" agree that traffic growth is a major factor.

The first two installments appeared yesterday:
Part I (http://blogs.spectrum.ieee.org/riskfactor/2008/11/the_risk_of_midair_collision_a.html) and Part II (http://blogs.spectrum.ieee.org/riskfactor/2008/11/the_risk_of_midair_collision_a_1.html).

The first part (http://blogs.spectrum.ieee.org/riskfactor/2008/11/the_risk_of_midair_collision_a.html) discusses some issues, and gives links to the Flight International articles on flightglobal.com (http://www.flightglobal.com/). The second part (http://blogs.spectrum.ieee.org/riskfactor/2008/11/the_risk_of_midair_collision_a_1.html) is a short history of the U.S. air safety measures prompted by midair collisions involving commercial transport aircraft, which ended with Congress mandating TCAS installation in the commercial fleet.


Bernd

[punkalouver]

"TCAS sometimes causes problems. A reported airprox (an event in which two aircraft come too close to each other) over Trasadingen in Switzerland in 2000 was triggered through excessive pilot reactions to TCAS advisories (it is an advisory system: any responsive actions are still human), and the report has just come out about an incident on 16 November 2006 over South Korea, in which the pilot's maneuver in response to an advisory was violent enough to injure 20 people on board his aircraft, four of them seriously. (Anecdotes and reports abound of “excessive” reactions.)"


That is not TCAS causing problems, that is pilots causing problems by not following proper procedures just like in the midair over Germany. In that midair and in the excessive manouvering incidents, TCAS worked as advertised.

[bsieker]

punkalouver,

1/ Yes, TCAS was a causal factor, because, by the simple counterfactual test, had TCAS not been installed and working, there would have been no incident.

2/ Yes, there is a problem with TCAS.

So, it worked "as advertised". More precisely, it was found that it worked as specified. (Advertising is quite a different matter ...)

And that tells us that the problem is not so much an implementation problem. For all we know the implementation is adequate with respect to the specification. But, as we've been saying for years, there are problems with the specifications, as they do not (always) serve the intended purpose.

See Ueberlingen, where no reversal RA was given, although both aircraft were still closing. A modification to the specification addressing this specific problem is finally being mandated (CP112E, which, in an earlier form without the "E" (extended) has been on the table since before Ueberlingen).

This is old news. And this is a problem with only the technical part of TCAS. The wider implications of the entire socio-technical system TCAS/Flight Crews/ATC have been discussed at length in this and other threads.


'nuf said.


Bernd

[ATC Watcher]

Absolutely correct Bernd, the only problem I still see is the (until now ) refusal of the FAA to consider mandating the CP change and any new TCAS version for that matter.
Possibly the new US Administration will change some of the FAA top, but even with new people I have my doubts that the attitude will drastically change overnight.

Punkalouver , you've made your points already many times here , we noted them. Why don't you just follow your RAs as the book says and come back here when you have something new to say?

[punkalouver]

Punkalouver , you've made your points already many times here , we noted them. Why don't you just follow your RAs as the book says and come back here when you have something new to say?

I always do follow the RA's and that is why I am able to come back here and say something. Someone didn't and a lot of people died.

As for a TCAS(that worked as advertised) being installed causing the accident, well, that is true, just like if planes had never been invented these perfectly working planes would never have crashed, so it is the planes fault. Hardly an intelligent argument.

If a GPWS gives a valid warning and the pilot overreacts by stalling it is ridiculous to fault the GPWS. Then to say ignore it may be logical because of some remote unlikely scenario is a dangerous statement, as is the similar statements made on this thread by those saying similar about their remote unlikely reason for ignoring a TCAS warning in the German midair several years ago.