TCAS philosophies

[ATC Watcher]

The " value of X "

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]

Originally Posted by 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?

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.

Originally Posted by airmen

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]

Originally Posted by ATC Watcher

" 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]

Originally Posted by Capt Pit Bull

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]

Originally Posted by 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.

Originally Posted by Andu

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]

PBL

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]

Originally Posted by GlueBall

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.

Originally Posted by GlueBall

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]

PBL wrote:

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.

Originally Posted by oceancrosser

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]

Originally Posted by 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?

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.

Originally Posted by FullWings

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?

Originally Posted by FullWings

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.