PBL,

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 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

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.

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.

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.

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.

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.

Sorry, not quite there yet :}

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,
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

@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.

The 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?

PBL,

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. :)

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,

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

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:

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?

Oh dear oh dear !
hedgehog-in-fog:
I sincerely hope that the date on top of this FCOM page is before 2002 ..
CDN_ATC,:
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.

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

2006
Sorry
:ugh:

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/...?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.

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:
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/publ...rts/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

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

I think they would have missed.

PBL

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).

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

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.

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.

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

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.

Yes.

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.

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.

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

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

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

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.

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.

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.

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.

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.

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

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

PBL,

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 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,

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?

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.

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.

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

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

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

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

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

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.

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)
(emphasis added).

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

PBL,

Replying to your latest post first:

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.

FullWings,

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

I am not assuming anything at all.
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.

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

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.

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

joernstu,

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.

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.

And these cases form what percentage of the threat population in an ATC radar environment?

At least we agree on something. :)

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

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...

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

As opposed to: "...you get a climb 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'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...)

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?

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

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...

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.?

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

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

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.

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

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... ;)

FullWings,

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 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

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:

and

I think that covers the conflict case?

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