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,
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? 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. And yet, the Ueberlingen accident would not have happened, if neither aircraft had been equipped with TCAS. 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. |
Originally Posted by bsieker
And yet, the Ueberlingen accident would not have happened, if neither aircraft had been equipped with TCAS.
Originally Posted by FullWings
Not possible I'm afraid. We all have to have it.
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). Bernd |
Originally Posted by FullWings
Where does the idea of an "unknown conflict" come from?
Originally Posted by FullWings
What does the TCAS have to say about it? Follow the commands to resolve the conflict. Done.
Originally Posted by FullWings
To reverse the question, why would I not follow the RA?
Originally Posted by FullWings
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
PBL |
Originally Posted by FullWings
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.
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. |
Originally Posted by FullWings
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 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 |
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: 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. |
FullWings,
Originally Posted by FullWings
Yes. ... Does the absolute direction of the avoiding manoeuvre matter if it resolves the conflict?
Originally Posted by FullWings
The danger in aviation comes mostly from the aircraft you can't/haven't seen or aren't aware of
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...
Originally Posted by FullWings
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
Originally Posted by FullWings
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
I am much less convinced than I was about your understanding of TCAS scenarios. PBL |
Originally Posted by FullWings
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.
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. 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 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. 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. 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. |
Originally Posted by FullWings
In the event under discussion, primary means of separation had or were shortly going to fail [...]
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 |
In contrast, the manoeuvre in Scenario 2 resolves the conflict with the unseen 2 o'clock traffic. 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... ..you get a climb RA for your visual target at 10 o'clock. 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. |
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! I don't think you're advertising that course of action. But what, then, is your point? Bernd |
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). 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? An open question: Can you point me in the direction of any hard evidence in support of the 'danger' hypothesis? Links, etc.? |
Originally Posted by PBL
In contrast, the manoeuvre in Scenario 2 resolves the conflict with the unseen 2 o'clock traffic.
Originally Posted by FullWings
Because you haven't come close enough to the other traffic to trigger an RA yet.
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...
Originally Posted by PBL
I am not assuming anything at all.
Originally Posted by FullWings
No? Then I'm having problems with the meaning of this sentence:
Originally Posted by PBL
..you get a climb RA for your visual target at 10 o'clock.
Originally Posted by FullWings
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.
Originally Posted by PBL
Or it is a military aircraft, or a NORDO, or an older bizjet, or a Legacy whose pilots just kicked off the transponder ...
Originally Posted by FullWings
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?
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 |
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. 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,
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 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. 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 |
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. 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. 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.) Off flying across the Atlantic now with my trusty TCAS, EGPWS and sextant. |
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