TCAS philosophies

[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]

Originally Posted by punkalouver

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]

Originally Posted by punkalouver

[...] 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 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]

Originally Posted by blueplume

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]

Originally Posted by Intruder

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. 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?