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TCAS RA in a hold

Old 20th Sep 2021, 12:32
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TCAS RA in a hold

During a random chat the other day, an interesting question came up: suppose you are in a holding area within TMA, with a few aircraft below and above you, and TCAS suddenly says "Descend descend". What shall you do? A TCAS RA in such a situation is fairly likely to be irrelevant - for example, this Eurocontrol study, albeit old, analysed 35 RAs in a hold and found 6 useful ones, 19 nuisance ones (i.e. dictated by TCAS logic but operationally unwarranted), 3 false ones, 4 RAs replicating ATC instructions already issued and 3 with insufficient data. On the other hand, a sudden descent with a stack of aircraft below you can cause more danger than it mitigates.
My idea would be to descend ~200 ft (so as not to trigger alarms below me), level out and transmit: Pan-pan pan-pan pan-pan all stations, this is XX-XXX holding at YYY FL ZZ, getting TCAS RA to descend. All aircraft above FL ZZ over YYY check your altitude, then stand by to descend further or return to the assigned level. A critical discussion is welcome.
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Old 20th Sep 2021, 12:55
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This is a very thin ice, only those equipped with thorough understanding what TCAS actually does and is capable of should entertain the thought. Also what the TCAS cannot do.

For the rest, follow the RA. Use standard phraseology. The knowledgeable ones would do exactly that.

With 25 ft S mode reporting the pop-up RAs are pretty much impossible. Daring to play a hero, not reacting is a scenario the TCAS can work with quite safely.
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Old 20th Sep 2021, 15:06
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Let the autopilot do its work in following the RA. That is what it is there for. Yes, it could lead to the RA rippling through the whole hold, but we have all been trained to follow RAs to the letter for a reason, in effect an RA can only happen when both crews and ATC have not done their job correctly.
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Old 20th Sep 2021, 16:10
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That seems like a very long radio transmission when you’re maybe in a situation where seconds count.
Situational awareness is important in a hold and in a multi crew at least one should have the NAV(traffic) display in a useful scale.
Consider aircraft not only vertically but also laterally staggered in a hold.
As in the other aircraft is not exactly above or below you and if the stack is not too high everybody is flying the same speed.
If they’re all jet aircraft that is and I’m assuming that as everyone has TCAS.
If we fly 10 mile legs we could have somebody diving through our altitude with (at best) 10 miles separation horizontally and worst case we’ll never know.
In that sense the question is a little academic.
So my answer is descend in accordance with TCAS RA as I know where the traffic below me is.
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Old 20th Sep 2021, 16:23
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With 25 ft S mode reporting the pop-up RAs are pretty much impossible.
in effect an RA can only happen when both crews and ATC have not done their job correctly.
If they’re all jet aircraft that is and I’m assuming that as everyone has TCAS.
If we fly 10 mile legs we could have somebody diving through our altitude with (at best) 10 miles separation horizontally and worst case we’ll never know.
In that sense the question is a little academic.
The original discussion arose from an opposite premise, in connection with an incident where onboard equipment failure in one aircraft triggered RAs in several others. I was told it occurred at EDDM some 10-12 years ago, but I couldn't find any reports.
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Old 20th Sep 2021, 19:20
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A correctly flown RA will create 500ft separation from the intruder, the idea being it doesn’t create an RA for the next aircraft. This of course assumes both started in level flight, if an intruder aircraft climbs or descends through the hold it will inevitably cause a cascade.

That’s why in a manually flown RA the training is to stay just outside the red, and in TCAS 7.1 version two introduced the “level off, level off” command. Auto TCAS will do the same. If you went massively into the green on a manual RA you stand a good chance of causing problems for someone else.
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Old 22nd Sep 2021, 21:33
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Ultranomad your idea looks like a solution in search of a problem. You mention a single incident a decade ago which you cannot track down. Not really a concern. On the other hand, 40% of TCAS RAs are not followed properly. TCAS/ACAS is now a mature technology and a massive amount of feedback from the real world has gone into the latest version. The only common problem left is crews that cannot or will not follow the RA, putting both aircraft at risk. Instead of creating an unsafe homemade procedure and then inventing dubious strategies to mitigate the risk, follow the RA. In the vanishingly unlikely event that it turns out to be from equipment failure it will be short lived and no harm done. If it causes an RA with other nearby traffic, they will hopefully also follow their RAs and no harm done.

Safe Handling of TCAS Alerts | Safety First (airbus.com)
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