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TCAS RA in a hold

Not quite a GA topic, but hopefully interesting. The other day, a question came up in a random chat: 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.

LKBU (near Prague), Czech Republic

You follow the TCAS RA. “Callsign, TCAS RA”. Thereafter “Callsign, clear of conflict, climbing FL/ALT”.

always learning
LO__, Austria

NATS will show you a very interesting video or two, showing what happens when somebody gets an RA in a holding stack

The pilots take matters into their own hands and get out of the holding stack laterally.

Administrator
Shoreham EGKA, United Kingdom

Snoopy wrote:

You follow the TCAS RA. “Callsign, TCAS RA”. Thereafter “Callsign, clear of conflict, climbing FL/ALT”.

This is the only answer for any operator I have ever worked for.

United Kingdom

@Peter, do you mean you’ve really seen it?

LKBU (near Prague), Czech Republic

Yes, at a presentation in Swanwick, c. 10 years ago. The holding stack disintegrated, with planes doing orbits off to the outside of it. At Heathrow, IIRC. The radar display was shown.

Administrator
Shoreham EGKA, United Kingdom

Peter wrote:

Yes, at a presentation in Swanwick, c. 10 years ago. The holding stack disintegrated, with planes doing orbits off to the outside of it. At Heathrow, IIRC. The radar display was shown.

As aptly mentioned on another forum, one thing has improved since then: starting from the version 7.1, TCAS II has a Level off, level off advisory, which would provide for 500 ft vertical separation and thus largely prevent a chain reaction of RAs along the holding stack. It may not prevent the pilots from fleeing the stack, but will at least make the whole situation less messy.

LKBU (near Prague), Czech Republic

I don’t have time to scan the internet for other forums, generally

I understand that SOPs are SOPs and you risk your job if you don’t follow them, but it would take balls of steel to follow the SOP when you know those above and below you are 3rd world airlines, right on the edge of their ELP perhaps, and perhaps not exactly the best trained button-pushers who wrote the answers in the 14 ATPL exams on the back of the Jeppesen chart and applied that philosophy throughout their subsequent career

Especially if your traffic display shows somebody actually getting really close. If I see traffic on my TAS heading for me, traffic which doesn’t exist officially, I am not gonna just sit there and watch it.

Administrator
Shoreham EGKA, United Kingdom

That’s the whole point of TCAS. You don’t react / trust humans, you strictly follow the TCAS‘ logic.
The worst you can do is being the 1 of a 2 party TCAS arrangement who comes up with another course of action.

always learning
LO__, Austria

Snoopy wrote:

That’s the whole point of TCAS. You don’t react / trust humans, you strictly follow the TCAS‘ logic.
The worst you can do is being the 1 of a 2 party TCAS arrangement who comes up with another course of action.

That’s fine so long as you trust other pilot(s) involved to do the same. I think that @Peter’s point is that, essentially, that may not be the case.

I don’t think one can argue for exceptionalism for TCAS. It’s just another system for keeping aircraft apart (like ATC) and there are feasible scenarios where, principally because of the failings of others, it might not keep you alive.

Personally I can totally understand why pilots would flee the hold laterally. In this scenario you have one job and that’s to avoid a collision – so it’s better to able to argue the toss afterwards. Of course mine were not airliner/TCAS situations, but I have been in one or two difficult situations with close-proximity traffic doing unexpected things and my philosophy (and the action I took) is to get the hell out of Dodge – I simply fly away from the problem into an area that I can see to be clear. I don’t care whether in doing so I’m breaking a rule, protocol or even a law – I will simply be glad that I’m alive to argue about it afterwards!

EGLM & EGTN
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