And I never saw anything regarding selecting TA when limited on performance. When should we consider that situation? FCOM you said??? never read anything about that. Can you please tell us the chapter?
Some situations require TA only. In some cases, for parallel runway ops. Also for Engine Out or certain other malfunctions. There are solid reasons behind these situations.
However, due to this issue (Climb RA at ceiling) it seems some operators have come up with the concept of TA only during high altitude cruise. This is (imho) a deeply stupid idea that could only be thought up by someone that doesn't understand how TCAS works and is incapable of assessing the relative risk.
In coordinated encounters, the outcome is still highly likely to be safe even if one aircraft does not (is unable) to follow its RA. In a high altitude encounter, if you are receiving a climb then the other aircraft is getting a descend (or an 'adjust' to reduce climb). So although performance is an issue for you, it isn't for the other aircraft. The size of the RA and the alerting time is sufficient that compliance by
either aircraft is sufficient. (contrary to common opinion, not following an RA does not lead to a large increase in risk, as long as you don't manouevre in the opposite sense to it).
But if you go TA only, and every other aircraft is also at TA only, then you'd better hope that you don't meet another aircraft with the same ceiling. In this case you'll both be at TA only!
If you're at TA only and the other aircraft is at TA/RA, what happens if the other aircraft gets a descend RA, but ATC are giving you a descent for avoiding action* ? You'll follow it, which invalidate the other aircrafts RA. Yes, the other TCAS can strengthen or reverse its RA, so the situation may be salvaged, but clearly this is a riskier situation.
(* or you decide to descend based on visual contact)
So what's different about this and TA ONLY at engine failure? The rationale
sounds similar - we can't climb, so set TA only.
The issue during engine failure is the low level possibility where descent RAs are inhibited by radio altitude. In this case BOTH aircraft involved in the encounter are inhibited in one sense (down). In these situations, in a
coordinated encounter, one aircraft will get a Climb RA and the other, instead of getting a descend, will get a preventative to NOT climb. But if that aircraft can not climb, due to engine failure, then a collision is possible.
Whereas, if the engine out aircraft is at TA only, the encounter is
not coordinated. This means the serviceable aircraft will definitely generate a Climb RA and the encounter should be safe.
So, really, these situations are quite different. With engine out, we are worried about the aircraft being unable to manouevre in
either sense, so we set TA only. At high altitude, we only have a problem in
one sense, so all aircraft should stay ay TA/RA.
pb