alf5071h,
It is extremely hard, to quantify the probabilities of very rare events.
But here is the reason why I think that it is not too far-fetched, and is a realistic, if rare, scenario to be considered when advocating "Always follow TCAS!".
It is essentially the same scenario as happened at Ueberlingen, the only difference being that at Ueberlingen, the Tupolev did get an RA, but chose to follow ATC instead. And that scenario did result in a collision. In that case, too, the Boeing's TCAS would have monitored the Tuploev's course, and it issued an increase-descend RA.
It has been debated why the Boeing's TCAS did not issue a reversal RA ("CLIMB NOW! CLIMB!") in this instance, as would seem appropriate.
While it may be true that this particular accident could have been avoided, if both pilots had followed their respective TCAS RAs, the Bashkirian crew's action were rational in light of the perceived three-aircraft-conflict.
But let's look at the decisions of the individuals involved in the modified setup:
- TCAS-aircraft crew:
Get TCAS RA (possibly after a TA)
SOP: follow ATC, except when getting in RA, in which case follow RA
Rational choices available: follow RA
Rational choice taken: follow RA.
- non-TCAS-aircraft crew:
Get ATC instruction
SOP: follow ATC, except when getting in RA, in which case follow RA
Rational choices available: follow ATC
Rational choice taken: follow ATC (in the absence of an RA)
- ATC:
notice conflicting traffic at the same FL
SOP: separate traffic
Rational choices available: issue climb or descend instructions to either of the involved aircraft
Rational choice taken: issue descend order to non-TCAS aircraft
So, ATC could take any of four different choices, but without knowing the circumstances, neither is favoured over the others (perhaps descend instructions are favoured, because it is unclear if the aircraft can climb quickly enough, or if it is even at its operational ceiling). Other factours may make the descend instruction to the non-TCAS aircraft the preferred one.
Are there data availabel on the availability of TCAS? What, thus, are the chances of two aircraft with inop TCAS "meet" each other?
Bernd