The obvious question to ask here is "how often might this sort of scenario occur".
When we instituted our TCAS training program this scenario was one that seemed in need of consideration.
My thoughts were that TA only was the way to go. After all, if you do that you are (at worst) no worse off than in the pre TCAS era, versus a scenario compelling you to manouevre opposite. General strategy here is that when a scenario occurs that prevents full manouverability, TCAS should go to TA only. The aircraft we flew at that time had "TA only" in several abnormal checklists, e.g. flying control malfunctions.
TA only mode is effectively a 'priority' button (as long as, as others have pointed out, your general operating environment is one of TCAS use by other traffic). I could talk more about this, but it would be mostly repeating others observations so I'll get back to the case in point.
I really felt that we should be pre empting this sort of scenario. In '99, as part of an attempt to quantify the problem I got in touch with the CAA and asked them to do a MOR database search and send me a list, for the previous year, of MORs including the terms "emergency descent" and "RA" or "TA".
We were hoping we might get hit or two and see how things had panned out in real life. It was therefore somewhat sobering to receive back a list of several dozen!
(cautionary note - this was u6.04 days, before u7 's improved horizontal miss distance filtering)
So folks, the bottom line is these scenarios are happening! For real!
- TCAS has been mandated in the UK for what, 6 years now?
- These scenarios are entirely likely and predictable.
- Lack of a clear policy is, frankly, negligent.
If your company doesn't have a policy on this, it should do. Urge your training departments / standards folks / flight safety team to make it happen!
BTW I am available to consult for a very reasonable fee
The former pilot known as Pitbull