For my two penn'orth, some TCAS facts:
It doesn't provide a response with closure rates >10,000fpm and/or >1200kts - software limit. i.e. it won't 'see' traffic doing that.
Different kinds of RA's are inhibited at 'low level', of the order of 1400ft and below (rad alt), e.g. RA 'Descend' inhibited below 1100ft
TCAS broadly uses time to 'collision' (tau) as the measure of when to alert (TA or RA). The tau limits change with altitude - gets bigger - to compensate for the IAS/TAS relationship I think. A head to head RA is the same time to collision with a Typhoon at 800kts as it is with another airliner at 250kts. I recommend using your favourite search engine to find the FAA document 'Introduction to TCAS II, Version 7.1'. Especially if you're having trouble sleeping - actually has a few useful nuggets of info.
There may be some conflation with TCAS (the only current implementation of the Airborne Collision Avoidance System (ACAS) standard) and a Traffic Advisory System (TAS). TCAS tells you how to get out of the way (in pitch only) whereas a TAS just tells you where the other traffic is (+/- angle of arrival errors). Studies have indicated that cued lookout is much more effective than a '360' scan - no surprises there.
The point I'm trying to make is that the blanket use of the term 'TCAS' indicates a system that provides an RA that (in the civvy world) the pilot is obliged to follow. That may well not be the case in the military environment, where the system could easily be used in the TAS sense to cue lookout. It's not really important what it's called, just how it's used, but it is important not to get wrapped around the axle due to misconceptions caused by calling something TCAS when it may not be used in the same way or indeed be the same thing.
Also, pilots may be reluctant to use it but while MAC remains the number one risk the relevant AOC will no doubt be keen to explain why such reluctance may be misplaced ...
