"I dont think TCAS can work off of Mode A only, it requires Mode C replies to identify if the aircraft is actually a threat. The Mode S transponder on the TCAS equipped aircraft will interrogate non-TCAS equipped aircraft with Mode C only all-call."
Not true; TCAS certainly does work against Mode A only aircraft! Non-mode C aircaft can be a bit of a headache because TCAS assumes they are at the same level and gives a "TRAFFIC, TRAFFIC!
" advisory.
Then it turns out it was possibly 2700' below, for example. Very worrying if you can't see him at all, which often happens - and as we all know, it's the one you don't see that hits you - which tends to concentrate the mind more than a little
