In article , Ron
Natalie wrote:

Can TCAS issue an RA with only a mode A return from the other aircraft?

No, it can't. It can and will issue a Traffic Advisory if the target
is a possible threat, but since all an RA can do is guide you
vertically, it's not possible for it to issue an RA without altitude
information on the other aircraft.

--
Garner R. Miller
ATP/CFII/MEI
Manchester, CT =USA=