Originally Posted by
robinpiper
I fly quite a few aircraft all of the same type but with different transponders. How can tell what mode type they are? They dont have written on the front of them this is a mode X type transponder. Most are the simple ones you just dial up the code and switch to on. Can you be seen by TCAS equiped traffic with these types?
The practical advice that you have been given above is good. There's a technical aspect that I'll point out and you can subsequently ignore.
SSR interrogations from ground stations come in two Modes, A and C. Mode A says "Send back your 4-digit squawk" and Mode C says "Send back your altitude". Virtually all modern transponders reply to
both modes of interrogation. Those without an altitude encoder attached send a null reply to Mode C interrogations.
TCAS sends only Mode C interrogations. It will, however, see replies from transponders without altitude encoding, it just won't know what level they're at, because the response is null.
Thus although transponders without altitude encoding are commonly called "Mode-A transponders", at least by pilots, most of them do in fact respond to Mode C.