TCAS indication for Mode A traffic

[Specaircrew]

Originally Posted by swh

Please see page 5 of this https://www.faa.gov/documentlibrary/...%20booklet.pdf

TCAS 1&2 interrogates other transponders the same way SSR does.

Yes but Page 17 of that document states: ' Since TCAS does not use Mode A interrogations, the Mode A transponder codes of nearby aircraft are not known to TCAS.' The next paragraph goes on to explain how Mode A/C (Not Mode A only) transponders with inoperative altitude encoders are tracked by using the blank altitude data field framing pulses.

My understanding is that a basic (cheap) Mode A transponder will not be seen by TACAS II but a Mode A/C transponder that doesn't have a height encoding altimeter will be seen.