James Robinson writes:

TCAS only works with both transponders working. One was not working.

Are you sure? Even a TCAS-II system installed on a machine with an
inoperative transponder should still provide traffic alerting, no?

[...] "A Mode S transponder is required to be installed and
operational for TCAS II to be operational. If the Mode S transponder
fails, the TCAS Performance Monitor will detect this failure and
automatically place TCAS into Standby." [...]

How unfortunate that it can't just downgrade to TCAS-I or even plain
traffic alerting. No wonder big planes have multiple transponders.


- FChE