Originally posted by Graybeard ...
TCAS uses only altitude difference, distance, and rate of closure.
Not disputing any of that, other than in the case of this TCAS, it had a self checking routine that performed a "credibility" test.
From BEA Report #2:
This message indicates that the TCAS is inoperative. Without an associated fault message, it could be the consequence of an electrical power supply problem or of an external failure. Amongst the possible external failures, only one is compatible with the CFR received. This is a monitoring process internal to the TCAS which applies to the standard altitude parameter. The latter is received from the active transponder (it can thus be the altitude elaborated from ADR 1 or 2) and is submitted to a “credibility” test. In actual fact the TCAS elaborates an altitude prediction that it compares permanently with the altitude received. When these two parameters move too far apart, it stops operating and generates this ECAM message. Once the altitude becomes “credible” again, normal operation resumes and the message disappears.
We don't know if normal operation ever resumed. I doubt it.