Maybe it's time to shed some topics, such as VS and TCAS, into separate threads. That said, I'll continue here.
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Thanks to mm43 and HN39 for your responses on 8 April.
As a point of information, communication between avionics boxes is via a one way tertiary 32 bit data bus. Included in every word is a parity bit, and three validity states: Normal, No Computed Data, and Fail Warn.
From the report:
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.
This is illogical. Why would the TCAS have a credibility filter on altitude data from the transponder? It would be just as important for the transponder to have the same credibility filter, as it transmits own ship altitude to ATC and other aircraft. If there were such a filter, it should be in the ADIRU, or even the Air Data Module, whichever unit contains the correction factor for airspeed.
Again, when the ADM or ADIRU detects a fault, Fail Warn is transmitted on the data bus to the transponder. The transponder reverts to responding with Mode C, no altitude, which it also relays to the TCAS. The TCAS then stops, and annunciates OFF to the pilots. This is not a TCAS Fail.
Think about it: ACARS maintenance messages are transmitted to the destination. You don't want the mechanic to change a TCAS processor when it's a source that's faulted.
I'm sure BEA is mistaken. There was another reason for the TCAS Fail.
There is a use for an altitude credibility filter in older aircraft which have only Gilham code air data source. Gillham code is a dozen discrete wires, with no parity or validity bits. This credibility filter, if it existed, would logically be in the Mode S transponder, not the TCAS. Early in TCAS history, a KAL 747-1 with Gillham code source, was reporting 7,000 feet when they were at 10,000, and had a near collision with a JAL 747.
GB
My credibility? I was heavily involved with Mode S transponder and TCAS certifications at OEMs and airlines for over a dozen years. I had access to the avionics OEM specialists.