TCAS Fail?
Thanks for the chart MM43.
Altitude is fed to the TCAS via the transponder. The transponder looks at only one altitude input at a time. Alternate altitude can be switched in. There is no airspeed input to the TCAS or the transponder.
Altitude out from the ADR is modified by (reportedly) as much as 300 feet as a low airspeed correction. Therefore, you could expect a 300 foot altitude shift if the pitot ram pressure were suddenly blocked while the moisture drain remained open, yielding an airspeed approaching zero.
The BEA assumption is that the TCAS faulted due to internal logic from airspeed approaching zero. The supposed rationale for this logic: "TCAS was inhibited to prevent the potential for mayhem." The only mayhem would be Collision Avoidance being compromised.
If this logic exists, why isn't it in the transponder, instead of in the TCAS? The transponder didn't fail. You don't want a 300 foot error in your altitude reported to other aircraft, just as much as you don't want erroneous evasive maneuvers. The transponder is needed for the TCAS to function. TCAS is not needed for the transponder to function.
Therefore, from the data we have, the BEA explanation for TCAS Fail is mistaken.