Originally Posted by
Graybeard
The early failures reported by AF447 ACARS all seem to be explainable by iced pitot tubes, save for the TCAS Fail. TCAS does not use airspeed, just altitude via the transponder.
Air data Altitude failure will cause the transponder to revert to Mode A. I'm not sure if it would report failure of its altitude input to the CMS/ACARS, but it didn't report a Fault.
Transponder reversion to Mode A will not cause a TCAS Fail condition, but a TCAS OFF condition.
Therefore, we have a TCAS Fault that occurred nearly simultaneously with the airspeed sourced faults, but unrelated.
Question: does the entire ADR section of an ADIRU "fail" as one unit? That is, having detected that it is producing "unreliable" airspeed data (through the ADR DISAGREE method) is the
entire ADR output regarded as suspect by other systems?
If so then you don't need any fault on the altitude to trigger the TCAS message - the airspeed problems cause the other aircraft systems - including TCAS - to regard all ADR info from the identified "unreliable" ADIRU(s) as suspect.
From a quick scan of such AB docs as are available easily online, I can only find references that say things like:
Each ADIRU is divided in two parts, either of witch [sic] can work separately in case of failure of the other.
Which would tend to suggest that the whole of the ADR part is considered to fail as a unit.
I can see logic for either approach - once a source of data is suspect, you may wish to disregard it entirely, but there's also an argument for trying to use what may still be valid data streams from a partial;ly invalid source. Hopefully someone know which applies.