Originally Posted by Graybeard
Airspeed and altitude are separate and unique functions within the ADR, except at low speeds, and their output data bus words are separate and unique. An ADR may flag or put out erroneous airspeed without affecting altitude output.
Takata replied:
Each ADIRU module is separated in two: ADR+IR. If you are turning OFF the faulty ADR part because of unreliable airspeed, you will also reject all the associated static and AoA probes. Hence, if all three are rejected, you are only left with your standby instruments.
Do you really know the fault logic of the ADIRU, Takata? You're saying that faulty airspeed is turns off all air data and AOA outputs from the ADR? I think you have confused ACARS report with reality. The ACARS report is to aid the tech at destination in troubleshooting to the level of the offending LRU, Line Replaceable Unit.
Arinc 700 design was finalized in the late 1970s, and first adopted on a whole airframe in the 767. That design mandates multiple sensors; simple sensors if you will. The Sensors, such as LRRA, ILS receiver, ADR, IRU, etc., put out data words with Sign Status Matrix of Normal, or No Computed Data, or Fail Warn. Filtering and judgements within the sensors are not as extensive as they could be. It is up to the user of this data, FD, AP, display, to filter and compare the data from multiple sensors.
LRUs often contain unrelated functions to minimize the number of LRUs and associated wiring. The VOR receiver, for example, also contains the Marker Beacon receiver, which was a separate LRU prior to Arinc 700.
The LRU is required to put out valid data for each function when it is available. The ADR shutting off its altitude and AOA outputs when it suspects its airspeed input is faulty, is nonsensical, and would be unsafe. AOA is needed more than ever when the airspeed is faulty.
BEA stated that the TCAS Fail on 447 was due to its internal monitoring of altitude. That altitude comes from the transponder. BEA doesn't explain why there wasn't also ATC Fail. There should have been, if the altitude it received from the ADR was faulty.
Yes, my knowledge and experience suggests BEA was mistaken on its assessment of the TCAS Fail.