Originally Posted by
Graybeard
Mad(flt)Scientist, "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?"
The communication from the Air Data (Module, Computer) to the transponder, for example, is on an Arinc 429 serial digital data bus. There are several words transmitted in sequence, each with its own identifier and validity state. CAS, IAS, TAS, SAT, Pressure Altitude, Baro Altitude, Vertical Speed, etc. will be on the bus.
The transponder will pick off Pressure Altitude, and ignore the rest. The ADC transmits all words repeatedly. If the airspeed goes invalid, for example, Fail Warn will be indicated in the Sign Status Matrix of that word.
Therefore, an iced pitot will have no effect on the transponder or any other device that uses Altitude, nor on the TCAS, which is even farther removed.
GB
OK, but in this case is it not the FMGEC that's calling the ADR DISAGREE, not the ADIRUs themselves? After all, an iced pitot may present no useful symptoms to the individual ADM or ADIRU that can be used to detect the possible problem. It's only at a higher, integrated, level where the comparisons can be carried out between the different ADIRUs and a problem possibly identified. In this case the ADIRUs may all have been claiming their data was valid, since no system failure had occurred. (And as I understand it there's no ADIRU failure on the ACARS list - which implies the boxes themselves thought they were ok; it was the customer systems that identified the problem.
If the FMGEC (or whatever its called?) is spitting out an ADR DISAGREE, is that going to then cause either the ADR parts to consider themselves potentially invalid, or other systems to do so? (The FMGEC in effect is "policing" the ADR data in that model)