Originally Posted by A33Zab
IF ONE ADR IS CORRECT BUT THE OTHER TWO ADRs PROVIDE THE SAME ERRONEOUS OUTPUT OR IF ALL THREE ADRs PROVIDE CONSISTENT AND ERRONEOUS DATA:
The System will reject the 'good' ADR and will continue to operate using the two 'bad' ADRs. This situation can be met when. for example, two or three pitot tubes are obstructed at the same time, by the same amount, and in the same way. (Flight through cloud of volcanic ash, takeoff with two pitots obstructed by foreign matter (mud, insects)).
Human beings (the pilot) tend to use the same type of 'fault accommodation' principles to detect an erroneous IAS/altitude indication. Flight crews will tend to reject the outlier information, if the other two outputs are consistent. This choice is, in the majority of cases, correct; but, all flight crews should be aware of very extreme and unlikely situations where two (or even three) speed/altitude indications can be consistent and wrong.
A lightening strike at the right side of the cockpit could damage both FO and standby pitots as they are mounted close to each other.
Then we could easily have a situation as described above where the system is rejecting the 'good' ADR.