# 2225
The fourth item could also be challenging;
It can be assumed that MCAS is now well protected from a single AoA failure, but the coincident effects on other systems - ADC, speed corrections, Stick Shake, EFIS low speed awareness, are unacceptable during takeoff given the likely failure rate of the AoA input (accident history / investigation).
Why did the AoA fail at or before takeoff in the two accidents and not later in the flights?
A failure in a dual AoA system can be detected by comparison, and the disagreement alerted, but a correct value cannot be identified, which other systems could switch to. Hence rumoured requirement for a triple AoA system - or a much higher reliability sensor.
Similarly for flight without MCAS, which could be acceptable for rare occurrences, but if the AoA reliability was low then this too could be unacceptable for certification.