Originally Posted by
Interflug
As far as I understand logic, a 3rd AoA would have allowed the system to run a software algorithm that votes an erroneous AoA sensor input out against the other two. That would have solved EVERYTHING in those two crashes, no?
XL Airways Germany Flight 888T, quote below is from the wiki page:
The aircraft's computers received conflicting information from the three angle of attack sensors. The aircraft computer system’s programming logic had been designed to reject one sensor value if it deviated significantly from the other two sensor values. In this specific case, this programming logic led to the rejection of the correct value from the one operative angle of attack sensor, and to the acceptance of the two consistent, but wrong, values from the two inoperative angle of attack sensors.
Triplexing/Voting works on the assumption that a single failure is unlikely, and a failure that affects two parts simultaneously is therefore extremely unlikely. It does not take into account a single root cause failure (as in the XL airways incident) that affects two parts simultaneously.
As an example, on the Space shuttle, there were four identical computers which voted against each other in the case of discrepancy. However, there was a 5th computer (limited to ascent and reentry only) which was different hardware and different software in the event of something which had the same root cause in the software / hardware.