Originally Posted by
GordonR_Cape
I agree that such data would be very useful, but suggest two reasons why it has not been forthcoming:
1. If AOA fails nose down (50% odds), there will be no stick shaker or stall warning or MCAS activation, and it would probably be a non event, written up for maintenance, or reported to the NASA hotline. Conversely if AOA fails nose up on the 'non-MCAS' side (25% odds), there will be stick shaker activation, but it will not be flight critical, and the fault will be repaired. In neither case will the airframe be examined in any detail.
2. In the event of AOA failing nose up and MCAS activating (25% odds), there is an overwhelming fatality rate. There are no known survivors other than the preceding Lion air flight, and unfortunately that airframe was destroyed the very next day, so there is little evidence to strip down and examine (apart from the FDR).
Does this provide some context on the relative scarcity of such occurrences?
As was stated before the AoA vanes are not new equipment and are the same on all the NGs and probably the Classic. There are vast numbers of these flying and as you state they would be just maintenance issues, but all those issues go into analytics and Boeing would have known their failure rate on designing the Max. In knowledge of that failure rate they felt failures were improbable to highly improbable. They also felt that a crew given significant uncommanded nose down trim would immediately trim back nose up and a second uncommanded nose down would result in the crew trimming back nose up then switching off the Stab Trim. It was certainly not expected that a crew would attempt to fly with significant pull force and _not_ trim back. As someone upthread stated trimming is second nature. So add to your probabilities extremely rare failure of AoA vane and Full expectation that 99.9% of PF would trim back to in trim if MCAS did operate incorrectly giving significant uncommanded nose down trim and after 2 or at worst 3 trim back would switch off Stab Trim.