Originally Posted by
Takwis
Quite a few votes for "It's an unreliable airspeed problem." Also quite a few votes for "It's a stab trim runaway problem." I haven't kept track of which has more votes, but I will note that at one time or another, Boeing has made both of those statements.
Until the flaps were retracted, the AOA failure presents as an "Unreliable Airspeed" malfunction. Both Lion Air flight crews recognized the airspeed issue right off, while it appears the Ethiopian crew did not. The Unreliable Airspeed NNC basically calls for the flying pilot to turn off any automation and fly pitch and power settings until such time as reliable instruments can be identified.
Once the flaps are retracted, the AOA malfunction triggers MCAS which then looks like a Runaway Stab Trim problem. As previously discussed, the aircraft was controllable if the flying pilot was using aggressive electric trim inputs (first Lion Air flight Captain and First Officer, Captain of the second Lion Air flight) and not so much if the primary response was with elevator (second Lion Air First Officer, Ethiopian Captain). Setting aside the issue of the delay or inability to connect the malfunction with the Runaway Stab Trim NNC, an open question is why three of the flying pilots were able to maintain aircraft control and two were not. This outcome is possibly related to training issues, automation policy (comfort with hand-flying, etc.).
BTW, reference some previous comments about stopping the moving trim wheel manually, 1) not really a problem given proper technique, and 2) I know of no historical case where this actually became necessary.