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.
Still, even with the aircraft severely out of trim, with unreliable airspeed and altitude information, the airspeed around VMO, stick shaker and other distractions, the pilots managed too keep the aircraft in the air, fly it, and even gain significant altitude. The preliminary report for the Ethiopian flight mentioned reaching 13400 feet, so more than 5000 feet AGL.
At this point they decided to re-enable electric trim and they made two ANU adjustments. Until today I assumed that, since they found it impossible to trim manually with the trim wheels, their plan was to slowly bring the aircraft back into trim using the thumb switches, then disable electric trim again, which doesn't sound like such a bad idea.
However, when looking today at the DFDR traces, I noticed that, immediately after re-enabling electric trim and making the two manual ANU adjustments with the thumb switches, there is a blip on the AP Warn Capt. trace.
So it seems that their plan was actually to re-enable electric trim, then test if it works for a bit using the thumb switches, then re-enable the auto-pilot. Unfortunately the auto-pilot failed to activate, and instead MCAS made the final AND trim adjustment dooming the flight.
If that was indeed their plan and that's what they did, it sounds like a really bad idea.
Later edit: from what I'm reading the A/P can't be engaged while there is force applied to the control wheel or column, which makes this even stranger. Because I don't see anything indicating they released the pressure on the column around the time of that blip. The column remained in the aft position according to the DFDR trace from the preliminary report.