Originally Posted by Larry in TN

They did not cut out the stab trim until the trim position was nearly full nose-down and was flying at over Vmo (over 390kts vs Vmo of 340kts). When they turned the stab trim back on the captain repeatedly tried to engage the autopilot (which won't engage when out-of-trim) instead of using the primary electric trim, which might have had a chance of rolling the trim back up. That allowed additional unscheduled MCAS activations to roll it to the full nose-down stop.

The Captain didn't know what to do so he kept trying to engage the autopilot. He had tried the same thing less than 500' off the ground on takeoff (minimum autopilot engagement altitude on takeoff is 800'). In doing so, he briefly coupled the autopilot (CMD A) to the Flight Control Computer that was receiving bad data. This produced unstable pitch inputs from the autopilot and caused the autopilot to disconnect within 30 seconds.

While "STAB TRIM CUTOUT switches ... CUTOUT" is part of the applicable procedure, they did it far too late and without doing the previous steps--this one is the second-to-last step. Doing only one item on a checklist does not qualify as following the procedure.

They made many other procedural errors prior to that one including never just flying the airplane. i.e. when the nose gets heavy, trimming it back up with the primary electric trim. The Captain of the Lion Air accident flight kept the airplane in-trim through 21 unscheduled MCAS activations by doing just that. The trim didn't get significantly out-of-trim until he transferred control to the F/O who did not re-trim the airplane after each MCAS activation.

If you're interested in understanding the actions of both accidents crews, I'd recommend this (rather long) article from NY Times Magazine. It is the most detailed article that I have seen on the MAX crashes.

https://www.nytimes.com/2019/09/18/m...x-crashes.html