[alistairSH]

This is my understanding.

In a typical run-away trim, the trim wheel will move a great distance. It's very noticeable, both audibly (the wheel makes a clack-clack-clack noise) and visibly (there are white paint flashes on the wheel). And, obviously, the plane nose goes up or down by more than expected. The correct remedy is to disable the auto trim control via switches on the panel (located near the trim wheels).

The MCAS will adjust the trim in small increments every 10 (or is it 20?) seconds. Yes, a pilot should notice this, but because it's intermittent, it's more likely they don't "see" it as run-away trim, and just a slightly abnormal trim (EDIT - problem made worse because pilots were not informed MCAS existed - it's not a failure more they have trained on). They may attempt to remedy this with the manual trim control (a rocker switch on the control yoke). This does NOT disable MCAS, it only re-trim the plane. MCAS will re-engage due to faulty AoA sensors repeatedly until either the pilot disables all auto-trim with the switch on the panel OR the plane runs into the ground.

[cjbprime]

Yeah. I'm also not clear on the practicality of retrimming the aircraft at low altitude (Ethiopian was never >1000ft above ground) before impacting terrain after you disable the trim motor. You have to use a hand crank, because you just killed the trim motor altogether, and I think it takes several minutes (!) of cranking to get the jackscrew from one end to the other.

I wonder if they killed the trim motor and then failed to reach level flight before the ground got in the way.

[cmurf]

Fatigue as well. Looking at the flight track after the last and final nose down, it's they were overcome. Nose down -> more speed -> more stabilizer nose down force -> more nose down attitude even without a change in stabilizer -> more speed -> more stabilizer force -> more nose down attitude.

And then the ground got in the way.