We wont know for sure now until they release more data but we can assume they were fighting MCAS trim with opposite trim and stick input.The flight lasted 13 minutes.Stick input doesnt override MCAS(!!) only electrical main trim(stick input does override STS).Stabilizer trim operation like anything else mechanical has limitations.(13 minutes off for every 2 minutes operation) or possibly continual engagement of opposite direction trim by the crew burnt out the motor anyway.
The accident Captain surely would have read the previous commanders write up mentioning STS trimming wrong way.He knows nothing about MCAS and he may well remember from his training that the way to disengage STS is to deselect the AP STAB trim motor which apparently doesnt exist on the MAX but is now called B/U.If he disengages B/U (the one on the right) and not the PRI(the one on the left) he may think he has successfully disengaged the trim but the culprit isnt STS but MCAS and you have to disengage both switches(RUNAWAY NNC).But he doesnt know this.
What the accident investigation will have to decide is were the crew negligent in not killing ALL trim once their opposing trim/stick input proved unsuccessful.The previous crew(s) presumably did just that.Comes under fly the plane.They will definitely crucify Boeing for this but MCAS will only be a contributing factor in the final report I am guessing.The Pilots must fly the plane.
Other possibility is they finally figured it out and disengaged both trim switches but the THS was by this time at its limits and elevator authority wasnt sufficient nor was there time to trim up manually.
Last edited by Rananim; 21st Nov 2018 at 17:10.