Originally Posted by
positiverate20
I know there's little evidence at this stage for this tragedy, but to elaborate and hypothesise about MCAS function:
1. MCAS activated by errant AoA vane data. (This same data may also lead to warnings such as stall etc.)
2. So, by the time you run through checklist, source the problem and stop the MCAS from further inputs by flicking the Stab trim cutout, the stab is already at X° nose down.
3. At relatively low speed you're able to manage this by pulling back on the yoke, and the now fixed stab angle may even go unnoticed and forgotten about for a while
4. Additional engine power may already have been applied, but, if not, you do so now as you need some height, especially with advancing terrain and the loss of altitude that you'd suffered.
5. The effects of the engine cowling on aerodynamics, as stated in the comment above, helps lift, as does the thrust moment created by the engines
6. You think you have the problem somewhat under control compared to the situation you were in a minute ago. You've now been able to climb and seem to have relatively stable manual control.
7. Now you're at Y feet, (still with X° nose down stab trim) with an increased airspeed of Z, up until now has been controllable due to your elevator inputs being assisted by thrust moment and engine aerodynamics, but, at this new increased airspeed and increased altitude the yoke is becoming even more difficult to keep holding back. The stab is still in the same X° nose down it was when you switched the cut-outs, and up until now you mightn't have thought about it because you'd disabled it- in accordance with the checklist.
8. You think about returning and getting this back on the ground
9. Once you level off, or even before then, with that stab STILL at X° nose down (now with a much higher airspeed component) there's only one place you're going. Once this vertical direction change has momentum there's now no chance, no elevators in the world are going to help you.
Look at the memory item for stab runaway- i.e.- not told to manually wind back trim wheel, the instruction is to "grab and hold".
All this could be caused by one errant sensor? Madness that it was certified.
What happened to one of your first priorities on an airplane that requires the pilot to manage pitch trim? If steady column forces are present, pitch trim should be used to drive the stabilizer so that those column forces are relieved. I concur with the criticism that with MCAS as currently implemented we can get headed down this path as a result of one errant sensor, but I don't see that the eventual result should put the airplane so far out of trim that pitch control power via the column is exhausted. MCAS moves the stabilizer, but does not interfere with the pilots ability to override and move the stabilizer back to the proper trim position.