Originally Posted by
derjodel
Lion air's FDR show that the trim was moved to full of the available 7 notches (it's not clear how those translate to degrees). They would certainly still be alive if there as a limit. They kept it in air at ~ 3 notches. If max needs more to get out of stall, perhaps it does need a larger hstab/eleveator instead of a hacked together software?
Or perhaps the key really is the reset. Does mcas reset with each trim input and takes the current position as the new neutral? If AoA vane is sending wrong data it's easy to see how this would result in a runaway trim.
This
image shows the difference between a 737-800 (yellow) and MAX (magenta). Notice the bigger nacelles and same hstab size.
I thought 0.6 was a hard limit... e.g, we move this much and then it's up to the pilot. If it was 0.6 every 5 seconds it's still bad, but at least easier to manage.
It would appear that MCAS has no memory of previous activations after each reset its world starts again it is totally context free. So when it wakes up it looks at the AoA and if it is above limits (and perhaps other conditions pertain ) it activates and puts in 0.6 nose down on
whatever the trim currently is pilot blips the trim pickle switch and MCAS goes to sleep and 5 seconds later MCAS wakes up and repeats the exercise
as it is context free it has no knowledge of the previous activation. That would appear to be the fault in the system that means an incorrect AoA will result in
repeated nose down trim increments for which there is no need. A nose down trim sensor that would inhibit MCAS trimming more nose down than a parameter value that would be calculated based on nose down trim and speed would prevent the over application of nose down trim.