Old 13th Mar 2019, 14:40
  #150 (permalink)  
Join Date: Mar 2006
Location: England
Posts: 835
Yo gums, et al, #148
Further contextual aspects should be considered, particularly the airspeed. As stated, the pitch trim moves nose down at a rate of .27 degrees per second for a total travel of 2.5 degrees in just under 10 seconds. How much the stabilizer moves depends on Mach number. At higher Mach the stabilizer moves less, at slower speeds it moves more. The trim system under MCAS is not stopped by simply moving the control yoke. (attributed to Boeing)

This describes normal operation of MCAS at lower airspeeds ~ Flap0, min manoeuvring speed; 10 secs of trim (or less ?) provides sufficient change in stick force to meet stability requirements.

However, with erroneous trim input due to AoA failure, MCAS can repeatedly apply 10sec inputs, furthermore this is not limited to lower airspeeds (not discussing Mach restrictions here).
Thus the increased stick force due to 10 sec of trim at ~ 250 kts could be considerable, repeatedly additive, and potentially even higher forces as speed increases - due to nose down pitch change.

The drill assumes that the crew will deduce ‘abnormal’ operation, but this might only be possible after noting trim activity in excess of what might be judged a normal input; so the decision process starts after 10 sec, with further nose down trim input.
Any one have a rough measure of stick force for 10sec of trim at >250 kts

Whilst we don’t know if the 737 Max suffers trim ‘jack stall’ as in previous models, but even if not, the control forces could demand great piloting effort.
The tech discussions of this are in Boeing advice on "aerodynamically relieving airloads" using manual stabilizer trim

If these concerns apply to the Max, then system modifications must ensure failure resistance over the complete speed range.
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