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Old 1st May 2019, 13:04
  #4680 (permalink)  
Join Date: May 2010
Location: Boston
Age: 70
Posts: 443
Originally Posted by GordonR_Cape View Post
An interesting point! I can only assume that this has something to do with the autopilot not using AOA as an input like MCAS does, but rather gyro pitch (and other parameters).

I previously asked the question: How could the autopilot ever get into a high AOA situation? One answer was if the autothrottle is disabled. The implication being that the autopilot could keep increasing the nose up pitch until the stall warning activates, and the crew intervenes. I hope that scenario has been carefully tested?

This also touches on the question of whether the MAX autopilot was specifically programmed for the region of high AOA characteristics covered by MCAS, and whether it was tested under actual flight conditions?
It may be simpler than that, of course I could be wrong on autopilot details.

The autopilot 'knows' the desired column (and other controls) positions and puts them there, it does not rely on column feel for feedback.

It is manipulating the controls to achieve the desired aircraft state, when pitch is low set column position back until pitch OK at a defined 'gain', how much (not 'how hard) to pull is based on divergence.
This a classic feedback loop, the stick force is not part of the loop.

An imperfect analogy is cruise control in a car, it maintains desired speed directly whereas the driver uses more or less force on the accelerator to maintain speed.

Given the above I would assume (a tricky word in many fields) that no significant changes to autopilot for MAX were required, at most some tweaking of gains.

When I said 'critical' instability i referred to something like reversal of effect past a certain point or similar which would be catastrophic in a feedback loop.
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