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Old 27th Apr 2019, 10:34
  #4428 (permalink)  
bill fly
Join Date: Feb 2015
Location: The woods
Posts: 1
Originally Posted by GordonR_Cape View Post
A feel augmentation system that takes AOA as input, what could possibly go wrong? I'm sure others can give scenarios where this would be a bad idea...

I previously pointed out that any system that takes a small input such as AOA, and produces a large output (whether feel or trim), is inherently undesirable and unpredictable. MCAS creates a semblance of smoothness by applying trim over a 10 second interval. Applying such a delay into elevator feel would create all kinds of feedback lags, and the solution could be worse than the problem.

The cost issue with any non-software change is how many years that would take to design, test and certify, and what production would be done in the meantime? Boeing have painted themselves into a corner, and the only way out seems to be double or quits (to mix metaphors).
Hi Gordon,
Although I didnít mention AoA as an input in that post, if you need to know AoA in order to correct elevator feel, then clearly you have to measure it. Done properly (dual input and monitor) and maintained correctly it is a valid measurement as used for years on many aircraft.
You could also ask what could possibly go wrong with a pitot system. There are probably more examples of failure in that system and yet it is universally used.
At some point you have to trust something for your data, so make it as reliable as you can and maintain it well.
As for feedback lags and delays, a spring is a wonderful reliable input which can be linearly or dynamically calibrated, and is used in many control run applications. The ten second interval used by MCAS would not apply - as long as the condition remained, the spring would stay compressed. As the condition decreased, so would the spring force. Qed. There are other force generators available, based on hydraulics or pneumatics although a spring is simple and less reliant on other systems.
Main thing is to have the force generated within the control run rather than by potentially powerful MCAS stab. movement (which still needs AoA input).
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