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Old 27th Apr 2019, 11:19
  #4431 (permalink)  
Join Date: Dec 2015
Location: Cape Town, ZA
Age: 58
Posts: 412
Originally Posted by bill fly View Post
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).
I am not an expert on flight instrumentation, but everything I have read about AOA vanes on the B737 in this thread, indicates that it is not a particularly well suited input parameter for direct command of flight controls. Stick shaker yes, that's a warning system. Fighter jets, yes its necessary. FBW aircraft, yes if validated against other parameters. The proposed Boeing fixes to MCAS, imply that none of this is true on the B737. AOA disagree tolerance of up to 5.5 degrees, what kind of input is that into a critical flight system in a passenger aircraft?

An analogy which I drafted in my comment (but then deleted), explains part of the situation: Imagine a motor vehicle driving down a potholed road, with all the irregularities transmitted to the steering wheel. Normally there is some kind of damper to prevent harsh feedback forces to the driver. Imagine that kind of instantaneous feel in the control column while trying to fly an aircraft?

Given the need for some kind of smoothing of AOA, by definition it would involve a delay or lag between pilot inputs and the feel forces. From a control systems theory viewpoint, delay lags are never a good thing, since they can lead to pilot induced oscillation, and other side-effects.

The opposite scenario, of full-force applied when over a threshold, is harsh and equivalent to a stick-pusher, with its own side-effects, training requirements, and type certification issues.

AFAIK MCAS was designed to avoid both scenarios, by being slow and unilateral. It fulfilled the criteria of not being harsh, nor could it induce short-period oscillations.

Sorry, if I took your comment and ran it to a logical absurdity, but IMO it would not be a simple fix. It really can be hard to explain complex control systems feedback, and I'm sure others can do it better,
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