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Old 4th Dec 2018, 22:11
  #80 (permalink)  
Join Date: Jan 2008
Location: uk
Posts: 780
Originally Posted by safetypee View Post
From the descriptions in this thread and external inks, #97, and particularly, the STS is a ‘crutch’ on late series 737 to meet low speed stability requirements.
MCAS appears to be a similar ‘crutch’ but addressing more specific nose up issues when approaching or at stall (25.203), and when turning where the trim would be more nose up, and where there may be a greater pitching moment associated with the new engines -737 MAX.

There was/is a similar mechanism added to the NG as part of speed trim function, my guess is MCAS evolved from it and replaced it. The NG AMM says:

Near stall, the speed trim function trims the stabilizer to a nose down condition to allow for trim above the stickshaker AOA and idle thrust. The trim continues until the stabilizer gets to its limits or the aft column cutout position is exceeded
However it looks like the activation conditions for MCAS may be wider (more of the envelope) and the trimming is more aggressive and ignores the aft column cutout, there are probably other differences too. It isn't clear (to me) that the NG function can kick in from just one AOA.

Originally Posted by safetypee View Post
However, it is difficult to understand the effects of input failures (e.g. AoA) amongst the complex computations and interactions in these trim systems.

(Forgive the ‘Non PC’ quip, but a failure in heavily ‘crutched’ systems, literally leaves you with no leg to stand on.) - ‘close coupled’ systems.
I agree but I do think there is a wider more general issue (shown nicely in AF447) with close coupling / interdependence of airdata from "independent" probes that are in reality all of the same type and sat in the same outside environment. The redundancy is illusory, and the interdependence means that when we do get failures they cascade - bad AOA fails altitude and speed, bad speed fails altitude and AOA, etc. Tying everything together and correcting for everything to put "perfect" data in front of pilots is great when it works - when it doesn't, the pilots are left trying to disentangle multiple failures, with less data they can trust, and may end up focusing on the wrong thing.

Essentially the automation paradox again.

Is it possible that a false AoA will trigger a change in the trimmed - elevator neutral shift unit due to a false change in the feel / centering unit ?
Is slat extension possible based on a false AoA, but at much higher air speeds those normally expected ?
Been wondering about those two as well - can't be sure if it's possible or not on the NG from the AMM, MAX could do anything since this is an area we know they've changed.
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