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Old 4th Dec 2018, 16:11
  #79 (permalink)  
Join Date: Dec 2002
Location: UK
Posts: 1,874
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.
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.

The association of the Elevator Feel and Centering Unit with trim has been described, and the reasons for separating the (independent?) pneumatic ‘muscle’ from the electronic logic in the trim system - mechanical, elevator feedback.
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 ?
If so, then a consequence might be an erroneous trim datum, which may also be misplaced by MCAS and / or electric pitch trim, so that no stable trim condition can be achieved. A pilot will continuously ‘chase’ a stable pitch condition - there is no trimmed position for speed (trying to fly in-trim with the elevator offset).
Furthermore with similar mechanism, would trim / AoA offset cause the feel unit to position at it’s maximum extent, and so doing limit the available amount of stabilizer / elevator for control ? (and / or stick force limit).

Aerodynamic slat operation also appears as a connection within the various ‘trim’ computations, relating to AoA input.
Is slat extension possible based on a false AoA, but at much higher air speeds those normally expected ? Possibly with significant adverse pitching moments competing with pitch control and trim.
Or even with the ‘handed’, separate-side AoA inputs and computations, could there be asymmetric slat deployment.
(The slat thoughts above come from the apparent roll control issues in the Lion accident.)

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