Originally Posted by
Mad (Flt) Scientist
From the peanut gallery, sort-of, not totally 100%.
MCAS appears designed to give assistance to using the elevator to recover from a high AoA condition. Likely the scenario for which it is implemented also includes some combination of high thrust (causes nose-up moment) and stab trimmed to abnormally nose-up position. Both 737NG and, I think, at least one AB variant have had high pitch angle upsets in very similar conditions, and the concern for such a scenario has been the subject of more general discussions. Thus Boeing, perhaps prodded by FAA, seem to have implemented a "helper" to trim the stab a bit nose-down for such a case. While the MAX has more thrust than NG and thus more nose-up possibly in the scenario I don't think it's a black-and-white "problem on MAX not on NG" - I suspect it's shades of grey.
Thing is, even in the NG the STS would start trimming down if you are in an approach to stall scenario in manual flight (unless counteracted by manual trim of course). The MCAS just seem to do a similar job in slightly different scenarios and based on AoA alone.