[forgetcolor]

Why does MCAS have to take control of trim at all? Would it not be sufficient to alert the pilot of an imminent stall situation so they can adjust the angle of attack themselves? Is it because doing so would put the MAX too far afield from the old 737 such that it would require simulator training?

[cjbprime]

There's an airworthiness rule requiring monotonically increasing yoke backpressure as the plane approaches a stall. It appears that the MAX violates this rule aerodynamically due to extra lift at high power generated by the high and forward nacelles, combined with the yoke being mechanically coupled. It can get easier to induce a stall as the plane approaches critical AoA.

This isn't directly the same thing as saying the MAX will stall itself: if you aren't pulling back on the yoke near critical AoA then you don't stall. It's just easier to stall with the yoke than regulations say it must be. MCAS "fixes" that handling issue.

I don't think an audible warning would be sufficient to turn unairworthy behavior into airworthy, so if all the assumptions above are correct, that's why it has to use trim -- or a stick pusher, but perhaps that wouldn't have enough control authority and also I'm not sure the MAX has a stick pusher, as opposed to just a stick shaker. It's a very mechanical cockpit, in general the forces you feel are coupled to aerodynamics, in stark contrast to an Airbus (or even more modern Boeing airframes).

[cmurf]

Maybe it's 25.173(c) https://www.law.cornell.edu/cfr/text/14/25.173

In the working case, MCAS presumably triggers at a particular AoA, trims nose down (but is this a fixed amount and what is it?) which in effect requires more stick back pressure to maintain the angle of attack. Thing is, it seems like MCAS, again in the working case, has a trigger AoA and will incrementally nose down until it goes below some defined angle of attack - which is not really a stick force moderator alone. It's acting as a kind of AoA guardian.

[salawat]

>It appears that the MAX violates this rule aerodynamically due to extra lift at high power

High Angle of Attack. The power can create a pitch up in some circumstances, but the AoA is the primary driver of the extra lift.

[ralph84]

Yes. MCAS is designed to make the Max handle like previous 737s when manually flying at high angle of attack. Without MCAS the handling would be different enough to require simulator training.

[VBprogrammer]

The behaviour is not certifiable, no amount of simulator training can fix that.