Originally Posted by
zerograv
Thanks for your input !
Very likely this detail has been discussed before, but what is the reason that causes Boeing to be so afraid that the Max gets into a stall situation, to the point that they came up with MCAS ?
Goes into something similar to a super stall situation, that is impossible to get out of it, or is it something else ?
It's been a very long time since I last studied stability
MCAS was to provide an offset to a slight increase at high AoA of nose-up trim force in order to meet the FAA mandate that the force to hold a pitch attitude be proportional to the AoA. Basically, if one doubles the AoA then the pilot needs to pull twice as hard to maintain it. Because of the MAX aerodynamics, this was reduced, so a 2X AoA didn't require quite as much as 2X the force. So far that "not quite as much" graph has not been released, but simple examination shows that the plane will not diverge in pitch - it's not unstable at any operable angle of attack.The engine thrust line is higher than the NG so that contributes less to pitch-up than before.
The "fear" is that an aerodynamic solution would be expensive - adding weight, drag, cost, structural modifications. Instead they adapted the speed trim system (STS), which already modifies the trim to alter the control wheel forces, to do this. No doubt Airbus already does this to adapt; it's just less noticeable on Airbus as the only force feedback is a spring in the sidestick and they change elevator and trim to match that force.
If I were to guess why it didn't get more notice, it's because the correction it was to provide is small with far less influence during normal operation than STS. Tragically, no pilot should ever have had MCAS operate - it's only for manual flight, flaps up, far from normal AoA. Further, any malfunction in the trim system produces the same symptoms and corrective procedures were already in place.