Hello Bernd, thanks for your reply
Originally Posted by
bsieker
During pitch-up and high-AoA maneuvers (and in statically stable / dynamically damped aircraft also in neutral flight), the tailplane exerts a downwards force, even in a rearward CoG situation. If it stalls, the force pushing the tail down will reduce, and the nose-up moment will reduce, so it will not result in (additional) nose-up moment.
I would mostly agree but assert that static longitudinal stability in general requires the horizontal tail (stab+elevator) to be at lower AoA than the wing, but not necessarily in downforce. Drag is generally lowest (and efficiency highest) with COG loaded to a rear limit only slightly in front of the neutral point. In this configuration the tail is generating negligible or zero downforce (and hence minimal drag), but the aircraft still has positive but weak stability. The longitudinal stability can then be augmented for certification/handling (e.g. by STS)
Originally Posted by
bsieker
That must never occur. You cannot get civil airplanes certified otherwise. If it does (e. g. by massive load-shift), catastrophic things will happen, as the 747 crew experienced in Afghanistan a few years ago.
Bernd
Quite right, was just pointing that FAA 25.203 describes (and forbids) what can happen to an aircraft loaded to static instability in the stall, the tail operating at higher AoA stalling before the wing causing pitch up. This is not what occurs in the (-MCAS) MAX, but the fact that it pitches up just before the stall due to the nacelles might be construed as similar for certification.