bisieker #3350,
Ethiopian airliner down in Africa
Bernd,
’ … had always assumed that in all but extreme out-of-trim situations, mechanical trim would be available.’
This is a very important point as it appears that the same assumption was in the EASA - Boeing case for equivalent safety as a means of compliance.
Boeing applied for a regulatory dispensation for the inability of the electric trim to achieve a trimmed condition throughout the aircraft’s speed envelope. Although this was unusual, but of importance that it was not described as a failure case, the use of manual trim was an acceptable alternative with caveats of training and normal procedure (not made available for 737 MAX ?).
Thus in normal operation, at all speeds, it should be possible keep the aircraft in trim, and able to correct small deviations (accel / deceleration), with the use of the manual trim wheel.
However, the abnormal situation where the aircraft is grossly out of trim (trim runaway) appears not to have been considered - or assumed to be the same as normal operation. This appears to be incorrect.
The difference in air-loads on the horizontal tail and elevator (
https://www.satcom.guru/2019/04/stab...and-range.html) result in the inability of the trim wheel to overcome the forces and regain a trimmed state.
This situation was not posed in Boeing’s application, or it was assumed that FAA - EASA would consider it, possibly on the basis of similar approvals in NG (my assumption). Thus for trim runaways, the dispensation is insufficient to meet the regulatory requirements, but more important that there loss of all pitch control, trim and elevator effectiveness!
Either this represents a significant difference between the NG and MAX in the aerodynamics affecting the failure cases (likely), or that the NG is similarly non compliant in the failure case.
This could also question if Boeing actually tested the dispensation for the MAX in normal flight conditions - opposed to grandfathering it from the NG.
Also, to check that the abnormal NG procedures would still be effective in the MAX, again because of different aerodynamics (and apparently, without including the necessary procedures for recovery of a trimmed condition in the MAX manuals; the ‘roller coaster’ could not be flown).
These assumptions could leave some very big holes in the 737 MAX certification, not only being non compliant, but also un-flyable with some trim failures - where MCAS modification might only address one of them.