FullWings, # 8
Some certification aspects were discussed on Pprune during the 'early days'. However, reconsidering these with what is now known could change thoughts and understanding; not necessarily for the better.
EASA requested certification clarification (*);
'
Simulation has demonstrated that the thumb switch trim does not have enough authority to completely trim the aircraft longitudinally in certain corners of the flight envelope, … The main issue being that longitudinal trim cannot be achieved throughout the flight envelope using thumb switch trim only. '
The Boeing / FAA response to use the trim wheel as an alternative, was accepted:-
'The need to use the trim wheel is considered unusual, as it is only required for manual flight in those corners of the envelope. The increased safety provided by the Boeing design limits on the thumb switches (for out-of-trim dive characteristics) provides a compensating factor for the inability to use the thumb switches throughout the entire flight envelope. Furthermore, the additional crew procedures and training material will clearly explain to pilots the situations where use of the trim wheel may be needed due to lack of trim authority with the wheel mounted switches. The trim systems on the 737Max provide an appropriate level of safety relative to longitudinal trim capability. '
This acceptance appears to be based on the engineering simulation at that time, which subsequently has been shown not to cover all aspects ( Ethiopian accident report ).
If the simulation deficiencies only applied to the specific conditions of an MCAS induced trim deviation, additional stick force (trim wheel) due to 'feel shift', then the MCAS mod should negate that extreme.
However, if the revised trim model now shows that the previous acceptable position is questionable - that trim cannot be achieved, or that the additional safety from limiting electric trim input no longer applies (assumes elect trim runaway is possible beyond thumb switch limiting), then the certification aspect must be addressed by other means.
*
https://www.easa.europa.eu/sites/def...20ISS%2010.pdf (Page 15)