Old 19th May 2019, 09:34
  #8 (permalink)  
Join Date: Dec 2002
Location: UK
Posts: 1,847
MemberBerry, # correct, to the point

Of greater concern is the origin of the error.

Based on what is now known, it’s is unlikely that Boeing had flight tested the extreme condition, an offset stab (unable to control the aircraft). Thus the simulation may have extrapolated steady-state data from operation in the normal flight envelope, which was alluded to in the EASA query.
If so, this might clarify the choice of wording of the emergency AD drill; the belief that the trim wheel could be moved to recover the aircraft. A caveat on this is circumstance where the abnormal ‘yo-yo’ manoeuvre is required - when the trim could not be moved without relief. Furthermore, whether this circumstance is the same as previous variants (assumption in that what worked in the -200, would also work in the -700, NG, etc).

This point is also at the crux of the training / pilot error debate; how much time would the crew have before encountering excessive stick forces, and/or the trim not being available. (MCAS ‘failure’ was a pulsed input spread over a longer time period than might be assumed for a trim runaway - depending on trim rate).
The simulator demos in the media might correctly replicate the difficulty of control and trim, even with lower forces, but would be inaccurate because the difficulties are encountered earlier, a lower stab displacement, and at the stab limit the forces would be much higher.

An associated issue is if the simulated contribution of elevator stick forces were similarly too low; tail trim load is a combination of stab trim and elevator. This can be identified in flight test by mis-trimming the stab and counteracting with elevator, but only up to the point of excessive control force. Of interest this type of test is similar to that which identifies the longitudinal stability compliance - too little at low speed (need MCAS), too much at higher speed (need to reduce the stick force).

A conclusion from the simulator error could be that the 737 Max differs considerably from previous 737 variants, and that these differences are more than Boeing knew of, or had anticipated, - what was the extent of their assumptions.

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