At least two of our Level D B737-800 FSTDs exhibit the characteristic being discussed with regard to electric trim being ‘overwhelmed’ by aerodynamic loads… the scenario being AND ‘loaded’ trim situations, significant aft elevator deflection/input to arrest ROD/pitch down and higher KIAS (albeit within the overall AFM envelope, however, over flap limit speed in the given scenario). ANU electric trim stops in this scenario, unless the elevator input is neutralised. Centralised control wheel/column allows electric trim to re-start, progressively, similar to an aerodynamic unloading of the forces acting on the motor/screw jack. It is not like a ‘switch’.
This effect was discovered during mandated UPRT, a ‘Rostov’ type of scenario (from the interim report data).
I would add during these simulated scenarios (which do involve mis-handling to get there), without ‘unloading/neutralising’ the elevator and allowing AUN stab trim movement as KIAS increases the pitch attitude further lowers despite full aft elevator, the outcome being wholly predictable.
The FFS concerned are SPF1 and Non SFP (W) variants, both built by the same very creditable and long term FSTD manufacturer.
Questions have been asked as to validity of the simulation, however there does appear clear historical evidence (FCTM advice etc) that previous Boeing models had similar effects. Some have suggested that what’s being observed is some form of ‘historical’ simulator data and may well not be valid.