I don’t usually post here, however seems there’s pool of knowledge (some historical) regards the B737 trim system in this thread. The following observation for your interest..
A simulator experiment … Try this in the sim if you get chance, requires some ‘mishandling’ and in this order:
- Conduct GA (Flap 15) with A/T disengaged – set full TOGA N1
- ALT ACQ at say 2500’ AGL manual flight, leave thrust at TOGA N1
- Electric Trim forward for 10-12 seconds continuously (agreed not a normal trim run by any means, quite difficult to force yourself to run electric trim AND for this period)
- Speed continues to accelerates +/- 220-230 KIAS, SFP1 simulation in flap load relief to F10 (from F15)
- Close thrust levers to idle, aircraft will now pitch down, attitude below horizon, KIAS further increases.
- Elevator full aft required to arrest nose drop (which may not be possible)
Now ‘try’ to apply ANU electric trim, stab trim motor will NOT run ANU unless the elevator is moved to a neutral position. With aft elevator input removed the stab trim ‘progressively’ starts to run ANU with control wheel thumb switches held in ANU throughout this maneuver.
This effect occurs in a similar fashion with two different Level D B737-800 FFS devices it’s been tried in, one FFS is a SFP1 simulator the other straight -800.
Would have to assume from that the stab screwjack loading and it’s effect on the trim motor is part of these particular FFS’s aero modeling? Only these Level D devices or other B737-800 FFS too?