Originally Posted by
rudderrudderrat
autotrim will move the stabiliser without any input from PF. If the speed of the aircraft changes, then the trim will run to help the elevator.
Agreed.
Once PF had commanded a pitch attitude which resulted in a real (not just spuriously indicated) speed decay - even if he didn't touch the side stick again - the FBW computers would attempt to maintain that trajectory by moving the elevators and hence the stab trim.
Which was the point I was making. Autotrim was trying to compensate for the simultaneous fall-off in airspeed *and* the commanded nose-up attitude, both of which were directly attributable to the PF's backstick commands.
@franzl - If I had to guess, I'd say that the autotrim limitation might have been dropped for the widebodies due to criticism from some pilots over not having full authority through the PFCs in Alternate Law.