Originally Posted by
AGBagb
Is the QRF allowing for some purely mechanical "uncommanded trim" that can only be mechanically countered with the wheel (assuming noting's catastrophically broken...)?
The manual pitch trim on the 737 is a 100% conventional cable-and-drum arrangement that Lindbergh would probably recognise. I'm struggling to think of any possible failure mode that would result in uncommanded movement.
The "grasp and hold" action item makes use of the fact that there is a clutch in the trim actuator gearbox which ensures that manual trim inputs override any electrical inputs to the actuator from either the trim switches or the autopilot.