Machinbird;
That once you let go of it, the system would again move the trim to where it wanted to move it.
Others have stated that once you handle the manual pitch trim, it stays in manual for the duration of the flight and has to be reset on the ground.
May as well sort out this detail fully. If you remained in manual THS, it would have implications for proper flight control function on some of the "protections."
The text I have is not the easiest to read/interpret. One could gain the impression by reading some of the statements, that once the THS is moved manually through the trim wheels, that it is in "manual, mechanical mode", period. However, the relevant statements which make me believe otherwise, are:
A. Mechanical Control Operation
When the trim handwheel installed in the cockpit center pedestal is manually turned by the crew, the chain and cable loop move the input shaft on the THS mechanical actuator. The input shaft moves the mechanical servoloop mechanism through an override mechanism. The override mechanism, which is installed adjacent to the Pitch Trim Actuator (PTA), makes sure that the mechanical control cancels, through a cam and microswitch arrangement, the electrical control.
..(1) Operation of the Override Mechanism
.....(a) Operation of the Mechanical Control
...........1 The input shaft turns.
...........2 A cam turns.
...........3 The cam moves a roller and releases the brake which limits the output shaft of the Pitch Trim Actuator [PTA] and the mechanical input together.
...........4 At the same time a piston is pushed to operate the three override mechanism microswitches.
...........5 The mechanical control now comes on before the electrical control.
.....(b) Release of the Mechanical Control
...........1 The input shaft stops.
...........2 The cam in the PTA is released.
...........3 The internal springs connect the mechanical input to the output shaft of the PTA.
...........4 At the same time the piston moves back from the microswitches.
...........5 The mechanism is now set in the electrical control position.
I think that makes it pretty clear, that once manual input stops, autotrim is reinstated. It also makes sense from the POV of the safety of flight - you cannot have a 1g AFS function safely with manual trim requirement.
BOAC;
Regarding THS then - if you managed to maintain1g flight while reducing IAS by 100kts maintaining enough pitch 'authority' with the elevators, the THS would not move - or would it simply move to maintain a neutral elevator position as a 'human' would do?
The THS would move, doing just as we would do in a non-AFS airplane...trim control column forces out to keep "elevator forces neutral", (bearing in mind that the forces aren't real, being masked by hydraulic servos and are artificially introduced by springs,
.
The question that the engineers had to come to terms with however is, how do you do that in a FBW system?
For the above reasons suggested by
Machinbird, you can't just rely on human input to do the trimming in a FBW automated system.
Maintaining "1g" flight is one solution. Using the elevator, (short term pitch control) as speed changes means that according to Nz Law the THS, (long term pitch control) would change position to maintain full elevator authority.
Maintaining 1g is an indirect response to increases and decreases in airspeed. A decrease in speed causes slightly < 1g as the aircraft pitch reduces, an increase, the opposite. Even in manual flight, the THS responds, both to stick and Nz laws.
Clearly, there are circumstances which you would either not want autotrim or autotrim would not be available in system or sensor failures.
Our human feedback loops are completely transparent to us but are extremely sophisticated and are the "loops" in a non-fbw airplane that we "tune" when we learn how to fly.
But those "loops" must be made visible (from their "automatic transparency!), then designed and built into a FBW system independent of humans for such a system to even function in a rudimentary way, and decisions must be made about such loops about the information signalled back to the servocontrols, the AFS and the crew.
Otherwise, any autoflight system would quickly become unmanageable, highly fatiguing and fundamentally unsafe.