Purpose of the AutoTrim
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I suspect the answer may be type dependent, but for the CRJs, the stab retrims when the AP servo sees a load above a threshold, so the design purpose of the trim motion is to relieve AP servo load while the AP is engaged.
This means that when the AP is disengaged, the aircraft will be close to in-trim - but by no means fully in-trim - but this is merely a useful side-effect of the operation, not its intent.
The only stab motion when the AP is disengaged is either pilot commanded (hardly auto-trim) or is a consequence of configuration or Mach number changes, and is not particularly intended to relieve forces, but rather to provide the desired force feel characteristics to the pilot.
So for those aircraft, the true answer is "neither", but the second is closer to the behaviour than the first option.
This means that when the AP is disengaged, the aircraft will be close to in-trim - but by no means fully in-trim - but this is merely a useful side-effect of the operation, not its intent.
The only stab motion when the AP is disengaged is either pilot commanded (hardly auto-trim) or is a consequence of configuration or Mach number changes, and is not particularly intended to relieve forces, but rather to provide the desired force feel characteristics to the pilot.
So for those aircraft, the true answer is "neither", but the second is closer to the behaviour than the first option.
Bottums Up
Perhaps it's to keep the aircraft on the programmed flight path. People wandering up and down the aisle off to the loo; cabin crew moving galley trolleys up and down the aisle; weather upsets that may change the pitch/roll/yaw attitude and thus the flight path.
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None of the above?!
I would say neither. It is to remove standing forces on the control surface itself as well as the servo. The Stab trims out such that the servo demand is reduced to a null. Sensing the servo demand is a way of detecting control surface loading, it is the effect not the cause. The reason for doing this are various.
1. Relieving loads on the airframe/stab componants increases reliability.
2. Maintains almost full elevator authority irrespective of flight profile.
3. Ensures a/c in trim and therefore will NOT experience any MAJOR attitude change in the event of an autopilot disconnect (failure of auto trim must be annunciated so crews are aware there may be a change in attitude if they disconnect)
Hope this helps.
1. Relieving loads on the airframe/stab componants increases reliability.
2. Maintains almost full elevator authority irrespective of flight profile.
3. Ensures a/c in trim and therefore will NOT experience any MAJOR attitude change in the event of an autopilot disconnect (failure of auto trim must be annunciated so crews are aware there may be a change in attitude if they disconnect)
Hope this helps.
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On the B737, the autopilot controls the elevators.
If the elevator deflection is held for a certain amount of time (15secs I think) then the autotrim trims back the elevators to neutral. There is a "Stab out of trim" light on the panel to tell you if this has not happened.
(Usually ex main base because you have to trip the C/B to do certain A/P checks!)
The elevators are kept faired to avoid drag.
If the elevator deflection is held for a certain amount of time (15secs I think) then the autotrim trims back the elevators to neutral. There is a "Stab out of trim" light on the panel to tell you if this has not happened.
(Usually ex main base because you have to trip the C/B to do certain A/P checks!)
The elevators are kept faired to avoid drag.
