Refer to figure.
An aeroplane is trimmed when it will maintain its attitude and speed without the pilot having to apply any load to the cockpit controls. If it is necessary for a control surface to be deflected to maintain balance of the aircraft, the pilot will need to apply a force to the cockpit control to hold the surface in its deflected position. This force may be reduced to zero by operation of the trim controls.
The aircraft may need to be trimmed in pitch as a result of:
- changes of speed.
- changes of power.
- varying CG positions.
- changes of configuration.
A Trimmable Horizontal Stabiliser (THS) enables a larger CG and speed range than a conventional tailplane with stabiliser, elevator and trim tab, because the whole stabiliser may be moved for trimming the aeroplane, instead of a relatively “small” elevator and trim tab only (a larger surface is much more effective). This also leads to more control difficulty due to a jammed stabiliser.
An aircraft on final apporach is configured and trimmed for low speed, i.e. nose up trim. This means that, in case of go-around, the pilot should be prepared to apply nose down or trim nose down to prevent excessive pitch up caused by the additional power.
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