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Which of these statements are correct, as regards trimming in an irreversible flight control system of a conventional aeroplane equipped with a trimmable horizontal stabiliser?

1. The zero force position of the control column changes when using the pitch trim
2. The zero force position of the control wheel changes when using the aileron trim

  • A
    Both 1 and 2 are correct.
  • B
    Both 1 and 2 are incorrect.
  • C
    1 is incorrect, 2 is correct.
  • D
    1 is correct, 2 is incorrect.

As you can see at the bottom of this explanation, for the aileron and rudder the “zero force point” will be adjusted when trimming. This is not the case for elevator trim.

Primary flight controls
Are controls which movement causes a rotation about one of the three axes of the aircraft, causing a change in the aircraft’s flightpath or attitude. The elevator, rudder and ailerons are considered to be the primary flight controls. The primary flight controls in a manually controlled system are reversible. This means that a force applied to the controls is a force which moves the control surface, and also a force which moves the control surface will move the pilot’s controls. This means that the air pressure on the control surface can be felt in the pilot’s controls. When the flight controls are fully power operated this is not the case and is called irreversible. This means that if there is a force moving the control surface this will not move the control columns, and there will be no natural feel on the controls.

Because of this there will need to be an artificial feel unit installed in a fully powered or irreversible control system. This artificial feel unit should apply force to the cockpit controls in proportion to the control deflection and in proportion to the aircraft’s speed.

The artificial feel system is placed in parallel with the hydraulic actuator, so the movement of the flight controls in the cockpit will cause the hydraulic actuator to operate. During this same movement the artificial feel system will apply pressure on the controls appropriate to the airspeed of the aircraft.
This artificial feel system will also work with a trim system, but this works different than a normal trim surface does. A movement of the trimwheel will result in a movement of the electric trim actuator, and so the control surface, but not in the movement of the control column (this is for elevator control). It is different for aileron and rudder trim.

Elevator trim: zero force position of the control column does not change (control column/horn/stick does not move)
Aileron trim: zero force position changes (control wheel turns in the direction of the trim)
Rudder trim: zero force position changes (pedals move in the direction of the trim)

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This question has appeared on the real examination, you can find the related countries below.