Transparency and override facilities

Automatic flight control systems that feature outer loop modes need to be carefully designed so that the pilot is able to make control demands without having to overcome an opposing input from the AFCS. Ideally movement of the controls in the cockpit should cause the relevant hold to be disabled, either permanently or temporarily, depending on the circumstances. If well designed the functioning of the basic holds (attitude, heading and height) can be transparent to the pilot.

Of particular importance is the blend between heading hold and turn co-ordination or more simply heading hold on and off. In forward flight the initiation of a turning manoeuvre is signalled by the application of lateral cyclic and this is used by the AFCS designer to unlock the heading hold and use the yaw series actuator (and parallel if fitted) to help generate the necessary yaw rate for a balanced turn. Thus a natural blend is achieved and, as far as the pilot is concerned, the helicopter changes transparently from maintaining a heading in straight and level flight to performing a Rate 1 turn for example. In the low speed regime the situation is different since during a lateral transition or side-step the pilot will expect the heading to be maintained. Therefore some form of airspeed switch is required to prevent heading hold unlock at low forward speed.

Rotorcraft AFCS without automatic turn co-ordination or an airspeed switch present the designer with more of a challenge in that he can no longer use lateral cyclic to unlock the heading hold. Instead a sensor is placed in the yaw control run so that if the pilot attempts to initiate a yaw rate by application of pedal the heading hold is disengaged. Unless this is carefully engineered the AFCS may suffer a lack of transparency that in extremis makes the helicopter harder to turn with the AFCS on than with it off.

Some AFCS provide the pilot with a range of programmed manoeuvres such as automatic transitions, auto-ILS and waypoint steerage, that often require large move­ments of the controls within the cockpit under the action of a parallel actuator. It is usual, therefore, for the pilot to be provided with an override or cut-out button so that in the event of a runaway, some other failure or a change of mind, he can disengage part or all of the mode either permanently or temporarily as the situation dictates. So for example the Westland Sea King ASW helicopter is provided with both cyclic and collective cut-outs to disengage parts of the automatic transition programme. Similarly helicopters fitted with height holds have cut-outs, usually located on the collective, to disengage the hold and some have a manoeuvre button to allow temporary disengagement of the hold as a new datum altitude is set.

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