Mission modes

Mission modes are included in autopilots intended for specific roles. As such they are not usually included in AFCS fitted to civilian helicopters unless they are expected to have a SAR capability as well as being suitable for single pilot IFR.

Auto-transition mode. Helicopters intended for ASW or SAR operations will usually have an autotransition mode provided by the autopilot. From a start point at a radio altitude of about 200 ft and airspeed close to that for minimum power, the AFCS will execute a programmed manoeuvre using both the pitch and the collective channel. Thus under that action of the autopilot the helicopter will decelerate and descend to hover at pre-selected radio altitude. It is usual to arrange that appropriate movements of the longitudinal cyclic or collective inceptor by the pilot will cause the programming of either altitude or airspeed to cease thereby enabling him to fly a modified profile or abort the manoeuvre. At the end of the program the helicopter will usually enter a hover with plan position held using signals from the Doppler and/or inertial/GPS system. Radio altitude will be maintained using the basic altitude hold mode. Certain autopilots have an additional mode that programs the helicopter from higher altitudes, up to 2000 ft, and higher speeds to a flight condition from which the basic autotransi­tion mode can be engaged.

Hover position hold. Hover position hold is achieved through the pitch and roll channels. The sensors used to generate the error signals will depend on the type of hover required. Plan position can be maintained accurately using a combination of signals from longitudinal and lateral accelerometers (translational acceleration), Doppler receivers (translational rate) and GPS (present position and translational rate). Hover position relative to a dipping sonar is often maintained using cable angle sensors. In some helicopters a rudimentary hover hold capability is provided using the basic attitude hold mode.

Translational rate command. Translational rate command (TRC), in the form of an auxiliary hover trim (AHT), has been available for a number of years on certain AFCS designed for SAR operations. Deflections of the inceptor, located at a crewman’s station, commands a rate of horizontal translation. Centralizing the inceptor will cause the system to maintain the selected rate as sensed by the Doppler radio receiver. Full deflection usually commands a fairly modest rate, typically 10 kts, in keeping with the hover trimming function of this mode.

Automatic circuit. A relatively new autopilot feature, again provided for SAR opera­tions, is the automatic circuit or Mark on Target (MOT) mode. The pilot overflies a desired point on the surface, usually the location of a survivor, and engages the mode. The existing flight path control functions of the AFCS then turn the helicopter downwind and initiate a deceleration and descent to the autotransition gate conditions. Some time later, using Doppler information, the helicopter turns into wind and at the appropriate point the autotransition mode is engaged and the aircraft descends and slows to a hover at an altitude pre-selected by the pilot. The navigation system will usually arrange for the hover to be established a short distance from the MOT point so that the AHT mode can be used for final plan positioning.

Dip-to-dip. Another relatively new AFCS mode is the so-called ‘dip-to-dip’ function. This uses a combination of hover-position hold, automatic transition and Doppler/ IN/GPS based navigation to move the sonar equipped helicopter from one dipping location to another. The aim of this mode is to move the helicopter in the shortest possible time commensurate with the available performance and the ambient wind conditions.

Chapter 7

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