Sensors are required in order that the flight control computer is aware of the state of the aircraft and position of the primary flight controls. The simpler stability augmentation systems require information from rate gyros whilst attitude stabilization requires a signal from a vertical gyro, or from integrating the rate signal. A gyrocompass will be used for heading hold. More complex autopilot systems will require more sensors, many of which may also be used to provide the pilot with flight information. Sensors include:
• a pitot/static system for speed and height holds (for safety reasons this system is usually separate from that which feeds the first pilot’s instruments);
• the navigation suite for waypoint steering;
• lateral accelerometers for turn co-ordination;
• ILS receivers for coupled approaches;
• Doppler or GPS receivers for automatic hover position hold.
A fuller list of the sensors currently used in helicopter automatic flight control systems is given in Table 6.3.
Table 6.3 Automatic flight control system – parameters and sensing systems.
Rate gyroscope or differentiated attitude
Vertical gyroscope or integrated rate
HADS or other low airspeed sensor
Yaw vane or differential static pressure
Static system or separate aneroid Radar altimeter Doppler radar, IN or GPS Doppler radar, GPS or cable angle ILS, VOR, DME, GPS or IN Position pick-off or force sensor Microswitch