In order for an autopilot to work successfully it requires authority over the relevant flying controls. This is usually exercised through an existing, and in most cases separate, AFCS that acts as a SAS and/or ASE when the autopilot is not operating (see Section 7.5). Although it was stated above that some ASE installations include heading hold and altitude hold, these functions will often be provided by the autopilot circuitry when one is fitted, simply because other, more complex, modes will make use of heading and altitude control. Before progressing it is necessary to review the terms used by certain AFCS manufacturers. The term ‘autopilot’ has been defined herein to mean an AFCS offering automatic flight path control, navigation and mission-related modes. Unfortunately the term ‘autopilot’ is used by certain manufacturers to refer to the SAS/ASE since it is this system that flies the helicopter when under automatic control. Others refer to this device as a ‘helipilot’ for much the same reason. The device that drives the SAS/ASE is often called a ‘coupler’, or ‘flight path computer’, since it takes navigation information and couples it to the flying controls. Most couplers offer a lesser mode whereby the flight path information is fed to command bars on the ADI and HSI in order to guide the pilot.
There are two basic types of autopilot: 3-channel or 4-channel. A 3-channel system controls the helicopter in pitch, roll and yaw with the pilot providing the necessary compensation in the collective channel. A 4-channel system provides automatic control of all flying controls and no compensation is required. The typical system will have series actuators operating in pitch, roll and yaw (for rate damping and initial attitude stabilization) and parallel actuators (or AFCS operated trim motors) operating in all channels for autopilot control and/or trim follow-up. Vertical gyros, and possibly rate gyros, will be fitted to provide signals for the SAS /ASE. The autopilot computer will require signals from the gyro-compass, pitot-static system, lateral accelerometer or sideslip ports, radar-altimeter and navigational radio/satellite receivers. Other equipment may be required depending on the mission modes available, these include Doppler receivers and inertial/GPS navigation systems. The modes provided by a typical autopilot can be divided into three groups: forward flight holds, navigation modes and mission modes. The navigation modes are often provided to enable single pilot IFR operation and are therefore usually civil orientated. Mission modes will consist of modes peculiar to the role of the helicopter, such as autotransition, hover position hold and programmed search patterns.