Forward flight holds
Altitude hold. If the autopilot is a 4-channel system then the altitude hold will operate with the collective parallel actuator making corrective inputs in response to altitude deviations sensed by either the static system, a separate barometric capsule or a radio altimeter. A 3-channel system, however, operates somewhat differently with altitude corrections being made through the pitch channel. Since pitch attitude is being used to control altitude it is not possible to retain control of airspeed. Compensation for the actions of the altitude hold on the airspeed is left to the pilot. Indeed if the pilot were to raise the collective the airspeed would increase but it would be difficult to estimate by exactly how much. Obviously, the altitude hold mode of a 3-channel system will only work satisfactorily above Vimp and therefore the hold is often disabled below a certain speed, typically 60 KIAS.
Airspeed hold. In all autopilots airspeed hold is achieved using the pitch channel. Once engaged the pitch parallel actuator will make corrective inputs in response to airspeed deviations, sensed by the pitot-static system, from the value set at the instant of engagement. Once again if a 3-channel system is installed the pilot will have to compensate for the effect of the autopilot by making appropriate collective inputs, this time to maintain altitude, and as before the hold will only operate satisfactorily above Vimp. It is clear that a 4-channel system is required if simultaneous operation of altitude hold and airspeed hold is desired.
Vertical speed hold. The operation of the vertical speed hold is very similar to the altitude hold. A 4-channel system will use collective and a 3-channel system will use the pitch channel. In a 3-channel system the pilot will have to apply collective to maintain airspeed as the autopilot controls the rate of climb or descent. Static pressure signals will be used by the autopilot to generate the appropriate error signal.
Heading hold. Naturally heading hold is achieved through the yaw channel, although most autopilots use the roll parallel actuator for large heading corrections, greater than 2° for example, with the yaw actuator maintaining the helicopter in balance in response to signals from a lateral accelerometer or sideslip ports. The yaw channel is used primarily because it is assumed that any deviations from the datum heading will be small and therefore sufficient control can be exercised through this channel without any changes to the position of the roll parallel actuator. Some AFCS make a more positive distinction between heading hold, which is retained as the yaw channel ASE/ ATT mode, and heading selection, or steerage, which is an autopilot mode achieved primarily through the roll axis. Most systems alter the control law at some value of forward speed to take account of the increasing effectiveness of the fin.