Auto Throttle Actuator

An autothrottle (automatic throttle, also known as autothrust, A/T) is a system that allows a pilot to control the power setting of an aircraft's engines by specifying a desired flight characteristic, rather than manually controlling the fuel flow. The autothrottle can greatly reduce the pilots' workload and help conserve fuel and extend engine life by metering the precise amount of fuel required to attain a specific target indicated air speed, or the assigned power for different phases of flight. Autothrottle and AFDS (Auto Flight Director Systems) can work together to fulfill the whole flight plan.

There are two parameters that an Autothrottle can maintain or try to attain: speed and thrust.

In speed mode the throttle is positioned to attain a set target speed. This mode controls aircraft speed within safe operating margins. For example, if the pilot selects a target speed which is slower than stall speed, or a speed faster than maximum speed, the autothrottle system will maintain a speed closest to the target speed that is within the range of safe speeds.

In the thrust mode the engine is maintained at a fixed power setting according to the different flight phases. For example, during takeoff, the Autothrottle maintains constant takeoff power until takeoff mode is finished. During climb, the Autothrottle maintains constant climb power; in descent, the A/T reduces the setting to the idle position, and so on. When the Autothrottle is working in thrust mode, speed is controlled by pitch (or the control column), and not by the Autothrottle. A radar altimeter feeds data to the Autothrottle mostly in this mode.

In addition to the usual modern autopilot facilities, the VC10 autopilot provides Mach-number and IAS locks, and adjustable datum for the height, Mach and IAS locks; automatic throttle control of airspeed on the approach; automatic flare-out control of the pitch channel; and yaw stabilization. The automatic throttle control is for use only at low speeds, and it can be used under manual control as well as under the autopilot. A friction coupling in the autothrottle drive to each powerplant control linkage enables the pilot to override the system manually without electrically disengaging.

The autothrottle system is a dual system, either half of which can be engaged independently of the autopilot and can be used with either autopilot. The pilot’s instinctive disconnect button will, however, disengage the system in use.

The throttle system drives the four throttle levers and controls the engine thrust in accordance with the required speed dialled on a digital counter located on the throttle dual controller. The Autothrottle Dual Controller is a single panel unit mounted on the flight deck centre console. It carries the switches and controls necessary for selection and engagement of the dual throttle system (System No. 1 or No. 2).