An autothrottle (automatic throttle) 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. These systems can 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. A/T and AFDS (Auto Flight Director Systems) work together to fulfill the whole flight plan and greatly reduce the pilots' work load.
There are two parameters that an A/T (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, A/T maintains constant takeoff power until takeoff mode is finished. During climb, A/T maintains constant climb power; in descent, A/T retards the throttle to IDLE position, and so on. When A/T is working in thrust mode, speed is controlled by pitch (or the control column), and NOT protected by A/T. A radar altimeter feeds data to the autothrottle mostly in this mode. (In Boeing it feeds data to the autothrottle every time. See below.)
On Boeing-type aircraft, A/T can be used in all flight phases from takeoff, climb, cruise, descent, approach, all the way to land or go-around, barring malfunction. Taxi is not considered a part of flight, and A/T does not work for taxiing. In most cases, A/T mode selection is automatic without the need of any manual selection unless interrupted by pilots.
According to Boeing-published flight procedures, A/T is engaged BEFORE the takeoff procedure and is automatically disconnected two seconds after landing. During flight, manual override of A/T is always available. A release of manual override allows A/T to regain control, and the throttle will go back to the A/T commanded position except for two modes (Boeing type aircraft): IDLE and THR HLD. In these two modes, the throttle will remain at the manual commanded position.
A primitive autothrottle was first fitted to later versions of the Messerschmitt Me 262 jet fighter late in World War II. Today it is often linked to a Flight Management System, and FADEC is an extension of the concept to control many other parameters besides fuel flow.
US patent US3362661, Booth George C, Post Morris H, Prilliman Floyd W, Redmond Jr William G, "Autothrottle", issued 1968-1-9