Autothrottle
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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 directly controlling 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 phrases of flight. A/T and AFDS (Auto Flight Director System) work together to fulfill the whole flight plan and greatly reduce pilots work load.
Simply put, when engaged AutoThrottle replaces manual throttle input with an electronic (or in older systems, mechanical) feedback loop that controls fuel flow to the engines. This reduces pilot workload as well as being more accurate than manually adjusting fuel flow.
[edit] Working Modes
There are two parameters that an A/T (autothrottle) can maintain, or try to attain: Speed and Thrust.
In Speed mode, A/T, without going beyond its limitations, moves the throttle to a position to attain target speed, meanwhile to protect the speed within the aircraft safety margin. For example, if the pilot selects a target speed which is slower than stall speed, or a speed faster than maximum speed, A/T will maintain a speed that is within the safety region though to the most extend to close to target speed. In another word, when the A/T is in Speed mode, speed is protected, and maintained to the target speed provided the target speed is a reasonable one.
In Thrust mode, A/T, disregarding speed, maintains a fixed power setting according to the different flight phrases. For example, during Takeoff, A/T maintains a constant Takeoff power till Takeoff mode is finished. During Climb, A/T maintains a constant climb power; Descent, A/T retards throttle to IDLE position, etc. When A/T is working in Thrust mode, speed is controlled by pitch (or control column), and NOT protected by A/T.
[edit] Usage
On Boeing type aircraft, A/T is able to work the whole flight phrases from Takeoff, Climb, Cruise, Descent, Approach, all the way to Land or Go-around provided there is no malfunction. Taxi is not considered as a part of flight, and A/T does not work for Taxi. Under most cases, A/T mode selection is automatic without the need of any manual selection unless interrupted by pilots.
According to Boeing published flight procedure, A/T is engaged in BEFORE T/O procedure and is automatically disconnected 2 seconds after landing. During flight, manual override of A/T is always available. A release of manual override allows A/T to gain the control back, and the throttle will go back to the A/T commanded position except for 2 modes (Boeing type aircraft): IDLE and THR HLD. In these 2 modes, throttle will remain at the manual commanded position.
[edit] Origins
A primitive autothrottle was first fitted to later versions of the Messerschmitt 262 jet fighter late in World War II. Nowadays 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.
