Autothrottle

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An autothrottle (automatic throttle) allows a pilot to control the power setting of an aircraft's engines automatically. This helps conserve fuel and engine life by metering a precise amount of fuel to attain (usually) 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, AutoThrottle is a replacement of manual throttle input. It controls engines power output by metering fuel input. It works the same way as pilots do, and always more accurate.

[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 collum), 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 happens. 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.