, an autothrottle
is an automatic engine control system that can be part of a complete flight management system
. It can be described as a cruise control
for airplanes, although it performs its job during parts of flight other than just cruise
Typically, an autothrottle consists of a computer, a servomechanism (connected to the manual throttles), and various sensors. Unlike the automobile cruise control, it is critical for the aircraft's manual throttles to move so that the pilot does not become confused when manually changing power settings - hence the servomechanism to move the throttles. In addition, multi-engine airplanes have a much more complex control law problem to solve - the autothrottle must take into account thrust balancing. Unlike in automobiles, aircraft power settings do not translate directly into speeds, so the autothrottle must be able to handle varying pitch angles and climb rates, and factor those into the desired speed setting. It must be restricted from operating in certain situations for safety - such as when the aircraft is on the ground, or would undergo radical power changes in flight, for example.
Some autothrottles can differentiate between indicated air speed(IAS) and calibrated air speed; most commercial ones (such as those on civil air transports) simply indicate either IAS or Mach number depending on how fast the aircraft is traveling.
Typically, an autothrottle will have several standard controls. There will be a 'Master arm' switch, which when OFF will fully disengage the autothrottle from the engine controls. This is a safety feature to both allow the autothrottle to be disengaged when not in use as well as to allow a single point response if it malfunctions. There will be a speed select control and display, allowing the user to set the desired speed for the autothrottle to hold the aircraft at. If the autothrottle is part of a full flight management system, there will likely be a control to connect or disconnect the autothrottle from the autopilot, to allow speed to be linked (or unlinked) from navigation by the computer. There may also be a simple 'HOLD' control, which would perform the same function as the 'SET' button on an automobile cruise control, commanding the autothrottle to hold the present speed.