Autopilot

Description

An autopilot is a device used to guide an aircraft without direct assistance from the pilot. Early autopilots were only able to maintain a constant heading and altitude, but modern autopilots are capable of controlling every part of the flight envelope from just after take-off to landing. Modern autopilots are normally integrated with the flight management system (FMS) and, when fitted, the autothrottle system.

Autopilot software, which is integrated with the navigation systems, is capable of providing control of the aircraft throughout each phase of flight. If an autothrottle/autothrust system is installed, the appropriate thrust may be automatically set during take-off, and is then adjusted automatically as the climb progresses, while the aircraft climbs at the appropriate speed for its mass and ambient conditions. The aircraft then levels at the required altitude or flight level while the power is adjusted to achieve and maintain the programmed speed. At the same time, the aircraft follows the flight plan route. If an autothrottle is not installed, the pilot must make all power adjustments appropriate to the autopilot mode and phase of flight.

On commencing the descent, the power is adjusted and the aircraft descends at the appropriate speed and on the required routing, leveling as required in accordance with the flight clearance until the approach is commenced. If this is to be a Category III Instrument Landing System (ILS) approach with Autoland, the autopilot controls the aircraft flight path so that it follows the ILS glide path and localiser, adjusting the power to maintain the appropriate speed and commencing the flare as required to achieve a safe landing without the runway being visible until the final stage of the approach. On some aircraft, the autopilot can then guide the aircraft so that it maintains the runway centre-line until it stops.

At any stage of the flight, the pilot can intervene by making appropriate inputs to the autopilot or the FMS. In an emergency, the pilot can disengage the autopilot and take over manual control, usually by pressing a switch mounted conveniently on the control column (although alternative means of disengaging the autopilot are available). Modern aircraft have another switch or throttle position which allows the pilot to change instantly from approach to go-around mode if necessary. If the aircraft is not fitted with an automatic go-around function, pilots must disconnect the autopilot and fly the missed approach manually.

The safe and efficient operation of automatic systems relies on clear understanding of the capabilities and the design philosophy of the equipment. Failure to achieve this level of understanding has resulted in several fatal accidents.

Related Articles

Further Reading

Accidents and Incidents

The following events involved airworthiness issues associated with the autoflight system:

Events in the SKYbrary database which include AP/FD and/or ATHR status awareness as a contributory factor:

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