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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 take-off to landing. Modern autopilots are normally integrated with the flight management system (FMS).
Autopilot software, which is integrated with the navigation systems, is capable of providing control of the aircraft throughout each phase of flight. For example, on take-off the power is set to the correct setting and adjusted 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 the desired flight characteristics. At the same time, the aircraft follows the flight plan route.
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 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. 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 override 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 which allows the pilot to change instantly from approach to go-around mode if necessary. Aircraft not fitted with a go-around switch 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 do so has resulted in several fatal accidents.
Accidents and Incidents
The following events involved airworthiness issues associated with the autoflight system:
- RJ1H, vicinity Zurich Switzerland, 2011 (LOC HF AW) (On 20 July 2011, the flight crew of a Swiss European Avro RJ-100 on a positioning flight from Nuremburg to Zurich responded inappropriately to an unexpected ‘bank angle’ alert in IMC. Near loss of control followed during which a PAN was eventually declared. The situation was resolved by a belated actioning of the QRH checklist applicable to the failure symptoms experienced. The subsequent investigation attributed the event to inappropriate crew response to a failure of a single IRU and poor manual flying skill whilst the situation was resolved.)
- B735, vicinity Perm Russian Federation, 2008 (HF LOC AW FIRE) (On September 13 2008, at night and in good visual conditions*, a Boeing 737-500 operated by Aeroflot-Nord executed an unstabilised approach to Runway 21 at Bolshoye Savino Airport (Perm) which subsequently resulted in loss of control and terrain impact.)
- A319, en-route, London UK, 2005 (AW) (On 22 October 2005, an Airbus A319-131, being operated by British Airways from London Heathrow to Budapest in night VMC, experienced a major electrical failure. The subsequent investigation made a number of recommendations but did not establish the cause of the failure.)