Autopilot Maintenance

Autopilot Maintenance By Jim Sparks September 2000 Oil change, replacement of ignition plugs and checking the pressure of tires are some of the many things often associated with routine aircraft maintenance. Most of us will automatically...

Autopilot authority
Autopilot authority is the speed or range of flight control movement. This speed or range is often adjusted as a result of aircraft speed or even the position of flaps or slats. In other words, when the aircraft is in a slow flight condition, the flight controls need to move faster and possibly further than with the aircraft in a clean, high-speed configuration. Often, external switches are fitted to secondary flight controls such as flaps strictly for autopilot sensitivity biasing. These devices are frequently confronted with the brushes and spraying equipment of aircraft cleaners or even possibly the application of deicing fluids. Often, the only purpose of this device is autopilot control so an internal electrical failure may not be easily recognizable.
Virtually all automatic flight control systems have some means of disconnect so that the human pilot can take control. Some of these systems are automatic, while others require some specific action of the pilot in command. One example may be if the aircraft is flying with the autopilot holding altitude and the pilot were to introduce a change in elevator trim or horizontal stabilizer angle of attack. The autopilot may respond with a command to the elevator so the aircraft will continue to hold altitude. This is a "Mis-trim" condition and could also occur if the pilot were to apply a force on the control column without first advising the auto-flight computer. Once again, the computer may attempt to override the pilot and a severe pitch oscillation may result. On many aircraft, the autopilot is inhibited anytime the pilot is using the manual trim systems. Should the crew need to make a pitch change and not want to disconnect the automatic system, switches on the control wheel such as "Pitch Sync" (PS), "Touch Control Steering" (TCS), or "Control Wheel Steering" (CWS) may be actuated. Once the manual maneuver is completed, the switches are released and the autopilot regains control of the aircraft. "Go Around" (GA) switches may also serve as an autopilot disconnect while on some aircraft they serve as an input to the flight guidance system as an automatic response to a missed approach to landing.

Testing for overuse and underuse
It is very important to understand all the interlocks in a specific aircraft that will cause the autopilot to become disabled. In many situations, flight crews get used to using only one means of isolating the auto-flight system and none of the alternate methods get exercised. Most of us in the aircraft maintenance field realize that the lack of use of a device causes almost as many problems as overuse. If not already incorporated in the aircraft normal maintenance schedule, it would be worthwhile to include a designated time where all the autopilot functions can be tested. Most of the later technology digital auto-flight systems include a maintenance test where all the concerned switches are exercised and monitored.
When performing even the most basic maintenance on an aircraft, it is worth considering not only how it could affect the pilot but also how it could affect the autopilot.

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