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Simplified Aircraft Autopilot Operation

October 2012

When customers call in for help troubleshooting an autopilot problem, I frequently hear the comment “I do not like troubleshooting autopilots.” This makes it difficult to know where to begin. There are many different components that make up an autopilot system and all of them must work together to make the aircraft fly successfully. It can be somewhat intimidating to isolate the cause of the problem, but it can be done with the right information.

When flying an aircraft, a pilot knows what control inputs are required to maintain a straight and level path by looking at the horizon or, in the case of IFR conditions, the artificial horizon. With the aid of an altimeter and vertical speed indicator, a pilot can establish and maintain a constant stable altitude. With the aid of heading compass and/or navigation radio or GPS receiver, a desired course can be followed to the aircraft’s destination. The pilot processes sensory inputs from the brain and makes the necessary corrections to maintain desired operation.

Basic Autopilot Operation

The autopilot can be thought of as “the brain” of an aircraft during flight. It requires input signals from numerous sensors, simulating much the same as what a pilot would see. It can then control electric servo motors mechanically linked to the controls to achieve the same results the pilot strives to achieve.

To keep an aircraft upright and flying level, the flight computer needs input, or reference. The most critical reference comes from the vertical gyro, sometimes called the attitude gyro. Not only does it provide attitude information, but it also indicates how fast the attitude is changing, allowing the computer to control the rate of command for smooth operation. Another requirement for smooth operation comes from the servo motor. The computer needs to know when and how fast the servo is turning in order to control the speed to produce a smooth transition to whatever new position is required to control the desired path of the aircraft. This information is produced by a position sensor or generated rate signal as the servo turns. While the pilot can rely on his senses and anticipate the reaction based on prior experience, the autopilot computer is dependent on good feedback from sensors to "anticipate" the command needed to smoothly control the aircraft.

Basics of Autopilot Troubleshooting

When attempting to troubleshoot an autopilot squawk, it’s a good idea to start with the basics of the system. If the autopilot holds the aircraft level and responds smoothly to manual command inputs, you can quickly eliminate the vertical gyro and servo as the cause. Move on to other sensor inputs that may be bad or missing, depending on the failure in the system.

Of course the best way to get accurate and complete information is to fly with the aircraft and see first-hand what is happening. However, in most cases, this is not an option. The next step would be to debrief the pilot-in-command (PIC) and get as many details as possible. Inquire about all modes of operation, even if it may seem irrelevant to the failure squawked.

A common problem is the squawk is passed down from pilot to maintenance to technician who is expected to work the problem. As you can imagine, it is easy for the description to change from one hand to the next. Many times, the actual squawk turns out to be completely different than what was given to the technician. A lot of time and effort can be saved by having the correct and accurate information from the start.

Another good tool in some cases is to have the pilot take a video with a smart phone. Sometimes, I find a picture is worth more than a thousand words, as the popular saying goes.

For a more in depth training on the basics of Autopilot Troubleshooting, attend the next Duncan Intelligence Live IA Renewal Seminar in Van Nuys, California, on September 12, 2012. Click here, to register.

Duncan Intelligence Live IA Seminar

Troubleshooting the aircraft autopilot systems can be complicated. At the next Duncan Intelligence LIVE IA Renewal Seminar, Curt Campbell, Avionics Tech Rep, goes into more depths on common autopilot squawks, such as Altitude Hold INOP and A/P Will Not Engage.

Register now for this face-to-face forum discussion with industry experts and earn 8 hours of Inspector Authorization (IA) renewal credit.