Plane & Pilot
Tuesday, October 8, 2013

Your Best Friend Or Worst Enemy

How well do you know your autopilot?

The first time I ever engaged an autopilot, it did something completely unexpected, and I quickly turned it back off. My checkout in the plane had covered just about everything—except the autopilot, so I was left on my own to figure it out. Since then, I've found that many pilots have had a similar experience when it comes to auto-pilot training. Even in more advanced training programs, questions and misperceptions abound.

There are times when knowing how to use an autopilot (AP) can save your life, and there are other times when it can kill you—particularly if you don't know what you're doing. Remember, an autopilot doesn't necessarily do what you want it to do: It does what you tell it to do.

So, let's review some of the basics of autopilot operations—components, basic functions, and a few important do's and don'ts. One warning: There are probably as many autopilot models as there are airplanes, so it will be hard to describe the exact operation of your particular autopilot in your airplane with your avionics. It's your responsibility to carefully review the Pilot Operating Handbook and AP manuals for your airplane to understand how your particular system works and its limitations. This is a big subject, so we'll only look at some of the more common functions that you might find in GA aircraft.

Components Of The Autopilot
All autopilots rely on a device that can sense orientation regardless of acceleration or gravity loads. The first autopilot was invented in 1909 by Elmer Sperry, who used a spinning gyroscope that has been in widespread use up until about 10 years ago. Today, modern GA systems are based on AHRS (attitude heading and reference system) technology that incorporates miniature vibrating MEMs components to sense orientation. Interestingly, physicists can't easily explain why spinning or vibrating components try to maintain a fixed orientation relative to the distant stars, so we'll just have to accept it as a useful fact that helps us navigate. A controller uses the signal from the attitude sensor to manage a feedback loop that drives servomotors attached to the aircraft control surfaces to achieve the desired flight attitude. Although older analog autopilot controllers work well, newer digital controllers have improved performance and expanded capabilities. A simple wing leveler might control only the ailerons, while a more sophisticated three-axis system controls the elevator and rudder, as well.

Most autopilots include some or all of the following components: a control unit (sometimes called the AFCS for auto-flight control system); an interface to a navigation system such as an FMS (flight management system); VOR receiver or a GPS navigator, which normally runs through a directional gyro (or a HSI—horizontal situation indicator) and various special-use switches and indicators. The switches may include a big red disconnect button on the control yoke (or stick), an electric trim controller, a control wheel steering (CWS) button on the yoke and possibly a go-around (GA) button. You may or may not have all of this stuff in your airplane, but we'll briefly review each feature so that you'll be prepared the next time you encounter something new.

Flight Director. This is a feature that has become more common with the advent of glass. The FD is displayed on the attitude indicator and can take on different forms—most commonly a set of triangular command bars. One bar shows the attitude of the aircraft, and the other bar moves to show the autopilot command attitude. By programing the AP, engaging the flight director and following the command bars, any pilot can hand fly, as well as the autopilot.


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