4By Bob Hart – www.AvionixHelp.com

Contemplating an autopilot upgrade? SLOW YOUR ROLL! Reviving a “tired” original may be the smarter, more economical option.

This article was written in 2015. Read newer articles on Cessna avionics here.

If you’ve been following my articles, you know that I believe an autopilot—even a basic wing leveler—belongs in any aircraft that intends to spend time in the clouds in IFR flight. The less time a pilot has spent in IFR conditions (e.g. a low time pilot with a fresh IFR ticket), the more an autopilot is needed.

Cessna first introduced an optional autopilot in the 1962 172C model, and their choice of manufacturer for factory-installed autopilots and avionics was Cessna’s own division known as ARC. Aircraft Radio Corporation (ARC) got its start in 1924 and played a significant role in the development of early avionics. You may be aware of the famous flight by Jimmy Doolittle in a Consolidated NY-2 aircraft in which “blind-flight” was first demonstrated. That was possible through the efforts of ARC, Sperry, and others. ARC also went on to develop avionics used in military aircraft in WWII. Then, after the war, Cessna Aircraft Corporation purchased ARC and changed its name to Aircraft Radio and Control (thus keeping the ARC “handle”) and a whole new division of Cessna was created for the sole purpose of developing, manufacturing, and supplying avionics and autopilots to be installed exclusively in Cessna aircraft.

Obviously, autopilot technology in general aviation aircraft (including ARC autopilots) has evolved since then. In this segment, we’ll identify Cessna “Legacy” autopilots and review their respective strengths and weaknesses, but let’s start with some autopilot basics.

First, autopilots are available with single-axis (roll) or two-axis (roll and pitch).

NOTE: Some autopilots offer a third axis or yaw damper (or separate yaw damper) option which is designed to compensate for excessive adverse yaw caused by aircraft design (as in the case of the V-tail Bonanza) or excessive turbulence. These are most often found in twins and charter aircraft to dampen yaw and make the flight more comfortable for passengers.

Autopilots are also available as either rate-based (where the turn coordinator is the primary sensing device) or attitude/position-based (where the artificial horizon serves as the sensor for roll and pitch). There are pros and cons to both types. Turn coordinators are electric and considerably more reliable than a vacuum-based artificial horizon because a vacuum pump failure is more likely to occur than a total electrical failure. In short, an all-electric, rate-based autopilot has fewer pathways to an inflight failure. Today, all STEC autopilots are rate-based. On the other hand, attitude/position-based autopilots are reportedly “smoother” and better able to handle turbulence.

Single-axis (roll only) autopilots keep the wings level and most can track an Omni, GPS, or localizer. Some, when linked to a directional gyro with autopilot interface (heading “bug”), can track a heading. This is a real nice feature if you spend a lot of time in controlled airspace where vectoring is frequent. Roll only autopilots, however, have no way to sense or control altitude.

Conversely, two-axis autopilots not only provide full roll control, but they can also maintain a heading (with DG option), and hold an altitude. In fact, better two-axis autopilots offer additional altitude features like altitude pre-select and vertical speed control. Some even have glideslope tracking capability and, frequently, automatic trim—or at least an “out of trim” warning. It should be fairly obvious that the more sophisticated the aircraft, the more sophisticated the autopilot.

Now, starting with the early days, let’s look at the autopilots that were available from Cessna:

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