Garmin Testing GFC 500 Autopilot On Piper’s Twin Comanche


Garmin has announced it is flight testing its GFC 500 autopilot on the first light twin aircraft, the PA-30/39 Twin Comanche. Designed for light piston aircraft, the digital GFC 500 includes self-monitoring features and requires far less maintenance than “older generation autopilot systems,” according to the company. “As Garmin continues to roll out GFC 500 autopilot supplemental type certification (STC) approvals at an increasing rate, spanning more than 200 aircraft models to date, the Twin Comanche marks the first light twin aircraft currently in [the] certification [process].

Installing the GFC 500 in light twins will necessitate using Garmin’s GI 275 electronic flight instrument to provide ADAHRS input. The installation can also interface with Garmin’s G500 TXi flight displays when the GI 275 is used as a standby attitude indicator. The autopilot also includes Garmin’s Level Mode (LVL), which returns the aircraft to straight-and-level flight “with the push of a button.”

Traditional autopilot features, including altitude hold, vertical speed and heading modes, are joined in the GFC 500 by advanced capabilities such as altitude preselect, VNAV2, underspeed and overspeed protection and more. The system also supports select coupled instrument approaches, including GPS, ILS, VOR, LOC and back course procedures, as long as the autopilot is paired with a compatible Garmin GPS navigator.

Mark Phelps
Mark Phelps is a senior editor at AVweb. He is an instrument rated private pilot and former owner of a Grumman American AA1B and a V-tail Bonanza.

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    • It’s both weight and engine power.

      A light twin doesn’t have enough power to climb on one engine in a meaningful way, hence all the reported fatalities.

      For example, on a hot day, an engine failure on takeoff would mean hitting any trees at the end of the runway, or if you try to avoid them, a Vmc roll (classic fatal stall/spin.)

      The only exceptions would be a well-maintained almost-empty Piper Aztec or Cessna 414, etc. which have a payload in the 2,000 pound range.

      Two (half) of my twin instructors were killed in light twin accidents, Ray Walkwitz (fatal takeoff accident in Bahamas) and an Asian MEI operating out of Reid-Hillview (fatal crash into a well with 2 student pilots.)

      At Walkwitz, a twin training mill that advertised nationally that had about 10 Apaches and a Baron, we considered a light twin to “have 2 engines, but really the performance of 2 halves of a single engine” and did balanced-field calculations for stopping distance after an engine failure on takeoff. Our alternate was the NASA Cape Canaveral runway nearby. We were using mogas, which would vaporlock on warm days, causing actual engine failures on takeoff. Keep an eye on that. 🙂

      A Hawaiian FBO asked me if they should buy a Piper Apache or Geronimo, and I told them it would just kill their students and instructors.

      • Actually, a “Light Twin” is DEFINED by FAA as a twin weighing less than 6,000 lbs. It has nothing to do with it’s climb performance on one engine.

        • You raise some interesting points.

          “Light-twin” is not an FAA regulatory term, but there are requirements for twin-engine performance above and below 6,000 pounds.

          You can find this info in various FAA publications:

          “The term “light-twin,” although not formally defined in the regulations, is used herein as a small multiengine airplane with a maximum certificated takeoff weight of 6,000 pounds or less.”


          “6,000 pounds or less maximum certificated takeoff weight and Vso of 61 knots or less. The single-engine rate of climb or climb gradient at 5,000 MSL must simply be determined. The rate of climb could be a negative number. There is no requirement for a positive single-engine rate of climb at 5,000 feet or any other altitude.”

          So it’s fair to say light-twins weigh less than 6,000 pounds and have no guaranteed climb performance.