We’ve gotten a few calls lately with customers installing Garmin G-600 systems in smaller aircraft and calling us for advice on how to interface the autopilot. This system consists of a large EFIS panel, a single GRS-77 AHRS and a GAD-43 for autopilot interface. The Garmin literature indicates the AHRS/GAD can replace existing gyros. Let me clarify that.
In order to upgrade to AHRS and remain in compliance with TSO C9c, in Part 25 aircraft the autopilot should have a separate source for each aircraft axis it controls. The worst case scenario would be a three-axis autopilot requiring three separate AHRS computers.
If the autopilot is a two-axis autopilot or has a separate source for yaw damper operation (its own rate gyro or accelerometer), then two AHRS computers would be acceptable, providing the autopilot uses information for only one axis per AHRS computer. For example, autopilot pitch attitude information would be provided from the first AHRS computer, while autopilot roll attitude information would be provided by the second AHRS computer. Directional gyro (Azimuth) information can come from either AHRS computer, but in the case of many Honeywell autopilots, which derive yaw rate from the directional gyro, a separate source would have to be provided in addition to the two AHRS computers. The C-14 would have to be retained for yaw rate sensing.
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