Automatic electric pitch trim is a popular option on Garmin’s popular GFC 500 flight control system, especially for higher-end aircraft. For these existing installations (both in certified and experimental aircraft) there’s a potential problem lurking that can cause the system (which uses Garmin’s GSA 28 pitch trim servo) to run uncommanded.
Garmin has issued Service Alert 22109, revision A, associated with the system’s STC SA01866W. The bulletin says that the GFC 500 retrofit autopilots that are installed in Cirrus SR20 and SR22 models (under the same STC) are not affected. The bulletin describes the problem: “An uncommanded automatic pitch trim runaway may occur when the autopilot is first engaged.”
To comply with the Service Alert yourself, you have no choice but to disable the entire GFC 500 autopilot by pulling the circuit breaker and putting a placard on the panel that says the autopilot is inoperative. It may be possible to bring the aircraft to a Garmin dealer so they can disable the pitch trim system, while retaining functionality of the rest of the autopilot. You’ll need to obviously trim the pitch axis by hand, but it may be better than operating with no autopilot at all.
If you have a GFC 500 installed in an experimental or light sport model, you’ll need to follow Garmin’s Service Bulletin 22112—a mandatory bulletin that must be accomplished before using the pitch trim/autopilot combination. For these aircraft you can pull the circuit breaker or disable the trim motor control in the interfaced G3X/G3X Touch and G5 system configuration mode. The bulletin directs you to the appropriate manuals for instructions on doing so.
In its Alert, Garmin says the problem will be corrected in a future software update that’s expected in Q4 of 2022, which may be by the time you read this. Still, it’s worth contacting your Garmin dealer so the shop can add you to its schedule for when the patch is available. The GFC 500 has been a popular seller and shops are reporting heavy scheduling delays for repairs and installations.