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.”