Garmin has received FAA approval for its software update and service bulletin to correct a previously reported possible runaway trim issue with its popular GFC 500 autopilot. In December, the FAA alerted GFC 500 operators who have the optional GSA 28 pitch trim installed of the possible problem, which could lead to loss of control. Software updates to accommodate installations involving Garmin GI 275 instruments are expected within weeks, according to Garmin.
In December, Garmin issued Service Alert 22109 Revision A for pitch-trim-equipped GFC 500 digital autopilots installed in accordance with Garmin’s Supplemental Type Certificate (STC) SA01866W. That configuration is approved for certified, experimental and light-sport aircraft.
Under the terms of the service alert, operators of certified aircraft with the STC were required “to pull and collar the autopilot circuit breaker and placard the autopilot as ‘inoperative’ prior to further flight.” To comply with this month’s approved software update requirements, owners must complete the software update in the next six months or at the next scheduled service interval, whichever comes first. The warranty reimbursable period closes out at the end of June 2023.
I think there may be some confusion as to which GFC500 installations are affected. I believe this issue is specific to those aircraft that have two GSA-28 servos acting upon the pitch axis, one directly actuating the elevator and one directly actuating the pitch trim.
Most installations have a single GSA-28 directly actuating the elevator, but most of these feature an auto-trim function whereby the servo itself can actuate other pitch trim servos such as the Ray Allens installed in many experimentals to neutralize any continuous GSA-28 torque so that the aircraft is in trim if tge autopilot is disconnected. The GSA-28 is also capable of sensing airspeed and modulating the rate of actuation of Ray Allen and other electric trim servos based upon airspeed when the autopilot is inactive but powered up. I do not believe that aircraft so configured are potential subjects of the runaway trim issue.
If I am mistaken here I welcome correction.