What was found
DURING DESCENT FROM BIG BEAR, THE GARMIN AUTOPILOT ABRUPTLY DISCONNECTED FOLLOWING FAILURE OF THE GSA 28 PITCH SERVO. IMMEDIATELY PRIOR TO THE FAILURE, I OBSERVED A MOMENTARY PITCH-TRIM-DOWN INDICATION. THE AIRCRAFT USES MANUAL PITCH TRIM. UPON SERVO/AUTOPILOT DISCONNECT, THE AIRCRAFT PITCHED UP AGGRESSIVELY AND AIRSPEED DECAYED RAPIDLY. IMMEDIATE AND SUBSTANTIAL FORWARD CONTROL INPUT WAS REQUIRED TO LOWER THE NOSE, RECOVER AIRSPEED, AND MAINTAIN AIRCRAFT CONTROL. THE FAILED GSA 28 WAS WITHIN THE POPULATION IDENTIFIED BY GARMIN IN A SERVICE BULLETIN ADDRESSING POTENTIAL SERVO FAILURES. REPLACEMENT WAS RECOMMENDED BUT NOT MANDATORY. REPLACEMENT ULTIMATELY COST APPROXIMATELY $2,500. I AM CONCERNED THAT THE FINANCIAL AND LABOR BURDEN ASSOCIATED WITH CORRECTING A KNOWN SAFETY-RELATED FAILURE MODE MAY DISCOURAGE COMPLIANCE WITH A VOLUNTARY SERVICE BULLETIN, LEAVING AFFECTED SERVOS IN SERVICE UNTIL FAILURE. GIVEN THE SIGNIFICANT FLIGHT-CONTROL UPSET RESULTING FROM THIS FAILURE, I RECOMMEND FAA REVIEW OF WHETHER VOLUNTARY CORRECTIVE ACTION PROVIDES ADEQUATE MITIGATION OR WHETHER MANDATORY CORRECTIVE ACTION IS WARRANTED.