Internal Failure Of The Autopilot Pitch Servo Actuator That Caused The Autopilot To Command A Nose Down Movement

Location: Bellefontaine, Ohio Occurrence Number: ENG22LA004
Date & Time: November 12, 2021, 09:02 Local Registration: N9035Q
Aircraft: Beech F33 Aircraft Damage: Minor
Defining Event: Sys/Comp malf/fail (non-power) Injuries: 1 None
Flight Conducted Under: Part 91: General aviation - Personal

Analysis: The pilot reported an uncommanded nose down pitch trim movement. The nose down pitch trim occurred immediately following the pilot’s engagement of the autopilot. The uncommanded nose down trim was repeated when the pilot engaged the autopilot later in the occurrence flight as well as during postflight troubleshooting. 

During the NTSB’s autopilot examination on the airplane, the uncommanded nose down pitch trim movement occurred repeatedly upon autopilot engagement. When the Garmin GSA 28 pitch servo actuator was removed and replaced with another GSA 28 servo, the uncommanded nose down pitch trim movement ceased.

The pitch servo actuator examination at Garmin resulted in test element failures (related to monitoring of the pitch servo torque) that, with the servo installed as part of the airplane’s autopilot, would have caused the autopilot pitch down movement, and, after several seconds, an automatic disconnect. Garmin’s analysis showed that an internal failure on the pitch servo actuator circuit card could cause uncommanded autopilot pitch trim movement before the autopilot would automatically disconnect.

As a result of the investigation, the FAA issued a notice of proposed rulemaking (NPRM) for various airplanes modified with the Garmin 500 GFC Autopilot System. The proposed Airworthiness Directive (AD) would require a mandatory software upgrade to the system. 

Probable Cause and Findings: The National Transportation Safety Board determines the probable cause(s) of this occurrence to
be -- The internal failure of the autopilot pitch servo actuator that caused the autopilot to command a nose down movement of the pitch trim.

FMI: www.ntsb.gov