What happened
While climbing through FL200, the autopilot pitch trim master caution warning light and the autopilot pitch trim nose down master caution warning light illuminated. The stabilizer trim became stuck at four units of trim. Following established AFM procedures, the flight crew disengaged the autopilot to address the malfunction. To maintain level flight, the crew was required to maintain approximately 2 inches of forward pressure on the flight control column. The first officer noted that while the stabilizer trim could be manually adjusted up or down, it would not move below the four-unit position.
The investigation
Maintenance crews replaced the Horizontal Stabilizer Trim Actuator (HSTA) and the Motor Control Unit (MCU). Following these replacements, the aircraft underwent diagnostic checks and a functional flight test, which showed no further mechanical anomalies. During the removal of the HSTA and MCU, a component was heard moving inside the HSTA.
A teardown of the HSTA revealed that the Channel 1 bell gear had separated from its four mounting tabs. Additionally, investigators found that two of the main spur gear bolts had backed out and were loose and out of position within the HSTA, while the remaining two bolts were only finger-tight.
Findings
Investigation into the component failure revealed a history of similar issues. Service Bulletin 8396-27-02 was developed following approximately 66 instances of bell gear failures over a six-year period; this SB recommended replacing the existing bell gear with a stronger version. Furthermore, Service Letter 8396-27-01 addressed the issue of main spur gear bolts backing out in service, mandating the use of Loctite and new torque values for the bolts. A review of the operator's FAA-approved Operating Specifications indicated that compliance with these manufacturer service bulletins and service letters was not mandatory.