America West Express Flight 6088

While climbing through FL200, the autopilot pitch trim master caution warning light and the autopilot pitch trim nose down master caution warning light illuminated, and the trim froze at four units of trim. The autopilot was disengaged in accordance with AFM procedures to resolve the problem. The flight crew had to maintain about 2 inches of forward pressure on the flight control column to maintain level flight. The first officer reported that the stabilizer trim could be manually adjusted up and down, but at no point would the trim go below four units of trim. The Horizontal Stabilizer Trim Actuator (HSTA) and Motor Control Unit (MCU) were replaced, and the airplane underwent a series of diagnostic checks and a functional flight test with no mechanical anomalies noted. During removal of the HSTA and MCU a component was heard moving inside the HSTA. Teardown of the HSTA revealed that the Channel 1 bell gear had separated from its four mounting tabs. It was also noted that two of the main spur gear bolts had backed out, and were loose and out of position in the HSTA. The other two main spur gear bolts were finger-tight, but remained in place. SB 8396-27-02 was developed as a result of repeated bell gear failures, about 66 in the past 6 years. The SB called for replacement of the existing bell gear with a new stronger bell gear. SL 8396-27-01 resulted from repeated main spur gear bolts backing out in service. The main spur gear bolts now require Loctite be used with the new bolts and new torque values be applied to the bolts. Review of the operator's FAA approved Operating Specifications disclosed that compliance with manufacturers SB's and SL's is not mandatory.

Verbatim NTSB analysis and probable cause from the NTSB dataset