What happened
While maintaining level flight at FL290 during a scheduled international passenger service from San Francisco, California, to London, England, the aircraft experienced erratic elevator pitch control. These malfunctions caused altitude oscillations of up to 1,000 feet on either side of the assigned base altitude.
In response to the instability, the crew disconnected the autopilot and established communications with maintenance control. The flight crew decided to divert the aircraft to the nearest airport capable of accommodating the flight with runways exceeding 10,000 feet in length. The aircraft subsequently performed an uneventful precautionary landing at Denver, Colorado, with no injuries reported.
The investigation
An examination of the stabilizer trim components was conducted following the incident. The inspection revealed discrepancies within both the trim module and the elevator feel computer. These discrepancies were attributed to normal wear and tear of the components.