What happened
The aircraft was climbing through smooth air at approximately 500 feet per minute with the No. 1 autopilot engaged when it experienced a sudden and hard uncommanded 2g pull-up. During this event, the control yoke moved rapidly aft. The captain responded by grabbing the control yoke, disengaging the autopilot, and leveling the aircraft.
The event resulted in 4 serious injuries, involving three flight attendants in the aft galley and one passenger in an aft lavatory. The flight attendants reported a sequence of intense vertical forces, describing being pulled to the floor, thrown into the ceiling, and slammed back down against the floor in a movement similar to a roller coaster before the aircraft reached a steady state.
The investigation
Review of the digital flight data recorder (DFDR) showed that as the aircraft passed through 29,200 feet, four pitch cycles occurred over a 15-second period. These were accompanied by vertical accelerations ranging from 1.84 to -0.12 g's. The initial uncommanded nose pitch-up was preceded by an autopilot-controlled movement of the left inboard elevator; subsequent pitch excursions were caused by the pilot's manual control inputs.
A review of maintenance records from the preceding year identified more than 50 discrepancies related to the autopilot system, including uncommanded disconnects, failures to engage, and uncommanded pitch-ups.
While most autopilot systems passed functional checks, testing of the first officer's control wheel sensor unit revealed an out-of-tolerance drifting null signal in the strain gage that provides pitch signal input to the No. 1 autopilot. The signal became noisy and jumped to values as high as 4 volts, with several spikes occurring at levels that would trigger an automatic autopilot disengagement.
Upon microscopic examination of the pitch strain gages, investigators found a foreign silver-based conductive substance bridging the terminal lug ends. This material was located beneath a factory-applied sealing layer and had been introduced during manufacturing. Additionally, the solder on the lugs and the wire used between the lugs and terminals did not meet manufacturer specifications.