Event Description
While climbing in smooth air at approximately 500 feet per minute with the No. 1 autopilot engaged, the aircraft experienced a sudden and hard uncommanded 2g pull-up. The control yoke moved rapidly aft. The captain immediately grabbed the control yoke, disengaged the autopilot, and leveled the aircraft.
Three flight attendants in the aft galley and one passenger in an aft lavatory sustained serious injuries. The flight attendants described the onset as 'being pulled to the floor by what felt like a strong pull of gravity.' The force then suddenly reversed, throwing them upward into the ceiling. Another force reversal followed, slamming them down against the floor. Subsequently, a 'roller coaster' type movement occurred, which quickly damped into a steady state.
Maintenance and Flight Data Recorder Findings
Review of the aircraft's maintenance records for the preceding year revealed over 50 discrepancies for the autopilot system, including uncommanded disconnects, uncommanded pitch-ups, and failures to engage.
Analysis of the Digital Flight Data Recorder (DFDR) data showed that as the aircraft passed through 29,200 feet, four pitch cycles were recorded over a 15-second period. These cycles were accompanied by vertical accelerations, with the most severe ranging from 1.84 g to -0.12 g. The initial uncommanded nose pitch-up was preceded by an autopilot-controlled movement of the left inboard elevator. Subsequent elevator movements and pitch excursions were due to the pilot's control inputs.
Postaccident Examination
All autopilot systems passed functional checks except for the captain's and first officer's control wheel sensor units. Postaccident testing of the first officer's control wheel sensor unit revealed an out-of-tolerance and drifting null signal for the strain gauge that provides pitch signal input to the No. 1 autopilot. After about three minutes, the signal became noisy and jumped to values of up to 4 volts several times. Many spikes were also observed at values under 3.5 volts; signals above 3.5 volts trigger an automatic autopilot disengagement.
Further examination of the pitch strain gauges under optical magnification found a foreign black-gray metallic-like substance bridging the terminal lug ends. Analysis identified the material as a silver-based conductive substance, located below a factory-applied sealing layer, introduced during manufacture. Additionally, the solder on the lugs and the wire between the lugs and terminals did not conform to the manufacturer's specifications.