Aircraft Accident/Incident Report

Los Angeles, California 90045
Thursday, May 21, 1998 13:05 PDT

The aircraft was climbing in smooth air about 500 feet per minute with the No. 1 autopilot engaged. The captain reported that the aircraft began a sudden and hard uncommanded 2g pull-up, with the control yoke moving rapidly aft. He 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 aft galley flight attendants described the onset of the event as 'being pulled to the floor by what felt like a strong pull of gravity.' The force suddenly reversed and the three were 'thrown up into the ceiling.' Another force reversal followed and the three were 'slammed down against the floor.' The flight attendants said that a 'roller coaster' type movement then occurred, which quickly damped into a steady state. Review of the aircraft's maintenance records for the year preceding the accident revealed over 50 discrepancies for autopilot system uncommanded disconnects, uncommanded pitch-ups, and failures to engage. Review of the DFDR data revealed that as the aircraft passed through 29,200 feet, four pitch cycles were recorded over a 15-second time period and were accompanied by vertical accelerations, the most severe of which was between 1.84 and -0.12 g's. The initial uncommanded nose pitch-up was preceded by an autopilot controlled movement of the left inboard elevator. The subsequent elevator movements and resultant pitch excursions were due to the pilot's control inputs. With the exception of the captain's and first officer's control wheel sensor units, all autopilot systems passed functional checks. Postaccident test of the first officer's control wheel sensor unit showed an out of tolerance and drifting null signal for the strain gage which provides pitch signal input to the No. 1 autopilot. After about 3 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 over 3.5 volts trigger an automatic disengagement of the autopilot). Subsequent examination of the pitch strain gages by optical magnification found a foreign black-gray metallic-like substance bridging the terminal lug ends. Analysis showed the material was a silver based conductive substance, lying below a factory applied sealing layer, which was introduced during manufacture. The solder on the lugs and the wire used between the lugs and terminals was found not to be consistent with the manufacturer's specifications.

The contaminated strain gage, which resulted in shorting of the strain gage's terminal lugs which lead to excessive autopilot initiated elevator movement, and excessive elevator actuation during recovery by the captain. Contributing factors were the failure of the airline maintenance department to diagnose and correct a historical problem with the autopilot system and the manufacturer's inadequate quality assurance program.