19980521015609A

Smooth air autopilot on. Pitched up suddenly. Cutoff auto pilot. Put nose down. Crew and passenger injury. Sign on. (.4)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. Reveiw of the dfdr data revealed that as the aircraft passed through 29,200 feet, four pitch cycles were reocrded over a 15-second time period and were accompanied by wertical 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 anf resultant pitch excursions were due to the pilot's control inputs. With the exception of the captain's anf first officer's control wheel sensor units, all autopilot systems passed functional checks. Postaccident test of the first officer's control wheel sensor units 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 th e 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. (-23)during interview, privacy data omittedstated that #1 autopilot was engaged, seat belt sign still illuminated, aircraft was climbing to FL 330, and flight conditions were VMC and smooth. At approximately FL 310 the aircraft pitched up rapidly with simultaneous aft control yoke movement. privacy data omitted Estimated approximately a +2 g pitch up, altitude gain of 1,000 to 1200 feet, and a loss of approximatley 30 knots airspeed. privacy data omitted Countered the aft yoke movement, disengaged the autopilot, and slowly recovered the aircraft attempting to minimize negative g's. he stated the aircraft was intrim, autothrottles stable in climb power, and fight conditions were smooth after pitch up. He noted no wind velocity or direction change on the long range navigation wind read out that would indicate tha the pitch up was caused by clear air turbulence.