What happened
While in cruise flight descending from flight level 350, the crew began reconfiguring the aircraft's electrical system from a Category III landing configuration to a Category I landing configuration. As the flight engineer closed the number one bus-tie-breaker, the airplane experienced an uncommanded pitch-up along with several momentary instrument failures.
During the event, 12 occupants were injured. The aircraft was operating with the "A" autopilot system engaged, which remained connected throughout the maneuver. The pilot responded by disconnecting the autopilot, leveling the aircraft, and subsequently re-engaging the autopilot to complete an uneventful landing. During a later ferry flight, maintenance personnel successfully duplicated the sudden pitch-up behavior using the "B" autopilot system while closing the number two bus-tie-breaker. The crew also noted that the airplane felt light in pitch.
The investigation
Data retrieved from the flight data recorder and optical quick access recorder showed an electrical discontinuity coinciding with the pitch-up event. A physical inspection of the aircraft revealed that the pitot connections for the number 1 and 2 elevator feel computers had been capped.
Maintenance records indicated the aircraft had recently undergone an "inter 2 check" at a British Airways maintenance facility. During this maintenance period, technicians had disconnected the pitot-static lines to the elevator feel computer to perform system checks. The investigation determined that leaving these lines disconnected caused the elevator control surface to undergo more extreme travel than intended. While the expected autopilot elevator authority was approximately 4 degrees, the actual deflection during the incident reached 6.87 degrees nose up and 6.97 degrees nose down.
Further review of the Boeing basic airplane maintenance manual showed a specific test designed to confirm the reconnection and functionality of the elevator feel computer. However, this specific test was missing from the maintenance manual used by British Airways.