Incident Overview
During cruise flight at Flight Level 350, the aircraft initiated an uncommanded descent and pitch-up maneuver. At the time of the event, the flight engineer was reconfiguring the electrical system from a Category III to a Category I landing configuration. The specific action that triggered the anomaly was the closing of the number one bus-tie breaker. This resulted in numerous momentary instrument failures and an uncommanded pitch-up. Twelve occupants sustained injuries during the sequence.
Crew Response and Recovery
The aircraft was operating with the "A" autopilot system engaged. Upon experiencing the pitch-up, the pilot disconnected the autopilot, leveled the aircraft, and subsequently re-engaged the system. The flight crew completed an uneventful landing following these corrective actions. During a subsequent ferry flight, maintenance personnel successfully duplicated the sudden pitch-up by utilizing the "B" autopilot system and closing the number two bus-tie breaker. Crew reports indicated that the aircraft felt light in pitch.
Investigation Findings
Data from the flight data recorder and optical quick access recorder confirmed an electrical discontinuity coinciding with the pitch-up event. A physical inspection revealed that the pitot connections for the elevator feel computer, specifically for numbers 1 and 2, were capped. Maintenance records indicated the aircraft had recently undergone an "inter 2 check" at a British Airways facility. During this maintenance, personnel disconnected these pitot connections to perform pitot-static system checks but failed to reconnect them properly.
Technical Analysis
The disconnection of the pitot-static lines altered the elevator feel computer's function, leading to more extreme travel of the elevator control surface than intended. Calculations showed that 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. A review of the Boeing basic airplane maintenance manual identified a required test to confirm the reconnection and functionality of the elevator feel computer. However, this specific test was absent from the maintenance manual utilized by British Airways, which was provided by Boeing. While the exact source of the initial pitch-up command to the autopilot was not determined, the resulting pitch characteristics were within expected values when the system was properly configured.