Flight 179 · Boeing 747-236

The airplane was in cruise flight when it began a descent from flight level 350. At the same time, the flight engineer was reconfiguring the airplane's electrical system from a Category III landing to a Category I landing. When the flight engineer closed the "number one bus-tie-breaker," the airplane experienced an uncommanded pitch-up, accompanied by numerous momentary instrument failures. Twelve occupants were injured. The airplane was utilizing the "A" autopilot system, which remained engaged. The pilot disconnected the autopilot, leveled the airplane, re-engaged the autopilot, and continued to an uneventful landing. During a ferry flight, maintenance personnel were able to duplicate a "sudden pitch-up" while using the airplane's "B" autopilot system, and closing the "number two bus-tie-breaker." Additionally, the flight crew reported that the airplane "felt light in pitch." Examination of data obtained from the flight data recorder and optical quick access recorder revealed an electrical discontinuity around the time of the pitch-up. An inspection of the airplane revealed that the number 1 and 2, elevator feel computer pitot connections were capped. Review of the airplane's maintenance history revealed that the airplane had recently undergone an "inter 2 check" at a British Airways maintenance facility. During that time, maintenance personnel disconnected the pitot connections to the elevator feel computer in order to perform pitot static system checks. The effect of the disconnected pitot-static lines on the elevator feel computer would have resulted in a more extreme travel of the elevator control surface. The calculated expected autopilot elevator authority for the accident flight was about 4 degrees. The estimated actual elevator deflection during the accident sequence was 6.87 degrees nose up, and 6.97 degrees nose down. Review of the Boeing basic airplane maintenance manual section that detailed the pitot-static system checks revealed a test to confirm that the elevator feel computer was reconnected and functioned. The test was not present the maintenance manual utilized by British Airways, which was provided by Boeing. The source of the pitch-up command to the autopilot was not determined; however, when the autopilot system was properly configured, the pitch-up characteristics were not objectionable and within expected values.

Verbatim NTSB analysis and probable cause from the NTSB dataset