What happened
On 5 September 2000, a Boeing 747-367, registration AP-BFY, was conducting a scheduled passenger flight from Karachi to Manchester. During the climb, as the aircraft passed approximately 20,000 feet over the Arabian Sea, the crew detected a medium frequency vibration throughout the airframe. While the crew attempted to troubleshoot the issue by adjusting autopilot and air conditioning settings, the vibration ceased and did not return for the remainder of the flight. The aircraft landed at Manchester without further incident, and no injuries were reported among the 464 persons on board.
Upon post-flight inspection, significant structural damage was discovered on the right side of the aircraft. The impact affected the inboard and outboard flaps, the high-speed aileron, and the movable flap track fairing. Additionally, the No 3 engine showed evidence of internal damage, specifically scrape marks within the bypass duct, and two combustion section side fairings were missing.
The investigation
Investigators examined the aircraft's flight data recorders and cockpit voice recorder. While the recorders could not pinpoint the exact moment the vibration began due to a lack of anomalies in engine parameters between FL180 and FL220, they confirmed the crew's efforts to troubleshoot the vibration during the climb.
Technical analysis focused on the No 3 engine's bypass duct. The investigation found that the two missing fairings, which cover the engine's combustion section, had likely detached during the period of vibration. The inspection of the remaining top fairing revealed a bent projecting lug, suggesting that the front left hook was the final point of separation.
Engineers noted that the No 2 igniter plug on the No 3 engine had been replaced shortly before the flight. Accessing this plug requires the removal or displacement of the combustion section fairings. The investigation also reviewed the maintenance history of the Rolls-Royce RB211-524C2 engines, noting that a previous Service Bulletin had been issued to prevent mal-assembly of these components, yet similar detachment incidents had been recorded in the industry.