What happened
On 6 December 1999, a Boeing 757-236, registration G-BMRC, was operating a scheduled public transport flight from London Heathrow to Manchester. While climbing through FL100 in the vicinity of Woburn, the flight crew felt a minor jolt. Shortly after this event, cabin crew notified the cockpit that a passenger had heard a loud thud originating from the right side of the aircraft.
Despite the noise, the aircraft handling and all onboard systems remained normal. Due to the darkness at the time of the occurrence, the crew could not perform a visual inspection of the exterior. Upon landing at Manchester, an engineering inspection was requested. Initial checks by the first officer and engineers failed to identify any defects, leading the crew to believe the noise may have been caused by shifting baggage.
The investigation
Following an air traffic control delay while the aircraft was parked at the stand, an engineer passing the aircraft noticed a missing panel on the wing-to-body fairing. The missing section was approximately 3 feet square. The investigation focused on the attachment points of the fairing to determine how the panel had detached.
Investigators found that the panel had separated from the aircraft along its lower edge, though the lower edge itself remained attached to the airframe. Because the panel was not recovered, the specific condition of all fasteners could not be examined. However, an inspection of the remaining hardware revealed that two anchor nuts for the leading edge fasteners had been cross-threaded. All other fasteners were found to be in serviceable condition with appropriate rundown torques.
Findings
- The investigation established that the panel detached along its forward edge.
- The attachments at the top and rear of the panel failed due to overload.
- Two anchor nuts for the leading edge fasteners were identified as being cross-threaded.
- The panel had been removed for maintenance in April 1999 and had undergone a routine security check in October 1999.