What happened
On 3 December 1999, a large metal component fell from a Boeing 757-236, registration G-BIKJ, while the aircraft was performing a public transport flight from London Heathrow Airport to Glasgow. The object, a strut-to-aft fairing seal assembly measuring approximately 1.5 metres by 0.5 metres and weighing roughly 25 lb, struck a house in Mansfield, Nottingham, causing damage to the rear door. The flight crew remained entirely unaware that the component had detached during the flight.
The investigation
The incident came to light following a report from a resident in Mansfield. The loss of the component was subsequently identified two days later during a routine ramp inspection at London Heathrow, where an engineer noticed the missing seal assembly on an engine pylon.
Technical examination of the assembly by maintenance personnel revealed that the retaining bolt, which is intended to secure the seal to the engine afterbody fairing, had not been engaged. The bolt itself was not recovered from the site. Records indicated that the engine on G-BIKJ had been replaced on 16 November 1999, marking the most recent time the seal assembly would have been disturbed.
Findings
- The primary cause of the detachment was the failure to install the seal retaining bolt.
- The assembly is designed to be secured via a bolt passing through a retention bushing into an anchor nut, a process that was not completed during the recent engine change.
- This specific failure mode had been observed previously on similar aircraft, specifically involving registration G-BIKF in 1997.
Safety action
Following a previous similar incident, the operator had implemented a modification to allow the bolt head to be wirelocked. At the time of this investigation, the operator was in the process of upgrading the remainder of its fleet to this standard and was also evaluating the implementation of a modified seal catch retainer to prevent future occurrences.