What happened
On 7 August 2012, a Boeing 757-2K2, registration G-LSAN, was conducting a post-maintenance airtest over the North Sea, approximately 85 nm north-east of Newcastle Airport. During the flight, which was part of a non-revenue commercial air transport operation, the crew identified a lateral fuel imbalance. The right wing fuel tank quantity was approximately 600 kg less than the left wing tank.
Shortly after this discovery, the EICAS FUEL CONFIG warning illuminated as the imbalance increased to 800 kg. Following the procedures in the quick-reference handbook, the crew performed an engine fuel leak checklist. An engineering observer on board confirmed seeing fuel leaking from the right engine. The commander requested a direct routing to Newcastle Airport and shut down the right engine. The aircraft completed a single-engine diversion and landed at Newcastle Airport without further incident. There were no injuries to the two crew members or the single passenger on board.
The investigation
Investigators examined the right engine's fuel system and identified the leak originating from the pump-end flanged joint of the fuel supply tube, located between the high-pressure fuel pump and the fuel flow governor. This specific tube had been recently replaced as part of a Service Bulletin.
Upon removal of the tube, it was discovered that one of the two attachment bolts was only finger-tight and the flange's O-ring seal was damaged, with a section missing. The investigation found that the maintenance had been performed while the engine was in a transport cradle rather than mounted on the aircraft pylon. This positioning, combined with the engine's framework, made access and visibility to the fuel tube joints significantly more difficult. Furthermore, the mechanic had used 'Nyco 65' petroleum grease to help retain the O-rings in their grooves, a substance that is not approved for use in these fuel systems as it may block small orifices.
Findings
- The fuel leak was caused by the O-ring seal becoming trapped in the joint between the tube and the pump body during assembly, leading to its failure under fuel pressure.
- The difficulty in aligning and tightening the bolts was exacerbated by the engine being mounted in a transport cradle, which restricted visual and physical access.
- The assembly sequence specified in the Service Bulletin contributed to the risk of O-ring displacement.
- The use of an unapproved lubricant (Nyco 65) was noted during the assembly process.
Safety action
- The engine manufacturer is revising the Service Bulletin and engine manual to reverse the required bolt-tightening sequence for the fuel tubes.
- The revised instructions will also recommend using a specific approved viscous assembly fluid (OMat 1069) to assist in retaining O-rings during installation.
- The maintenance organisation is issuing a Quality Advisory Notice to its staff regarding the critical stages of this task, including O-ring installation and torque application.