Incident Overview
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. The aircraft had undergone a C-check inspection and had accumulated about three hours of flight time when the flight crew identified a lateral fuel imbalance. The imbalance was approximately 600 kg, with the right wing fuel tank indicating less fuel than the left. Shortly afterward, the EICAS FUEL CONFIG warning illuminated as the imbalance increased to 800 kg.
Crew Actions
The flight crew carried out the 'Fuel Configuration' section of the Quick-Reference Handbook (QRH), which directed them to complete the 'Engine Fuel Leak' checklist. An engineering observer aboard the aircraft reported visible fuel leaking from the right engine, which was confirmed by the first officer. The commander made a PAN radio call to Scottish ATC and requested a direct routing to Newcastle Airport, the closest suitable airport. The crew then completed the 'Engine Fuel Leak' checklist by shutting down the right engine. The aircraft conducted a single-engine diversion and landing at Newcastle Airport without further incident.
Engineering Inspection
Inspection of the right engine's fuel system revealed that the source of the fuel leak was the pump-end flanged joint of the fuel supply tube running between the high pressure (HP) fuel pump and the fuel flow governor. This fuel tube had been installed during the C-check as part of a recommended Service Bulletin (SB) RB.211-73-G230, which replaced earlier fuel tube standards prone to leaks. When the fuel supply tube was removed, one of the two bolts attaching the flange to the HP fuel pump body was found to be only finger-tight, and the flange's O-ring seal was damaged, with a section missing. Both attachment bolts were new items fitted during installation. The bi-hex head of the lower bolt exhibited galling consistent with a socket slipping off during tightening. The threads on the lower bolt were damaged, as was the start thread of the wire-thread insert in the pump body, which was displaced upward with metallic debris.
Examination showed that the bolt had not been cross-threaded; rather, the start thread of the insert had 'picked up' during insertion, causing progressive thread damage. A trial at the HP fuel pump manufacturer's facility using a new bolt showed that similar thread damage occurred, yet the specified bolt torque (102 lbf-in) was achievable when the joint was fully fastened. It was thus unlikely that the damaged insert caused the lower bolt to remain only partially tightened despite correct torque application. The torque wrench used during installation was checked and found serviceable within calibration limits. The pump body face exhibited fresh galling and scratch marks around the lower bolt hole and flange interface, indicating assembly difficulty.
Maintenance Context
The Service Bulletin was accomplished while the right engine was removed from the aircraft and installed in a transport cradle, as both pylons had been removed for other maintenance. This placed the lower engine parts, including the fuel tube area, close to the ground and partially obstructed by the cradle's framework, making access significantly more difficult. The mechanic described access as 'not good' and experienced initial difficulty retaining the O-rings using fuel as a lubricant, eventually applying Nyco 65 petroleum grease, which is not approved for fuel systems due to potential blockage issues. The mechanic tightened the fuel tube bolts according to the SB's sequence: tightening three bolts at the fuel flow governor end before the two bolts at the HP fuel pump. Both flanged joints appeared properly seated. The engines were ground-run at full power post-maintenance with no leaks detected, and the aircraft flew two uneventful sectors before the incident.
Engine Manufacturer's Investigation
The engine manufacturer investigated the installation difficulty of the fuel tubes and found that it was significantly easier to align and torque the HP fuel pump flange bolts before tightening the fuel flow governor bolts—the reverse of the SB's specified sequence. Retention of O-rings in the grooves was also problematic, particularly at the HP fuel pump ends where access and visibility were limited. The manufacturer identified a suitable viscous assembly fluid (OMat 1069), approved for fuel system components, to aid O-ring retention.
Conclusion
The fuel leak was caused by the fuel supply tube's O-ring seal becoming trapped in the joint between the tube and the HP fuel pump body during assembly, before subsequently failing under fuel pressure load during flight. Two contributory factors were identified: embodiment of the Service Bulletin while the engine was mounted in a transport cradle, making access more difficult, and the bolt-tightening sequence specified in the Service Bulletin that exacerbated alignment difficulty and increased the probability of O-ring displacement.
Safety Actions
The engine manufacturer is revising the Service Bulletin and engine manual to require that the fuel flow governor supply and overspill return tubes have their bolts tightened in the opposite order to that presently specified. The modified text will also recommend applying a viscous assembly fluid (OMat 1069) to aid O-ring retention. The maintenance organisation is drafting a Quality Advisory Notice to communicate the findings.