Fuel Tube Installation Trouble (Jet2 B757 G-LSAN)
On 7 August 2012 Jet2 Boeing 757-2K2 G-LSAN diverted in flight after a fuel leak was detected. The UK Air Accidents Investigation Branch (AAIB) safety investigation report explains that approximately three hours into a post-maintenance check flight the flight crew identified a 600 kg lateral fuel imbalance. They determined this to be due to fuel leak from the right hand Rolls-Royce RB211-535E4 engine. The crew carried out the ‘Engine Fuel Leak’ QRH checklist, shutdown the right engine being shut down and diverted to Newcastle Airport.
Inspection of revealed that the leak was from pump-end flanged joint of the fuel supply tube running between the high pressure (HP) fuel pump and the fuel flow governor.
The fuel supply tube had been installed during the prior C-check as part of recommended Service Bulletin RB.211-73-G230. This SB was developed to the replace earlier standards of fuel tube that were the source of previous fuel leaks (e.g. as occurred to B757-28A G-TCBA on 12 June 2010).
Investigators found one of the two bolts that attached the flange to the HP fuel pump body were only finger-tight and the O-ring seal had been nipped on installation, resulting in a portion being missing.
The AAIB note that:
Both attachment bolts were new items that had been fitted during the installation of the fuel tube, however the bi-hex head of the lower bolt exhibited an unexpected degree of galling consistent with a socket slipping off the head of the bolt during tightening of the bolted-flange joint.
Examination showed the wire-thread insert had been ” ‘picked up’ during insertion of the bolt, causing a progressive roundingover of the bolt’s threads as the bolt was tightened”. However, trials done for the AAIB at an overhaul shop indicated that it was “unlikely that the damaged wire-thread insert caused the lower bolt to remain only partially inserted”. AAIB also checked the torque wrench used and confirmed it was properly calibrated.
AAIB did observe fresh galling and scratch marks both around the lower bolt hole and the periphery of the fuel tube flange interface. This indicated that difficulty was experienced installing the fuel tube. Further investigation revealed that both engine pylons had needed to be removed during the C-check, so the SB had been incorporated with the engine removed from the aircraft and installed in a transport cradle.
The lower parts of the engine, including the area where the fuel tubes were to be replaced, were close to the ground and partially obstructed by the cradle’s steel framework. These restrictions made access significantly more difficult than if the engine had been mounted on its pylon, or in an engine overhaul fixture.
Following the incident the engine manufacturer conducted an investigation into the installation difficulty of both the supply and overspill return fuel tubes as specified in the SB. This activity showed that for both fuel tubes, it was significantly easier for a mechanic to align and torque the HP fuel pump flange bolts before then tightening the fuel flow governor flange bolts, which is the reverse of the bolt tightening sequence specified in the SB. Retention of the O-rings within the grooves in the fuel tube flanges was also identified as a problem, particularly at the HP fuel pump ends where access and visibility are limited. Application of a viscous assembly fluid to specification OMat 1069, which is approved for use on fuel system components, was identified as a suitable measure to retain the O-rings during assembly of the joints.
Based on these findings, Rolls-Royce revised the SB and Engine Maintenance Manual to reverse the order that the bolts be tightened. Th SB now recommends the use of OMat 1069 as an assembly aid.
The maintenance organisation also classified this SB as a ‘Flight Safety Sensitive Task’. Consequently Independent Inspection were required during the critical stages of the task including:
- installation of the O-ring seals,
- lubrication with viscous assembly fluid,
- installation of the fuel tubes and
- torque tightening of the attachment bolts
The failure of the O-ring was insidious in that no leak occurred during the full power ground run and eventual failure of the O-ring under fuel pressure loading occurred only after 4 hours and 20 minutes of flight had elapsed.
The detachment of the pinched section of O-ring reduced the end load on the fuel tube flange, causing the upper attachment bolt to appear to be only finger-tight immediately after the diversion to Newcastle Airport
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Aerossurance worked with the Flight Safety Foundation (FSF) to create a Maintenance Observation Program (MOP) requirement for their contractible BARSOHO offshore helicopter Safety Performance Requirements to help learning about routine maintenance and then to initiate safety improvements.
Aerossurance’s Andy Evans was recently interviewed about safety investigations, the perils of WYLFIWYF (What-You-Look-For-Is-What-You-Find) and some other ‘stuff’ by with Sam Lee of Integra Aerospace:
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