After a 'C-check' maintenance inspection a Boeing 757 was on its third test flight after the maintenance was completed. The previous 2 post-maintenance test flights were uneventful and totalled 1 hour and 20 minutes. During this third and final test, prior to release to service approximately 3 hours had elapsed, when the crew noticed a lateral fuel imbalance. The right-wing fuel tank indicated 600 kilograms of fuel less than the left-wing tank. Not long afterwards the EICAS "FUEL CONFIG" advisory message came on, indicating an imbalance of 900 kg. (See editorial note below the blog)
The damaged seal and damaged insert (see the AAIB Report for more details)
The crew completed the necessary Checklist from the QRH (Quick Reference Handbook) for the advisory message, which further directed them to complete the 'Engine Fuel Leak' checklist of the QRH. This checklist called for a visual check of the suspected engine, initially, this was done by a member of the engineering team onboard the aircraft, who reported to the flight crew there was a fuel leak from the right engine. A check by the first officer was carried out to confirm the report of the engineer. The first officer confirmed the report from the engineer.
The captain subsequently made a Pan Pan call to ATC, requesting a direct routing to Newcastle Airport, 85 nm to the southwest of their position. Which he determined to be the closest suitable airport. The crew then completed the 'Engine Fuel Leak' checklist of the QRH by shutting down the right engine. The rest of the flight and the diversion to Newcastle, including the single-engine landing were uneventful. An inspection, by the engineers, 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 (Figure 1). This fuel tube had been installed during the C-check as part of a recommended Service Bulletin, RB.211-73-G230. This Service Bulletin recommends the replacement of earlier standards of fuel tubes that were the source of previous fuel leaks.
When the fuel supply tube was removed from the engine, one of the two bolts that attached 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.
Location of the supply and overspill fuel tubes between the HP fuel pump and fuel flow governor, including detail of the supply tube’s flanged end fitting
The incident was investigated by the Air Accident Investigation Board (AAIB) and in their report, detailed information is available regarding the cause of the fuel leak. In the report, the following is stated regarding the cause of the fuel leak;
"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 in the investigation:
The embodiment of the Service Bulletin whilst the engine was mounted in a transport cradle, which made access to the fuel tubes more difficult than if the engine had been mounted on its pylon,
The bolt‑tightening sequence specified in the Service Bulletin exacerbated the difficulty of aligning the fuel tube to the HP fuel pump and therefore increased the probability of displacement of the O-ring from the tube’s flange groove.
As a result of the incident, the referenced Service Bulletin was revised. The engine was repaired and the aircraft returned to service. The full AAIB report is available for the readers' reference, by clicking on the file below;
** EDITORIAL NOTE ** The AAIB report states the EICAS FUEL CONFIG warning came on. According to Boeing (click here for source) this is an EICAS advisory message. Therefore the blog follows the information from Boeing;
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