What happened
On 12 June 2010, a Boeing 757-28A, registration G-TCBA, was operating a commercial passenger flight from Milas-Bodrum, Turkey, to London Gatwick. While cruising at FL360, the flight crew identified a lateral fuel imbalance of 800 kg, with the right wing significantly heavier than the left. Upon attempting to correct this via cross-feeding, the crew noted a discrepancy between the fuel consumed and the fuel remaining, leading to the conclusion that fuel was leaking from the aircraft.
As the aircraft entered French airspace, a FUEL CONFIG warning appeared on the EICAS display. The commander requested a PAN call and was cleared for an immediate approach to Runway 26L at Gatwick without speed or altitude constraints. During the landing roll, the commander shut down the left engine as a precaution against fire. Due to the amount of fuel spilled on the runway, taxiway, and engine, the Airfield Fire and Rescue Services (AFRS) inspected the aircraft, and the airfield was temporarily closed to all movements.
The investigation
Following the landing, the aircraft was moved to a remote stand. Maintenance engineers discovered that the left engine cowl and engine interior were saturated with fuel. The investigation traced the leak to a pipe coupling at the high-pressure (HP) fuel pump oversproll return tube on the left engine.
Investigators found that the seal ring removed from the aircraft had been discarded according to the operator's procedures, preventing a direct examination of the component's condition. However, the investigation reviewed the maintenance history of the fuel supply tube, which had not undergone any maintenance since its installation eight months prior. The investigation also examined global engine data, noting that while the manufacturer had issued a Service Bulletin (SB) to widen the seal ring groove to prevent pinching and subsequent erosion, this specific aircraft had not yet received the modification.
Findings
- The source of the leak was the coupling between the HP fuel pump and the fuel overspill return tube on the left engine.
- The seal ring likely became pinched in the groove during installation, leading to erosion and a loss of sealing capability due to vibration and fuel pressure fluctuations.
- The aircraft had not yet been updated with the manufacturer's recommended modification (SB RB.211-73-G230) to increase the groove width.
- The removal and disposal of the faulty seal ring prevented a definitive physical analysis of the component's failure mechanism.