What happened
On 12 July 2016, a Boeing 767-322, registration N642UA, was operating a scheduled passenger service from London Heathrow to Chicago O’Hare. Roughly 35 minutes after departure, the crew was alerted by a passenger to a fuel leak originating from the right over-wing fuel filler cap. The crew declared a MAYDAY and diverted to Edinburgh Airport. To ensure the aircraft was within safe landing weight limits, the crew coordinated with Air Traffic Control to dump fuel before landing without incident. Following the landing, emergency services inspected the right wing and engine area, and the aircraft was moved to a remote stand for disembarkation.
Maintenance personnel at Edinburgh replaced the fuel filler cap and filled the tanks to capacity. No leaks were detected during the subsequent overnight period. However, during a ferry flight from Edinburgh to Chicago the following day, the flight crew observed fuel streaming from the same location on the right wing during the climb. A second MAYDAY was declared, and the aircraft returned to Edinburgh. Because the aircraft was already below maximum landing weight, fuel dumping was not required for this second landing.
The investigation
The AAIB investigation focused on the cause of the recurring leak. While the initial repair involved replacing the filler cap itself, subsequent maintenance inspections following the second diversion identified the true source of the leak. Investigators examined the over-wing fuelling port and discovered a damaged O-ring within the assembly.
Findings
- The investigation established that the primary cause of the fuel leaks was a damaged O-ring located in the over-wing fuelling port.
- The damage to the O-ring likely occurred because it was pinched or improperly seated during its last installation.
- There were no maintenance records indicating that the over-wing fuelling port had been serviced during the most recent heavy maintenance check in June 2015 or at any time since.