What happened
On 10 June 2004, a Boeing 777-236, registration G-YMME, was performing a scheduled passenger flight from London Heathrow to Harare, Zimbabwe. Shortly after departing from Runway 27L, ground personnel and other aircraft reported seeing a trail of smoke and smelling fuel vapour trailing from the rear of the aircraft.
As the aircraft climbed to 6,000 feet, the crew noted no internal flight deck indications of a leak, but the external reports of a two-mile vapour trail persisted. Suspecting a leak from the centre tank, the pilots declared an emergency and began dumping fuel to reach a safe landing weight. After 25 minutes of dumping fuel over the sea, the aircraft returned to Heathrow for an ILS approach. Upon landing, the Airfield Fire and Rescue Service observed vapour near the left main landing gear, though no active leaks were found. The aircraft was taxied to a stand, and all 151 passengers and 15 crew members disembarked without injury.
The investigation
Following the incident, engineers inspected the aircraft and discovered fuel drips on the left main landing gear. Upon opening the gear bay door, a strong smell of fuel was detected. The investigation revealed that the centre fuel tank purge door was missing from its proper position; instead, it was found hanging on a lanyard inside the tank. A plastic bag, which contained the screws intended to secure the door, was also found inside the tank and had collected fuel.
Investigators examined recent maintenance records and found that a heavy 2C maintenance check had been performed between 2 May and 10 May 2004. This check required access to the centre fuel tank for structural inspections. While the maintenance organisation had raised job cards for several access doors, no specific job card had been created for the removal or refitting of the purge door. Furthermore, the investigation found that the rear spar inspection had been performed incorrectly because the maintenance manual contained an outdated, incorrect diagram.
Findings
- The purge door was left unsecured and unattached, allowing fuel to escape during the climb-out phase.
- The maintenance procedure for draining and purging the tank did not explicitly reference the need to manage the purge door.
- Existing leak detection procedures for the centre tank were insufficient, as the required fuel level for a leak check (40 tonnes) was below the level of the purge door opening.
- The maintenance organisation's database contained an incorrect inspection diagram, which led to the failure to properly inspect the rear spar.