What happened
On 17 January 2008, a Boeing 777-236 ER, registration G-YMMM, was performing a scheduled passenger flight from Beijing to London Heathrow. The flight proceeded normally through the descent and approach phases until the final stages of the landing.
While established on the ILS approach for Runway 27L, the aircraft was fully configured for landing. At a height of approximately 720 ft, the thrust from the right engine reduced. Seven seconds later, the thrust from the left engine dropped to a similar level. Despite the flight crew advancing the thrust levers to the full forward position, the engines failed to respond to the increased demand.
As the airspeed dropped to approximately 108 kt, the autopilot disconnected. At 175 ft, the aircraft entered a rapid descent, making contact with the ground roughly 1,000 ft short of the paved runway surface. The impact caused the nose gear to collapse and the right main landing gear to separate from the aircraft. The departing gear struck the fuselage, penetrating the cabin, and damaged the wing-to-body fairing. The aircraft eventually came to rest on the paved surface in the undershoot area. While a significant fuel leak occurred, there was no fire, and the crew successfully supervised the evacuation of the 136 passengers and 16 crew members. The accident resulted in one serious injury and eight minor injuries.
The investigation
The AAIB investigation focused on the engine control systems and the fuel system. Investigators examined the engines and found no evidence of mechanical defects, bird strikes, or ice ingestion. Data from the Electronic Engine Controllers indicated that the engine control systems functioned correctly, responding to the reduction in fuel flow.
Examination of the fuel pumps revealed signs of abnormal cavitation on the pressure-side bearings and outlet ports, which may suggest a restriction in fuel supply or excessive aeration. The investigation also looked into the fuel temperature, noting that the aircraft had flown through a region of extremely cold air. While the minimum recorded fuel temperature was -34ºC, the actual freezing point of the Jet A-1 fuel on board was found to be -57ºC.
Findings
- The primary cause of the accident was a reduction in thrust from both engines that the crew could not recover through manual thrust lever movement.
- The right engine thrust reduced first, followed by the left engine seven seconds later.
- The fuel temperature in flight was significantly higher than the actual freezing point of the fuel, though the fuel itself met specifications.
- The separation of the right main landing gear caused structural damage to the fuselage and the rupture of the rear right wall of the centre fuel tank.
- The high-pressure fuel pumps showed evidence of cavitation, potentially linked to fuel supply issues or aeration.