What happened
On 17 January 2008, a British Airways Boeing 777-236 ER, registration G-YMMM, was performing a scheduled passenger flight from Beijing to London Heathrow. During the final stages of the approach to Runway 2 and 7L, the aircraft's autothrottle commanded an increase in thrust. While the engines initially responded, the thrust from the right engine subsequently decreased, followed shortly by a similar reduction in the left engine.
As the aircraft descended, the airspeed dropped to approximately 108 knots. At roughly 175 feet, the autopilot disconnected, causing the aircraft to descend rapidly. The aircraft struck the ground approximately 1,000 feet short of the paved runway surface, within the airfield boundary fence. The impact caused the nose gear to collapse and the right main landing gear to separate from the airframe, penetrating the fuselage and cargo hold. The aircraft came to rest in the undershoot area. While a significant fuel leak occurred, there was no fire. The evacuation of the 136 passengers and 16 crew was successful, though the incident resulted in one serious injury and eight minor injuries among the passengers, along with four minor injuries to the crew.
The investigation
The AAIB investigation examined the aircraft's engines, fuel systems, and flight data. Analysis of the Digital Flight Data Recorder and Cockpit Voice Recorder confirmed that the engine control systems functioned correctly according to the commands provided by the autothrottle. Investigators also examined the fuel system, finding no evidence of mechanical defects or foreign object ingestion in the engines. However, small pieces of debris, including plastic and paper, were discovered in various parts of the fuel tanks.
Engine examinations revealed signs of abnormal cavitation on the pressure-side bearings of the high-pressure fuel pumps, which may suggest fuel supply restrictions or excessive aeration. The investigation also looked into the aircraft's maintenance history and the configuration of the engine spar valves.
Findings
- The primary cause of the undershoot was the reduction in thrust from both engines during the final approach.
- The engines' electronic control systems responded correctly to the autothrottle commands, but the engines failed to maintain the required thrust.
- The right engine spar valve remained in the open position despite the crew's attempt to shut off fuel, because the fire handles were operated before the fuel control switches. This allowed fuel to continue leaking from the aircraft.
- The aircraft's wiring configuration had not yet incorporated a Boeing Service Bulletin intended to prevent the simultaneous loss of both power paths to the spar valves when fire handles are used.
- The aircraft sustained damage beyond economic repair, including a ruptured rear wall of the centre fuel tank and damage to passenger oxygen cylinders.