What happened
On 17 January 2008, a Boeing 777-236ER, registration G-YMMM, was performing a commercial passenger flight from Beijing to London Heathrow. During the final approach to Runway 2 .7L, the aircraft experienced a sudden and simultaneous reduction in fuel flow to both engines. At an altitude of approximately 720 ft, the thrust on the right engine dropped, followed seven seconds later by a similar reduction in the left engine.
As the autothrottle attempted to maintain the target approach speed, the aircraft's airspeed decreased significantly. The crew attempted to manage the descent, but the lack of available thrust led to a loss of control. The aircraft's nose pitch was reduced by the crew in an attempt to maintain the glideslope, but the aircraft ultimately impacted the runway with a high descent rate, bounced, and underwent a ground slide. The accident resulted in 4 minor injuries to the crew and 1 serious injury to a passenger, while the aircraft was damaged beyond economic repair.
The investigation
The AAIB investigation examined the aircraft's flight data, including the Flight Data Recorder (FDR) and Quick Access Recorder (QAR). Investigators analyzed the engine control systems, specifically the Electronic Engine Controllers (EECs) and Fuel Metering Valves (FMVs). The investigation also included extensive testing by the manufacturer, involving both small-scale 'beaker tests' and full-scale fuel rig testing in a climatic chamber.
Engine testing revealed cavitation marks on the high-pressure pump outlet ports, which were attributed to low inlet pressure caused by a restricted fuel flow. The investigation also reviewed the aircraft's maintenance history, noting that the left engine's EEC had been replaced two days prior to the flight. Furthermore, a large-scale data mining exercise was conducted, reviewing approximately 13,000 flights of Boeing 777 aircraft powered by Rolls-Royce engines to identify similar fuel flow anomalies.
Findings
- The primary cause of the engine rollback was a restriction in the fuel flow to both engines.
- This restriction was most likely caused by ice accumulating within the fuel feed system.
- The ice likely formed from water naturally present in the aviation turbine fuel, which froze during the long period of flight in an unusually cold environment, where fuel temperatures reached as low as -34°C.
- The restriction was likely triggered when the sudden demand for increased thrust during the final approach caused accumulated ice to be released or shifted, obstructing the fuel supply.
- The investigation found no evidence of pre-existing technical defects in the electrical, hydraulic, or flight control systems.
- The fuel used met all standard specifications, and no significant quantities of free water were found in the main tanks, though small amounts of water were present in the engine fuel filters.
Safety action
- Safety Recommendation 2008-047: The AAIB recommended that the FAA and EASA, in coordination with Boeing and Rolls-Royce, implement interim measures for the Boeing 777/Trent 800 combination to mitigate the risk of ice-induced fuel flow restrictions.
- Safety Recommendation 2008-048: The AAIB recommended a review of current certification requirements to ensure that aircraft and engine fuel systems are designed to be tolerant of potential ice build-up and sudden release within the fuel feed system.