What happened
On 17 January 2008, a Boeing 777-236 ER, registration G-YMMM, was performing a commercial passenger flight from Beijing to London Heathrow. The flight proceeded normally until the final approach phase onto Runway 27L. While the aircraft was configured for landing with the autopilot and autothrottle engaged, the autothrottle commanded an increase in thrust. While the engines initially responded to this command, a significant malfunction occurred at an altitude of approximately 720 ft. Seven seconds after the right engine's thrust dropped to 1.03 EPR, the left engine's thrust similarly decreased to 1.02 EPR. The aircraft sustained damage beyond economical repair, resulting in 4 minor injuries to the crew and 1 serious injury to a passenger.
The investigation
Investigators examined the aircraft's engine and control systems to identify the cause of the simultaneous thrust loss. The inquiry ruled out external factors such as bird strikes, wake vortex encounters, or engine core icing. There was also no evidence of electromagnetic interference affecting the systems. Analysis of the fuel system, including the spar valves and boost pumps, showed they were fully operational and free of contamination or water. However, a detailed inspection of the high-pressure fuel pumps from both engines revealed fresh cavitation damage at the outlet ports, which is consistent with the engines operating at low inlet pressure.
Findings
- The investigation identified that both engines experienced low fuel pressure at the inlet to the high-pressure pump.
- This low pressure was caused by restrictions within the fuel system located between the aircraft fuel tanks and the engine high-pressure pumps.
- Despite the engine control system detecting the reduced fuel flow and commanding the fuel metering valves to open fully, there was no significant change in fuel flow to either engine.
- While the aircraft flew through extremely cold air, with ambient temperatures reaching -76ºC, the fuel temperature remained at -34ºC, which was well above the freezing point of the Jet A-1 fuel used.
- The investigation continues to focus on the dynamics of fuel flow and the potential for restrictions to form under specific conditions.