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 27L, the aircraft was configured for landing with the autopilot and autothrottle engaged. At approximately 720 feet, the right engine experienced a significant reduction in thrust, followed seven seconds later by a similar reduction in the left engine.
In an attempt to maintain the glideslope, the commander selected flap 25, which caused the airspeed to drop. The aircraft's stick shaker activated at 170 feet, and a nose-down pitch input by the First Officer caused the autopilot to disconnect. The aircraft impacted the runway with a descent rate of approximately 1,400 fpm, bounced, and then underwent a ground slide. The incident resulted in 1 serious injury and 12 minor injuries among the passengers and crew. The aircraft sustained damage beyond economic repair.
The investigation
The AAIB investigation examined the aircraft's flight data, including the FDR, C2008/01/01, and the QAR. Analysis of the engine control systems showed that the Electronic Engine Controllers (EECs) attempted to compensate for the thrust loss by commanding the Fuel Metering Valves (FMVs) to the fully open position. However, the actual fuel flow remained significantly lower than required.
Investigators also reviewed the fuel system, noting that the fuel temperature had reached a minimum of -34°C during the flight. While the fuel itself did not reach its waxing point, the investigation focused on the potential for ice crystals from entrained water to cause a restriction. Testing by the manufacturer using a fuel rig and specialized engine test cells demonstrated that ice could indeed accumulate within the fuel feed system and cause a restriction in fuel flow.
Findings
- The primary cause of the engine rollback was a restriction in fuel flow occurring somewhere upstream of the high-pressure pump inlet.
- The restriction was likely caused by ice within the fuel feed system, which may have formed from water naturally present in the fuel during a period of low fuel flow and cold temperatures.
- The investigation identified two possible scenarios: either ice accumulated over time at a location downstream of the forward boost pump connection, or ice crystals were released from elsewhere in the system during the increased thrust demand of the final approach.
- The aircraft's fuel temperature reached -34°C, which is within the range where ice crystals can adhere to surfaces and potentially block components.