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 2SB, the autothrottles commanded an increase in thrust. While the engines initially responded, the thrust on the right engine dropped to approximately 1.03 EPR at an altitude of 720 ft, followed seven seconds later by a similar reduction on the left engine to 1.02 EPR. The aircraft experienced a dual engine rollback. Despite the loss of thrust, the crew managed to recover normal engine control, and the aircraft landed safely at London Heathrow. There were 142 persons on board, resulting in 1 serious injury and 4 minor injuries among the crew.
The investigation
The AAIB investigation, supported by the NTSB and engine manufacturer Rolls-Royce, examined the fuel system and engine performance. Investigators analyzed flight data recorder (FDR) and cockpit voice recorder (QAR) data, which showed fluctuations in burner pressure and fuel flow. Testing on a fuel rig was conducted to replicate the environmental conditions of the flight. This included simulating the temperature profiles of the fuel and the aircraft's attitude. The investigation also looked at a related incident involving a Boeing 777-200ER, registration N862DA, which experienced a similar right engine rollback during cruise.
Engine manufacturer testing revealed that oil pressure changes were consistent with a restricted Fuel Oil Heat Exchanger (FOHE). Furthermore, the investigation explored how ice might accumulate in the fuel feed pipes and subsequently block the FOHE face during changes in flight phase or temperature.
Findings
- The engine rollback was caused by a reduction in fuel flow resulting from ice accumulation and subsequent release within the fuel system.
- During the flight, fuel temperatures in the main tanks dropped to approximately -34°C.
- Testing established that ice can accumulate on the inside of fuel feed pipes when fuel is at low flow rates and cold temperatures.
- An increase in total air temperature (TAT) during descent, or changes in aircraft pitch, likely triggered the release of this accumulated ice.
- The released ice traveled through the system and caused a restriction at the face of the FOHE.
- While the investigation identified the mechanism of the blockage, it could not definitively rule out other contributing fuel system restrictions.