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's autothrottles commanded an increase in thrust. While the engines initially responded, at approximately 720 feet above the ground, the right engine experienced a significant thrust reduction to an Engine Pressure Ratio (EPR) of roughly 1.03. This was followed seven seconds later by a similar reduction in the left engine to 1.02 EPR. The event resulted in 4 minor injuries to crew and 1 serious injury to a passenger, with the aircraft sustaining damage beyond economic repair.
The investigation
The AAIB investigation focused on the fuel system and engine performance. Investigators examined flight data, including engine oil pressure and temperature, and conducted extensive fuel rig testing. The testing aimed to replicate the environmental conditions of the flight, specifically the low fuel temperatures and water concentrations. The investigation also looked into a related event involving a Boeing 777-200ER in the United States to identify common characteristics. Testing on a fuel rig demonstrated that ice could accumulate within the fuel feed pipes and subsequently release, causing a restriction at the Fuel Oil Heat Exchanger (FOHE).
Findings
- The primary cause of the engine rollback was a reduction in fuel flow caused by ice accumulation and subsequent release within the fuel system.
- During the flight, fuel temperatures in the main tanks dropped to approximately -34°C, placing the fuel in a "sticky range" where ice adheres more readily to pipe walls.
- Ice built up in the fuel feed pipes, particularly in the engine pylon (strut) area, and was released during the approach, likely triggered by changes in aircraft pitch, turbulence, or increased temperature in the strut area during descent.
- This released ice traveled through the system and blocked the face of the FOHE, restricting the fuel flow required to maintain engine thrust.
- The fuel used contained sufficient water concentrations to allow for this icing process.