Late last week,the UK Air Accident Investigation Board (AAIB) issued an Interim Report concerning the British Airways accident involving Boeing 777-236ER, registration G-YMMM which occurred on 17 January 2008 at London Heathrow Airport.
The interim reports seems to confirm the theory that ice was the culprit.
The report says, "The investigation has shown that the fuel flow to both engines was restricted; most probably due to ice within the fuel feed system. The ice is likely to have formed from water that occurred naturally in the fuel whilst the aircraft operated for a long period, with low fuel flows, in an unusually cold environment; although, G-YMMM was operated within the certified operational envelope at all times."
In addition, "Although the exact mechanism in which the ice has caused the restriction is still unknown, in detail, it has been proven that ice could cause a restriction in the fuel feed system."
The report further notes, "All aviation fuel contains water which cannot be completely removed, either by sumping or other means. Therefore, if the fuel temperature drops below the freezing point of the water, it will form ice. The majority of flights have bulk fuel temperatures below the freezing point of water and so there will always be a certain amount of ice in the fuel."
"To prevent the ice causing a restriction requires either: the fuel system must be designed in such a way that the ice in the fuel does not pose a risk of causing an interruption of the fuel supply to the engine or; prevention of the water from becoming ice in the first instance. Changes to the fuel system design could make the system more tolerant, but would take time to implement and would certainly not be available within the near term. Therefore, to reduce the risk of recurrence interim measures need to be adopted until such design changes to the fuel system are available."
As a result of the AAIB report, the 777 pilots have been told to change altitude periodically when fuel in the main tank is below minus 10 degrees Celsius and to advance the throttle to maximum for 10 seconds before the final descent when fuel has been below that temperature for more than three hours to clear any potential ice buildup from fuel lines.