On September 1, 2008, the Air Accidents Investigation Branch issued an interim report on the progress of the investigation. According to the report, the investigators have ruled out major mechanical problems with the aircraft, including the engines, the fuel pumps, and the electronic engine control system. All of these systems were found to be functioning normally during the flight.
The investigation has concluded that the reductions in thrust from the engines were most likely the result of reduced fuel flow to the engines due a physical restriction caused by ice build-up within the fuel system. Earlier in the investigation, fuel contamination with water was high on the list of suspected causes. This normally happens as a result of poor quality control on the part of the fuel company. However, according to the report, the water content level in fuel samples taken from the aircraft after the crash were within normal tolerances. Instead, the ice that caused this incident is believed to have built up over the duration of the long flight from trace amounts of water that are always present in the fuel.
This is the first time that a major incident such as this is believed to be the result of ice forming in this manner. It was noted in the report that the outside air temperature was unusually cold for a significant portion of the flight, and that as a result the fuel temperature was also quite cold (though not colder than regulations permit). The basic hypothesis is that these temperatures may have resulted in the gradual formation of ice crystals, which at some point began to adhere to critical structures within the airplane's fuel system, in turn creating a blockage and reducing the flow of fuel to the engines. It is not fully understood precisely where these blockages occurred, nor is it fully understood why they happened precisely when it did. It is also difficult to say if this occurred because of something unique to the 777's fuel system or whether all jet powered airliners might be vulnerable.