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On January 17 of 2008, British Airways Flight 038 was approaching London's Heathrow Airport after an uneventful but very long non-stop flight from Beijing, China. The twin engine Boeing 777-200ER entered the final approach path to Runway 27L and the crew configured the aircraft for an automatic landing. The approach proceeded normally until the aircraft descended through 720 feet above the ground. This would have been only 1-2 minutes prior to landing.
At this point, there was a significant loss of thrust in the right engine, followed a few seconds later by a similar loss of thrust in the left engine. The engines did not quit, but were producing much less than the level of thrust being commanded by the autothrottle (the electronic engine management system that is used during automated flight). The autothrottle attempted to overcome this by commanding even more thrust, but to no appreciable response. Finally, the crew manually commanded maximum thrust, but there was still no increase in thrust from the engines.
With the aircraft configured for landing (low speed, low altitude, and increased drag from the flaps and landing gear) the diminished thrust from the engines was insufficient to carry the plane the remaining distance to the runway. The pilots had no choice but to set the aircraft down as gently as possible onto the grassy field. Nevertheless, the aircraft touched down with a descent rate of 1400 feet per minute (a very hard landing) and bounced once before sliding to a stop. There were no fatalities or serious injuries.
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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.
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The investigation is not yet complete, but the AAIB has already made recommendations that further studies be carried out by the manufacturer on possible modifications to the Boeing 777. They also recommended that in the meantime, that precautionary procedures be put in place to reduce the likelihood of the problem repeating itself. These measure include the use of fuel additives, and also recommendations that aircraft crews should avoid flying the aircraft where colder temperature ranges are expected. This might be accomplished by flying at different altitudes, and or by modifying routes through areas where lower temperature levels are predicted.