Air bubbles in fuel could have caused the British Airways Boeing 777 crash-landing at Heathrow, investigators said on Monday night.
The Air Accidents Investigation Branch has focused its attention on the plane's high-pressure fuel pumps, after a study found they were damaged and there was debris in the fuel tanks.
Heathrow plane crash may have been caused by air in the fuel
The AAIB found no evidence of a mechanical failure
If air got into the fuel it would have prevented the pumps working properly. This could have led to inadequate fuel reaching the engines, which then failed to provide the thrust needed to land.
The plane, with 16 crew and 136 passengers on board, landed short of the runway. Other causes, including frozen or contaminated fuel, bird strike or computer failure, were ruled out.
Investigators now plan to recreate the damage and compare the black box recorders' account of last month's flight from Beijing.
Their report said: "Detailed examination of… fuel pumps revealed signs of abnormal cavitation (air bubbles) on the pressure-side bearings and the outlet ports. This could be indicative of either a restriction in the fuel supply to the pumps or excessive aeration of the fuel."
advertisement
"This could be indicative of either a restriction in the fuel supply to the pumps or excessive aeration of the fuel."
Investigators found no evidence of mechanical defects or any anomalies after examining the black box flight recorders.
However they did raise concern about the failure of valves to cut off fuel supply after the plane landed, which led to fuel leaking from the aircraft.
"This was not causal to the accident but could have had serious consequences in the event of a fire during the evacuation," the report noted.
The valves are designed to stop fuel supply immediately in the event of an accident or an engine fire.
They should be cut off first - either automatically or manually - before the fire handle, which works the onboard fire extinguisher, is turned.
But the original design of the wiring was found to prevent this happening, leading Boeing to advise airlines to modify this.
The Federal Aviation Authority in Washington has set a deadline of July 2010 for this work to be done for all 667 Boeing 777s in service.
However this modification had not been carried out on this particular BA plane.
As a result the two procedures were carried out manually by Peter Burkill, the captain, and John Coward, his first officer.
With time short, they did so simultaneously. This, however, led to one of the valves not being cut off, leading to a loss of fuel from the aircraft.
"It's a bit like turning a fire extinguisher onto an engine before you have turned off the ignition," one industry expert explained.
But, BA said on Monday night, the two men followed BA's standard procedures which had been agreed with Boeing.
However the AAIB has told Boeing to instruct airlines to make sure that the fuel valve is cut off before the fire handle is operated.
"We have already implemented these changes to reflect the latest advice," a BA spokesman said.
