'No evidence' of air crash defect

They also found the "black box" flight recorders indicated "no anomalies in the major aircraft systems", and ruled out ice, water or contamination of fuel tanks, a report from the Air Accidents Investigation Branch (AAIB) said.

But there was some damage seen to the engine high pressure fuel pumps and some small items of debris were found in the fuel tanks.

Not giving any cause of the January 17 accident, the AAIB said it was carrying out a full examination and analysis of the entire aircraft and engine fuel system.

The Boeing 777, with 136 passengers and 16 crew aboard, had flown to London from Beijing on January 17.

With the cockpit crew unable to get the required thrust from the engines as the plane approached Heathrow, the aircraft came down on the grass "some 1,000ft short of the paved runway surface and just inside the airfield boundary fence", the report said.

As the plane skidded across the grass and on to the end of the runway, the underpart of the aircraft collapsed. All occupants were safely evacuated, with one passenger suffering a broken leg and eight others receiving minor injuries. Four crew were slightly hurt.

Following an examination of the wreckage the report said: "Detailed examination of both the left and right engine high pressure 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. The manufacturer (Boeing) assessed both pumps as still being capable of delivering full fuel flow."

The report went on: "Investigations are now under way in an attempt to replicate the damage seen to the engine high pressure fuel pumps and to match this to the data recorded on the accident flight."