Heathrow crash mystery goes on as engine fault is ruled out

The Air Accidents Investigation Branch added that the engines of the British Airways jet had not been hit by a bird or by a piece of ice.

The special bulletin means that experts are no closer to solving the mystery of why the jumbo came down short of the runway on January 17 - passing within 20ft of cars on the A30 alongside the airport - and skidded across a field.

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All 136 passengers and 16 crew on the flight from Beijing survived but one passenger suffered a broken leg and eight others had minor injuries.

Four crew members were slightly injured.

A mechanical engine failure was always regarded as an improbable explanation, as it would have had to make both engines fail at the same time.

A problem with the software or electronics controlling both engines, or a fault with the fuel supply, are thought to be more likely causes for the crash, which saw senior first officer John Coward hailed as a hero for bringing the aircraft safely to a stop.

The AAIB report said there were no anomalies in the "major aircraft systems" and that the autopilot and autothrottle systems worked properly, as did the engine control systems.

"Examination of the engines indicated no evidence of a mechanical defect or ingestion of birds or ice," the report concluded.

There were no signs of fuel contamination or unusual concentrations of water in the fuel. There were, however, pieces of debris in the fuel, the relevance of which was "still being considered".

Investigators also found unusual air bubbles in the high-pressure fuel pumps.

"This could be indicative of either a restriction in the fuel supply to the pumps or excessive aeration of the fuel," the report said.

"The manufacturer assessed both pumps as still being capable of delivering full fuel flow."

The AAIB said an inquiry was now under way "to 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".