One theory is that low fuel temperature may have caused the crash
Investigators looking into the British Airways air crash at Heathrow in January are focusing on the plane's fuel systems and engines.
An interim report from crash investigators says the engines had low pressure at the inlet to fuel pumps.
The plane, which was arriving from China, came down on grass at the end of the runway.
A now-discounted theory was that radio signals from Gordon Brown's motorcade interfered with the Boeing 777.
The prime minister's motorcade was leaving Heathrow at the time, after dropping Gordon Brown off for a flight to China.
But the report from the Air Accidents Investigation Branch (AAIB) rules out electromagnetic interference.
The aircraft skidded on the grass and part of its undercarriage collapsed.
Of the 136 passengers and 16 crew, only one person was seriously injured.
The plane was operating in what are described as unusually low temperatures, and one theory is that this affected the fuel, though recorded temperatures were within safe limits.
The report also said that restrictions in the fuel system between the aircraft fuel tanks and each of the engine high pressure (HP) pumps may have resulted in reduced fuel flows.
And, according to the report, the HP fuel pumps from both of the aircraft's engines had "unusual and fresh cavitation damage to the outlet ports consistent with operation at low inlet pressure".
The engines are now being tested at Rolls Royce in Derby, the fuel system at Boeing in the US.