Partial interruption to the fuel flow still appears to be in the frame as a potential cause of the British Airways Boeing 777 crash-landing at London Heathrow airport on 17 January, according to a just-issued UK Air Accident Investigation Branch special bulletin (AAIB).
The Agency is careful not to state it as a cause, but concludes the interim report by saying that “a comprehensive examination and analysis is to be conducted of the entire aircraft and engine fuel system, including the modelling of fuel flows and taking account of the environmental and aerodynamic effects”.
The aircraft was descending through 720ft (220m) on final approach with the runway in sight when autothrottle, and then the crew, demanded an increase in thrust, but they only got a temporary response, following which the power reduced to the same level.
The report says: “By 200ft the airspeed had reduced to about 108kt (200km/h). The autopilot disconnected at approximately 175ft, the aircraft descended rapidly and its landing gear made contact with the ground some 1,000ft [300m] short of the paved runway surface.” At 108kt the aircraft was within a couple of knots of stalling speed.
The AAIB says it has tested all the electric and electronic engine and fuel system controls and found them to be working perfectly, and there is no indication in the flight data recorder of any such malfunction during the flight or the event itself.
Flight data shows that, at the moment of thrust reduction, the right-hand engine’s electronic control system was responding correctly to a reduction in fuel flow to the right engine.
This was followed by a similar response from the control system in the left engine when fuel flow to that powerplant also diminished.
The agency remarks, however: “Detailed examination of both the left and right engine high pressure fuel pumps revealed signs of abnormal cavitation 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 assessed both pumps as still being capable of delivering full fuel flow.”
The AAIB does not say whether it associates this evidence with the accident cause or not, but adds: “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.”
When the report refers to studying the fuel flow system “taking account of the environmental and aerodynamic effects”, it appears to be referring to the fact that the temperature at high level right from the start of the cruise until well beyond the Ural Mountains in Russia, was “unusually low compared to the average”.
The minimum recorded outside air temperature was minus 76°, and the crew was monitoring the fuel temperature carefully.
The bulletin makes clear that, although the fuel freezing point of a sample of the fuel taken from the aircraft was minus 57°, the minimum temperature the fuel dropped to during the flight was only minus 34°.
Insignificant quantities of water and no other contaminants were found in the fuel, says the bulletin, but there were a few small items of debris found in the tanks, and the AAIB says it will be examining this evidence to see if it was relevant to the accident.
“There was no evidence of a mechnanical defect or ingestion of birds or ice,” says the AAIB special bulletin.