Investigators examining the Boeing 777-200ER that crashed short of the runway at London Heathrow last month state that the twin-jet passed through two regions of particularly cold air during its flight from Beijing.
The Air Accidents Investigation Branch, in a progress bulletin on the inquiry, also says it is trying to identify the source of damage to the high-pressure fuel pumps in the aircraft’s engines.
Both engines on the aircraft failed to respond to auto-throttle requests for increased thrust during the final stages of the approach to Heathrow on 17 January.
During the approach, with the 777 at a height of just 720ft, thrust from the right-hand Rolls-Royce Trent 800 engine suddenly reduced, about 7s before thrust from the left engine similarly dropped.
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.
Recorded data has shown no anomalies in the aircraft’s major systems and investigators have discovered no evidence that the engines suffered a malfunction or that they ingested ice or birds during the approach.
The jet had 10,500kg of fuel remaining at the time. Scrutiny of fuel samples taken from the aircraft has shown no evidence of contamination or unusual levels of water content.
As part of its flight plan the aircraft was required to climb initially to FL341 and then – owing to extreme cold – descend to FL315 at a waypoint called POLHO on the Chinese-Mongolian border.
But to accommodate a request from air traffic control the aircraft’s crew accepted a climb to a higher altitude, FL348. The AAIB says the ambient temperature at FL348 was minus 65°C, but adds that the crew closely monitored the fuel temperature.
After the aircraft crossed the Ural mountain range in Russia it climbed further to FL380 where the ambient temperature dropped to as low as minus 76°C.
Analysis of aircraft data shows that the fuel temperature reduced to a minimum of minus 34°C. The AAIB says fuel temperature should not be allowed to fall below 3°C above the fuel freezing point. The fuel on board the 777 had a freezing point of minus 57°C.
Examination has found that both engines’ low-pressure and high-pressure fuel filters were clean, and both fuel oil heat exchangers were free of blockage. Visual examination of the fuel feed lines also revealed no blockages.
Inspection of the 777’s fuel tanks turned up a few small items of debris. The AAIB says: “The relevance of this debris is still being considered.”
Detailed examination of the engines’ high-pressure fuel pumps, however, has revealed signs of “abnormal cavitation” on the pressure-side bearings and outlet ports.
“This could be indicative of either a restriction in the fuel supply to the pumps or excessive aeration of the fuel,” says the AAIB. It adds that the manufacturer assessed both pumps as still being capable of delivering full fuel flow.
“Investigations are now underway 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,” says the AAIB.
“In addition comprehensive examination and analysis is to be conducted on the entire aircraft and engine fuel system, including the modelling of fuel flows taking account of the environmental and aerodynamic effects.”
Source: flightglobal.com's sister premium news site Air Transport Intelligence news