Reduced fuel flow to both engines was the reason the British Airways (BA) Boeing 777 lost power and landed short of the runway at London Heathrow on Thursday 17 January.
Air Accidents Investigation Branch (AAIB) issued its latest report yesterday (12 May) which reveals tests are focusing on the engines” high pressure (HP) fuel pumps which had ”unusual and fresh cavitation damage.”
Various theories appear to have been ruled out now, with extensive investigation uncovering no evidence that electromagnetic interference caused any unusual behaviour, nor were there any signs of a 'wake vortex encounter', bird strike, engine icing or fuel contamination.
Fuel pipes showed ”no unusual deterioration of physical blockages”, while the spar valves and fuel boost pumps were working during the flight.
So focus is on the HP fuel pumps, with damage found on the outlet ports ”consistent with operation at low inlet pressure” which indicates that both engines ”had low fuel pressure at the inlet to the HP pump.”
The report says information from the Flight Data Recorder, Access Recorder and Electronic Engine Controller indicate that at a height of about 720ft the right engine suffered a sudden severe loss of thrust, followed by the left engine seven seconds later.
The engine control system detected the problem and commanded the fuel metering valve to open fully, which it did ” but with ”no appreciable change in the fuel flow to either engine.”
The report concludes that it ”suspects” faults with the HP pumps resulted in reduced fuel flows between the aircraft”s fuel tanks and engines.
It also notes that during flight BA038 from Beijing to Heathrow there was a region of particularly cold air - with temperatures as low as -76” - between the Ural Mountains and Eastern Scandanavia. According to the Met Office this was unusual but 'not exceptional.' However, the fuel temperature only dropped to -34” and could have safely gone to nearer -47”.
The aircraft landed short of runway 27L, with all 136 passengers and 16 crew managing to evacuate, although one passenger is classified as suffering serious injury.
While further tests are conducted by Boeing in Seattle ” where attempts are being made to recreate the environmental conditions of a flight at 40,000ft - no operational changes are being recommended by either AAIB, engine-maker Rolls-Royce or Boeing.