Safety officials probing the circumstances leading to the January 17 accident of a British Airways (BA) Boeing 777 at London Heathrow are continuing to focus on the fuel system. In particular, they want to know why the aircraft lost power when it was on final approach.
The 777, operating as flight BA038, was flying nonstop from Beijing to London Heathrow. The flight was uneventful and the engine operation was normal until the final approach. As the 777 approached to land, the R-R Trent 895 engines initially responded to an autothrottle command for increased thrust. At a height of about 720 feet, thrust on the right engine reduced to about 1.03 times the engine pressure ratio (EPR), followed about seven seconds later by a similar reduction (1.02 EPR) on the left engine.
The airplane touched down 1,000 feet short of Runway 27L’s paved surface before coming to rest in the undershoot area astride a taxiway junction near the threshold. In the heavy landing, the nosegear collapsed and the left main undercarriage was punched up through the upper surface of the wing, while the right undercarriage six-wheel bogie detached from the aircraft. One passenger was seriously injured and 12 other occupants received minor injuries. The airplane sustained heavy damage and was subsequently written off.
The UK Air Accidents Investigation Branch (AAIB) has determined that in the case of both engines, thrust reduction resulted from reduced fuel flow, which was reflected in all subsequent engine parameters. Although the fuel-metering valve responded to an engine-control system command to open fully, there was “no appreciable change” in fuel flow. Evidence indicated that both engines had “low fuel pressure at the inlet to the high-pressure pump.”
Following “sustained interest” in the accident, the first such event involving a 777, the AAIB last month published a special bulletin that said Boeing Commercial Airplanes division was conducting fuel-system testing in Seattle. Engine manufacturer Rolls-Royce had earlier completed “extensive full-scale engine testing” in a cell modified to replicate the actual response of the accident aircraft’s engine fuel and control system.
AAIB investigators say the primary challenge was for Boeing to create the environmental conditions the flight experienced while flying over Siberia at altitudes of up to 40,000 feet. The flight over the region took the 777 through particularly cold air, as low as -76 degrees Celsius (-104.8 degrees F). The minimum recorded fuel temperature was -34 degrees Celsius (-29.2 degrees F) and subsequent tests of onboard fuel showed a freezing temperature of -57 degrees Celsius (-70.6 degrees F).
Investigators are principally concerned with understanding the potential for
fuel-system restrictions to have formed. Additional work has aimed to improve understanding of fuel-flow dynamics between the tank and the engine.
Systems consultancy Qinetiq is reviewing recorded data from a large number of similar flights. Qinetiq analysts are concentrating on the identification of abnormal combinations of parameters, since no single reading from the accident flight has been identified as outside previous type operating experience.