Monday, May 12, 2008
The investigation into the crash of a British Airways Boeing 777 (B777) written off on January 17 after landing short of the runway at London’s Heathrow International Airport has been updated. The crash of Flight 38 injured one passenger seriously and 12 others on board, including four crew members, as well as being the first hull loss of a B777.
The latest word from the United Kingdom’s Air Accidents Investigation Branch (AAIB) is that both engines had low fuel pressure in their high pressure (HP) fuel inlets, leading to the failure of both engines. Having departed from Beijing, China the aircraft was on final approach for landing at Heathrow when the thrust on the engines first reduced and became unresponsive, then ceased completely. The copilot, who was flying the aircraft at the time, received significant media praise for gliding the aircraft past housing and airport barriers to crash land the airliner onto the grass at the runway threshold, likely saving the 136 passengers and 16 crew from death or further injury as well as preventing potential ground casualties. The landing gear collapsed and the airframe, wings and engines were significantly damaged.
Each HP inlet also exhibited “unusual and fresh cavitation damage to the outlet ports consistent with operation at low inlet pressure”, according to the AAIB’s latest report on investigatory progress, which also states engine failure was definitely caused by low pressure and that the autothrottle had opened up appropriate valves fully in an effort to increase fuel flow, but to no avail. The report indicates the investigation has also considered an area of unusually cold air over Russia which the B777 passed through at 40,000 feet on its journey. Between the Ural mountain range and East Scandinavia the air was found to have been as low as -76°C. The AAIB has ruled out fuel turning to ice as the fuel temperature never fell below -34°C. This is compared to an average freezing temperature of -47°C for jet fuel and test showing the fuel in the B777’s tanks needed cooled to at least -57°C before it froze. Floating ice particles had previously been suggested as a possible causal factor.
However, it has been suggested outside of the investigation that although the fuel did not freeze, it could have become unusually thickened, restricting its flow through the HP inlets, although the temperatures did remain within what are currently considered safe levels. Another possible explanation was that the fuel was contaminated or of inferior quality, but the AAIB has ruled this out. Also disproved are suggestions that the plane suffered a bird strike or that it was disrupted by electromagnetic interference from jamming devices set up to protect a motorcade carrying Prime Minister Gordon Brown.
The report says that there was also no evidence to suggest any control systems failed, that the aircraft encountered a wake vortex or that there was core engine icing or other evidence that the engines suffered core lock. There was no damage to the fuel systems excluding that caused by the low pressure and no blockages, therefore it is suspected that fuel flow was restricted.
The investigation has also begun analysing previous flights on comparable aircraft in a large sample of flights. Thus far, no parameters present on Flight 38 have been found that differ significantly from anything previously encountered. The purpose is primarily to identify potentially troublesome combinations of unusual operational parameters.
The report concludes by stating that interested parties to the investigation, specifically the Federal Aviation Administration, the European Aviation Safety Agency, the Civil Aviation Authority and British Airways are being kept fully informed of the investigation’s progress.
Although this shows some progress, according to David Learmount, operations and safety editor at periodical industry magazine Flight International, “This report takes us absolutely nowhere, I think they still have no idea.”
The B777 has never had any other hull losses or any fatal accidents and is considered to have one of the world’s safest commercial airliners. According to one spokesman for British Airways “It has one of the best safety records of any commercial aircraft. At no stage since the accident has the AAIB, Boeing or Rolls Royce advised against the continued operation of the Boeing 777.”
The engines underwent analysis at manufacturer Rolls-Royce‘s facility in Derby, while airframer Boeing is studying the fuel systems in the United States. Rolls Royce’s engine test bed was modified to replicate control inputs from aircraft control systems, and Boeing are working at replicating the cold conditions experienced over Eastern Europe. The investigation is ongoing.