Saturday, September 6, 2008

The United Kingdom’s Air Accidents Investigation Branch (AAIB) has released an interim report into the crash of British Airways Flight 38. The Boeing 777-ER crashed early this year as it tried to land at London’s Heathrow Airport.

The report states that the AAIB believes the crash occurred when ice crystals formed within the fuel system. However, it should be noted that the report only serves as an update into the progress of the investigation and that the final report is yet to be published and may yet reach a different conclusion into the cause of the accident.

The flight from Beijing, China had been normal until final approach at Heathrow, at which point the aircraft was coming in for landing with the autopilot and autothrottle engaged. However, engine power became greatly reduced when the autothrottle requested more thrust, and the aircraft made a forced landing 1,000 feet from the runway. The aircraft suffered substantial damage as it slid across the grass to the runway threshold, where it came to rest, and was written off. There was only one serious injury and eight minor ones to the 136 passengers and 16 crew.

As the jet passed over Siberia it encountered significantly reduced temperatures. The AAIB has determined that the fuel was at a temperature below 0°C for an unusually long duration. This is believed to have caused water in the fuel – which met all relevant international standards – to have frozen into crystals. These were able to form undetected as the aircraft cruised with a low fuel requirement, and it was only when extra fuel was pumped in from the tanks for the landing that the crystals became a problem.

The report says that the current explanation being offered for the accident is an “apparently hitherto unknown phenomenon” and warns that other combinations of aircraft models and engines may also be vulnerable. The 777 involved was powered by Rolls-Royce Trent 800s, like 222 of the 736 777s currently operating. Data from 141,000 other flights of such aircraft has not yielded any other set of circumstances similar to the one in the crash flight.

In light of these results, a worldwide alert has been issued to the 11 airlines that between them operate all the aircraft identical to the lost jet. Boeing have instructed airlines to vary altitude regularly when fuel is below 10°C and to run the engines at max power for ten seconds before attempting a landing if the fuel has been at such temperatures for over three hours. The extra power is intended to clear out any buildup of water. To prevent buildup of water during ground operation at freezing temperatures all fuel pumps should be run at full power for one minute.

Air New Zealand have already confirmed they will introduce the new measures, and United States authorities are expected to make them mandatory within days. The affected aircraft are operated by US carriers American Airlines and Delta Air Lines.