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13 March 2009

Fixing the Problem That May Have Caused the British Airways 777 Crash

Recent findings from the AAIB accident investigation point to ice buildup in the fuel system as the key factor in the January 2008 crash of a British Airways 777 in London. On March 11th, 2009, the NTSB called for a redesign of the fuel system, and for the affected aircraft to have those changes installed within six months after the redesign is complete.

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In the January 2008 crash, the flight from Beijing to London was routine until the the aircraft was on final approach, when both engines had an uncommanded power reduction, or engine rollback, which caused the plane to land short of the runway. Although the aircraft was seriously damaged, only one of the 136 passengers was seriously injured, and there were no serious injuries among the 16 crew members.

The Air Accidents Investigation Branch in the UK headed the investigation, with the help of several other organizations, including the aircraft manufacturer Boeing, the engine manufacturer Rolls Royce, and British Airways.

The series of updates and interim reports from the AAIB, the most recent of which was released in early March 2009, revealed that the likely cause of the dual engine rollback was ice blockage in a critical fuel system component that led to a reduction of fuel flow to the engine.

These findings didn't come easily. The AAIB focused its efforts on the fuel system because of the unusual conditions of the accident flight. That flight was exposed to rather cold atmospheric conditions, so cold that the crew changed altitudes at one point to fly through warmer air.

The AAIB reviewed the minimum fuel temperature data from over 141,000 777 flights. About 13,000 of these flights were on aircraft operating with the Rolls Royce Trent 800 series engine, the same kind as on the accident aircraft. Of those 13,000 flights, only 118 had fuel temperatures at takeoff that were at or below the takeoff fuel temperature of the accident flight, and during the approach phase, only 70 had fuel temperatures that were as low or lower than the fuel temperature on the accident flight.

The two most likely accident scenarios that were investigated by the AAIB both involved ice formation within the fuel system, leading to a reduction of fuel flow. This ice formation was possible because the aircraft fuel did contain some water. This kind of contamination is normal, and in fact the fuel from the accident aircraft was tested, and found to be in compliance with the appropriate fuel specifications.

After an extensive analysis of the fuel system, the AAIB concluded that the most likely scenario for the engine rollback was that ice formed in the fuel pipes within the main fuel tank, and that during the latter part of the approach phase of the flight, factors such as turbulence, aircraft pitch changes, and increasing temperatures could have contributed to the sudden release of accumulated ice into the fuel feed system of both engines. This ice would have restricted the fuel flow through a component called the fuel oil heat exchanger and would have led to the engine rollbacks.

The AAIB recommended that Boeing and Rolls Royce review the aircraft and engine fuel system design, and make changes that would prevent ice from restricting fuel flow through the fuel oil heat exchanger.

In the US, the National Transportation Safety Board went further, recommending that within six months of completing the redesign, that it be incorporated in all 777 aircraft using the Trent 800 engines. Some of the airlines that fly Trent 800 equipped triple sevens include Air New Zealand, American Airlines, British Airways, Cathay Pacific, Delta Airlines, El Al, Emirates, Kenya Airways, Malaysia Airlines, Singapore Airlines, and Thai Airways. There are about 220 such aircraft currently in service.

The NTSB's recommendations were influenced by a second 777 rollback event. On November 26, 2008, a Delta 777, powered by two Trent 800 series engines, experienced a single-engine rollback while in cruise on a flight from Shanghai to Atlanta. The crew was able to address the issue and continued the flight without incident. Later analysis indicated that there was a blockage of the fuel oil heat exchanger on that engine that was likely due to ice accumulation. Although the engine rollbacks on the British Airways accident aircraft and the Delta Airlines incident aircraft occurred during different phases of flight, the fuel temperatures at the time of the rollbacks were about equal.

Taken together, these developments are good news for the aviation community, especially passengers and crews flying on 777s equipped with Trent 800 engines. The investigative authorities have determined the likely cause of the event, the changes to the fuel system that are needed are well understood, and the engine and aircraft manufacturers are well on their way to developing solutions that will prevent similar occurrences in the future.

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