Investigators studying last week's crash landing of a British Airways Boeing 777 are looking closely at the possibility that the accident...

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Investigators studying last week’s crash landing of a British Airways Boeing 777 are looking closely at the possibility that the accident was caused by an interruption in the flow of jet fuel to the engines.

“All possible scenarios that could explain the thrust reduction and continued lack of response of the engines” are being examined, the U.K’s Air Accidents Investigation Branch said Wednesday in an update on the January 17 crash at London’s Heathrow airport. “This work includes a detailed analysis and examination of the complete fuel flow path from the aircraft tanks to the engine fuel nozzles.”

Fuel flow is the only area of investigation singled out for mention in the report update.

The plane, carrying 136 passengers and 16 crew, lost power in its engines about 2 miles from touch-down at a height of 600 feet, above a residential neighborhood near the airport. It was the first crash in the more than a dozen years that airlines have flown the 777.

The cause of the accident has not been established, and even if it is a fuel flow interruption, that doesn’t necessarily exonerate Boeing.

But this focus of the investigation seems to increase the possibility of an external cause such as contamination of the jet fuel that was loaded before the 777 took off from Beijing, China.

Alternative scenarios, such as a software control system failure, would clearly be the manufacturer’s responsibility and could bring into question the safety of the airplane.

Wednesday’s update from the U.K. agency leading the investigation provides new detail on what happened.

Contrary to some initial reports, the jet’s two Rolls-Royce engines did not fail at precisely the same moment, and neither failed completely, the agency said.

The aircraft was approaching with the autopilot engaged when the autothrust system commanded the pilot to increase thrust from both engines.

“The engines both initially responded but after about 3 seconds the thrust of the right engine reduced,” the report said. “Some eight seconds later the thrust reduced on the left engine to a similar level.

“The engines did not shut down and both engines continued to produce thrust at an engine speed above flight idle, but less than the commanded thrust,” the report said.

The accident resulted in few injuries but came close to a disaster.

The co-pilot who was flying the airplane was credited afterward with heroic skill in keeping the plane aloft over the houses and putting it down onto the grass 1,000 feet short of the runway.

The pilots had no time to warn passengers that anything was wrong. The impact was sufficently heavy that the landing gear collapsed.

“During the short ground roll the right main landing gear separated from the wing and the left main landing gear was pushed up through the wing root,” said the initial AAIB report.

Although some fuel leaked from the aircraft, there was no fire. Passengers and crew evacuated quickly via the emergency exit slides with only some minor injuries. One passenger suffered a broken leg.

Investigators have ruled out the possibility that the plane ran out of fuel entirely. The new report states that plane had “adequate fuel” as it approached.

The report also said that “the autothrottle and engine control commands were performing as expected prior to, and after, the reduction in thrust.” This appears to imply that the primary software controlling the system functioned as it should.

But the singling out of fuel flow for investigation suggests investigators are zeroing in either on fuel contamination — perhaps from an external fuel source or maybe from some internal leakage of other fluid into the fuel tanks — or on some leakage in a fuel line serving both engines.

After fueling in Beijing, China, the plane flew non-stop to London. The fuel tanks would have been low as the jet came in to land at Heathrow. A contaminant at the bottom of the tanks could potentially have blocked the fuel flow to both engines.

If that turns out to be the cause, Boeing executives, engineers and mechanics will let out a sigh of relief.

As a large, twin-engine airplane, the 777’s ability to serve airlines on ultra-long-haul routes depends upon the reliability of its engines.

Engines have occasionally failed in flight for various reasons, and the affected 777s have continued flying safely on the remaining engine. No 777 has previously had two engines fail together.

The plane is certified to fly under ETOPS (Extended-range Twin-engine Operations) regulations for at least three hours from the nearest airfield. Some airlines operating north Pacific routes can fly routes just shy of four hours from the nearest airport.

Unlike a typical uncontrolled plane crash into the ground, the jet survived the crash landing largely intact, providing investigators a great deal of information on its condition. The 165-ton aircraft was moved from the accident site to an airport apron on Sunday evening.

Boeing is providing technical assistance to the investigation, said Boeing spokesman Jim Proulx.

Dominic Gates: 206-464-2963 or dgates@seattletimes.com