Cause of Jet Crash Remains Elusive

The plane headed over Siberia and skirted the Arctic, far from emergency airports. For years, regulators have generally limited such flights on twin-engine planes because they worried about engine failure. But engines have become so reliable that U.S. regulators have steadily loosened those limitations, including restrictions on flights over the North and South poles. The Boeing 777, especially versions designed to fly extremely long distances, has become the workhorse of many of those routes.

During portions of the British Airways flight, the outside temperature was as low as minus-49 degrees. Experts said such temperatures should not have been a particular concern to the crew.

The lowest recorded temperature of the plane's fuel was minus-29 degrees, well above the freezing point of minus-70. Investigators found that the fuel did not have "excessive" amounts of water in it, which can cause engines to freeze and block fuel lines.

Toward the end of an otherwise uneventful 10 1/2 -hour flight, the plane was nearing Heathrow about 10:40 a.m. London time. About 800 feet above the ground, the plane was configured to land, and the copilot had taken the controls.

About this time, the plane's auto-throttle system commanded the engines to increase thrust to keep the plane on its proper glide path to the runway.

The engines initially responded, but then the right engine's thrust subsided. A few seconds later, thrust on the left engine dropped off. The engines did not quit, but their thrust was too weak to keep the plane airborne. The plane slowed and lost lift. Then it smacked to the ground, according to investigators and witnesses.

At first, investigators thought the plane might have experienced a computer glitch that led to the reduction in thrust. Aircraft are becoming increasingly reliant on computers to control their movement and engines. The possibility of such a failure worried investigators, according to sources familiar with the investigation.

But the recorders quickly disproved that theory, revealing no errors in the computer signals sent from the cockpit to the engines, according to reports and sources. Meanwhile, investigators were looking at the plane's fuel supply. An empty gas tank would have led to a thrust reduction, but the plane's tanks still contained plenty of high-quality fuel, investigators said in reports.

When they took apart the engines, investigators found evidence on pumps that the engines had been starved for fuel in the moments before the crash. That led them to conclude that the fuel supply had become blocked somewhere between the plane's tanks and its engines.

Sources familiar with the probe said engineers suspect that ice collected in or near a fuel-oil heat exchanger on each engine, blocking the fuel supply. The heat exchanger uses cold fuel to cool hot engine oil, and the hot engine oil to warm fuel before it is injected into the engines. The heat exchanger is the only point in the system that engineers have not yet eliminated as the potential bottleneck, the sources said.

But the ice theory is not perfect. Plenty of jets fly through cold weather, and the 777's fuel contained additives designed to prevent it from freezing under such conditions.

Outside experts noted that the ice theory seems implausible for another reason: Each engine should have been drawing fuel from a separate tank. The chances of ice breaking off in separate fuel tanks and blocking the fuel supply in each engine at nearly the same moment is almost too tiny to comprehend, said John Goglia, a former member at the National Transportation Safety Board, which investigates air crashes in the United States.

"This isn't supposed to happen," Goglia said. "These are two independent systems."