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AA 777 engine incident: tests support 'impeded throttle' theory

Preliminary investigations into last week’s throttle-response failure on an American Airlines Boeing 777-200ER suggest that a simple inadvertent obstruction of the throttle lever might have been responsible.

The aircraft had been approaching Los Angeles on 28 February, with its engines at flight-idle power and the auto-throttle engaged, when its left-hand powerplant apparently did not respond to a command for more thrust. Although the right-hand engine behaved normally, the left remained at flight-idle for 10-15 seconds before its thrust increased.

American’s incident came just six weeks after an identical British Airways aircraft crashed short of the runway at London Heathrow when both engines failed to respond to a thrust-increase command. Both carriers’ 777-200s are fitted with Rolls-Royce Trent 800 engines.

But a source familiar with the American inquiry says that, despite the parallels, initial findings suggest the two incidents are “very different”.

Crucially, flight-data recorder information from the American 777 – which includes the angle of the throttle levers – shows that the speed-brake was extended, and the left-hand throttle remained around the flight-idle position while the right-hand throttle moved forward.

Investigators have discovered no evidence of mechanical or fuel-related problems. But simulations and nearly three hours of flight tests, combined with information from the flight recorders and the crew, have turned up a potential explanation.

Because the speed-brake on a 777 is located to the immediate left of the two throttle levers, the first officer must reach over the throttles to activate it. The flight tests demonstrated that the first officer on the American Airlines flight might have unintentionally obstructed the left-hand throttle lever, preventing its moving forward when commanded by the auto-throttle.

Just 700g (1.5lb) of pressure would be enough to impede the throttle, and the resulting differential thrust would have initially been corrected by the autopilot, through the rudder, and by the aircraft’s thrust-asymmetry compensation system.

If the pilots were concentrating on the approach, says the source, the combination of these circumstances would have made the onset of the problem “almost unnoticeable” until the resulting yaw became significant.

American Airlines has withdrawn the aircraft – a seven-year old example registered N799AN – pending completion of examination by the US National Transportation Safety Board, Rolls-Royce and other parties.

Investigators in the UK have yet to reach a conclusion on the British Airways 777 accident which occurred on 17 January as the aircraft arrived on a service from Beijing.

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