A simple inadvertent obstruction of the throttle lever may have been responsible for the recent throttle-response failure on an American Airlines Boeing 777-200ER.

The Rolls-Royce Trent-powered twinjet (N799AN) 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 10-15s before its thrust increased.

But despite the circumstantial parallels with January's British Airways 777 crash landing at London Heathrow, a source familiar with the American Airlines inquiry says that 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.

Because the speed-brake on a 777 is located to the immediate left of the two throttle levers, the co-pilot must reach over the throttles to activate it, so the American first officer might have unintentionally obstructed the left-hand throttle lever.

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.

Source: Flight International