Thursday afternoon, a B777, aircraft 7BH, en route from MIA to LAX, descended at flight idle power on the profile descent into LAX. At approximately 2000 feet, slowing to 170kts, the autothrottles moved forward to maintain the selected airspeed. The right engine responded normally, but the left engine remained at flight idle for approximately 10-15 secs. The left engine then responded to the demand for increased thrust. The rest of the flight continued with normal engine operation and the flight landed without incident. The aircraft was taken out of service and has been moved to a hanger for further inspection. In light of the recent events at British Airways, we have placed the highest priority on this investigation. Representatives from Boeing and Rolls Royce will participate. We will keep you informed of our findings.
-- Initial review of DFDR data by Rolls Royce indicates a very different event than what British Airways experienced. There were a number of markers in the BA event that are not present In our DFDR data. As it is a concerning event you will be kept informed as more details become available.
-- Boeing, Rolls Royce, AA Maintenance, TAESL Engineering and Flight Test have been unable to uncover any mechanical discrepancies which may have caused the left engines delayed thrust during approach into LAX. All parties reviewed DFDR data and performed extensive fuel and systems testing. At the time the left engine thrust failed to advance, the First Officer (PF) had the speedbrake extended slowing the aircraft from 220kts to 170kts, descending through approximately 3000ft. The DFDR data, which measures actual throttle angle, indicated the left throttle was at or near flight idle while the right throttle advanced.
The only theory that we could prove, which was done in the simulator and again during flight test on Saturday, was the possibility the First Officer may have had his left hand resting on the left throttle for leverage while holding the speedbrake extended, slightly impeding the forward movement of the throttle. Tests on both throttles during flight indicated 1.5 pounds of pressure was required to keep each throttle from advancing. With the left hand on top of the throttle, maintaining back pressure on the speedbrake handle and focused on a very demanding approach, the opposite throttle could advance to maintain airspeed momentarily unnoticed by the crew until a yaw was experienced from differential thrust. The TAC system and autopilot compensation with rudder makes the onset almost unnoticeable. Timed from RT throttle advancement, immediate recognition by the crew, to left throttle awakening and advancing to join the right throttle approximately 12 secs elapsed.
During 2.9 hours of flight test with 2 TUL flight test pilots, the Fleet Captain, and TAESL and Rolls Royce engineers all autothrottle functions and protections worked properly. Minus an unknown mechanical malfunction this scenario was the only plausible way we could recreate this event to match what the crew experienced and what the DFDR data and testing indicated at the time of the event.
This crew did a fabulous job handling this situation and we appreciate their cooperation in this investigation. We certainly are not placing blame, just attempting to explain a scenario that may have produced this set of observed and recorded circumstances.