Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 236.18; Flight Crew Total 33600; Flight Crew Type 10390.87
Aircraft Equipment Problem Less Severe; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew FLC Overrode Automation; Flight Crew Overcame Equipment Problem
Aircraft
During cruise flight in Russian airspace on a clear moonless night with no turbulence or wind shear; the auto throttle system very slowly and imperceptibly began to retard the thrust levers over a period of at least a minute or two. No warnings or status messages indicated a failure of any kind. I was shocked to look up and see our airspeed 20 knots below the indicated target speed bug (cruise was .82 Mach and about 280 knots indicated) and decreasing and the thrust levers half way to the idle position. I grabbed the thrust levers and pushed them forward; having to resist the force of the auto throttle which [was] trying to retard them at the same time. I turned off the auto throttle arm switches and set the thrust levers manually to return to normal cruise speed. We could find no reason for the auto throttle to depart from holding the cruise airspeed as they had since the beginning of the flight. I reengaged the auto throttle system; carefully monitoring the indicated airspeed hold and Mach hold functions; which operated normally for the remainder of the flight. The failure was entered into the aircraft logbook and a text message sent to maintenance. Maintenance responded to the text message that there was no previous write-ups on the auto throttle system; and that it would be looked at on the ground. I do not know if maintenance discovered a fault with the system in their post flight inspection.  In my 10;000 hours as a 777 captain I have never seen the auto throttle system fail to maintain cruise flying airspeed unless associated with turbulence or wind shear; none of which appeared was a factor this night; nor have I seen the auto throttle system react opposite to an airspeed excursion.. I am quite certain that the aircraft would have stalled or come close to doing so had I not physically intervened and would like to know the outcome of the post flight inspection by maintenance. It would be extremely disturbing to me if they did not find a problem with this particular aircrafts auto throttle system; because we clearly came close to having a major jet upset incident and I believe the problem should be researched to determine if it might happen again. I consider myself very lucky to have caught this incipient failure in a timely manner during what was otherwise a very quiet and lulling portion of cruise flight where the problem could [have] gone unnoticed and caused a significant departure from cruise flight.
A B777 flight crew; while in cruise; detected that the autothrottles were not maintaining .82 Mach and were retarding further even as airspeed was decreasing. The Captain disengaged the autothrottle system and manually set the throttles to maintain Mach .82. He then reengaged the autothrottle system and it operated normally for the remainder of the flight.