Narrative:

At FL410; initially at mach .834 with 80 kts of steady-state tailwind; we encountered a severe mountain wave: airspeed increased 25 kts within 5 seconds. As the PF; I rapidly went full idle and then deployed full speed brakes as it continued to accelerate. The aircraft sped into the barber pole with full idle and boards by 15 kts and remained there for 15-20 seconds before decelerating to within limits. Now on the performance decreasing side of the wave and in moderate turbulence; we asked for and were given a descent to FL390 with airspeed above the lower barber pole but within the amber foot. I achieved normal range airspeed in the descent and ascertained that there were no injuries in the cabin from the abrupt turbulence.these mountain waves are not uncommon with a strong tailwind eastbound from the rocky mountain range and are normally easily handled with throttle reduction below mach .84; but this one was very severe and could not be prevented even with an expeditious reaction. While the acceleration was adequately handled; little sleep on a turbulent night may have been a factor with me not better catching the deceleration on the performance decreasing side of the mountain wave and allowing the airspeed to enter the amber foot. Additionally; I was slow to catch the fact that the autothrottles had entered the 'hold' mode vs a speed mode from the initial throttle reduction; and that confusion caused a slower acceleration back to normal parameters than what I would be proud of; as I hand-flew the aircraft through the descent to FL390. It was safe; but could have been flown better. The fb (in the pm role) did an outstanding job of assisting me through the event.

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Original NASA ASRS Text

Title: B777 flight crew reported an aircraft overspeed for 15-20 seconds while in cruise at FL410 due to a severe mountain wave encounter.

Narrative: At FL410; initially at Mach .834 with 80 kts of steady-state tailwind; we encountered a severe mountain wave: airspeed increased 25 kts within 5 seconds. As the PF; I rapidly went full idle and then deployed full speed brakes as it continued to accelerate. The aircraft sped into the barber pole with full idle and boards by 15 kts and remained there for 15-20 seconds before decelerating to within limits. Now on the performance decreasing side of the wave and in moderate turbulence; we asked for and were given a descent to FL390 with airspeed above the lower barber pole but within the amber foot. I achieved normal range airspeed in the descent and ascertained that there were no injuries in the cabin from the abrupt turbulence.These mountain waves are not uncommon with a strong tailwind eastbound from the Rocky Mountain Range and are normally easily handled with throttle reduction below Mach .84; but this one was very severe and could not be prevented even with an expeditious reaction. While the acceleration was adequately handled; little sleep on a turbulent night may have been a factor with me not better catching the deceleration on the performance decreasing side of the mountain wave and allowing the airspeed to enter the amber foot. Additionally; I was slow to catch the fact that the Autothrottles had entered the 'HOLD' mode vs a speed mode from the initial throttle reduction; and that confusion caused a slower acceleration back to normal parameters than what I would be proud of; as I hand-flew the aircraft through the descent to FL390. It was safe; but could have been flown better. The FB (in the PM role) did an outstanding job of assisting me through the event.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.