B767 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 44; Flight Crew Total 1302; Flight Crew Type 1111
Situational Awareness; Workload
Deviation - Speed All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Weather / Turbulence
Flight Crew Regained Aircraft Control
Weather
We were in cruise flight in smooth air with the autopilot on and flying our cost index profile. The Captain was on his rest break. I was the non flying pilot in the left seat; and the Flying First Officer was in the right seat. We were at 37;000 ft. and M.83 in smooth air when the airspeed started climbing as we encountered a mountain wave. The flying First Officer immediately began to reduce the throttles and came all the way to idle as the airspeed kept increasing. At this point; I asked if he would like me to deploy the speedbrakes to assist in the effort to keep the aircraft from over speeding. I began deploying the speedbrakes; but the aircraft accelerated slightly above maximum operating to M.875. We maintained idle thrust and speed brakes until clear of the mountain wave.We then submitted a PIREP electronically and also notified ATC who said we were the first report. However; shortly thereafter another aircraft also reported mountain wave activity. Total airspeed fluctuation was plus or minus about 15 to 20 kts.
B767 First Officer reported the aircraft encountered mountain wave turbulence causing a speed increase above the maximum operating speed.