B787 Dreamliner Undifferentiated or Other Model
Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 96.43; Flight Crew Total 8443.00; Flight Crew Type 471.77
Fatigue; Human-Machine Interface
Deviation - Altitude Excursion From Assigned Altitude; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Loss Of Aircraft Control; Inflight Event / Encounter Weather / Turbulence
Flight Crew Returned To Clearance; Flight Crew Regained Aircraft Control
Human Factors; Weather
We encountered a significant unforecast mountain wave. Autothrottles were slow to react. I called out acceleration to Captain who was pilot flying. He acknowledged but did not react to the acceleration. We started to overspeed so I deployed the speed brakes and pulled back the throttles. Lost 500 feet in recovery. Reported altitude loss  and wave to ATC. Debriefed event and discovered pulling throttles back was counter to FM guidance. During debrief we also discussed late departure and long taxi with complications on ramp due to weather on departure as contributing to our reaction to this event due to being fatigued. Good debrief.
Air carrier First Officer reported an altitude excursion occurred when the flight encountered an unforecasted mountain wave.