B747-400
Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Communication Breakdown; Situational Awareness
Deviation - Altitude Excursion From Assigned Altitude; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Weather / Turbulence; Inflight Event / Encounter Loss Of Aircraft Control
Flight Crew Regained Aircraft Control; Flight Crew Became Reoriented
Weather
I was in rest in the bunk when I felt some turbulence. Soon after I felt that the aircraft was nose down and descending at a steep angle and I was rolled into my seat belt which prevented me from hitting the floor. I ran to the front of the aircraft to find the aircraft was 900 feet low and 20 kts slow and was slowly recovering. The altitude was amber as it was off FL400 and the auto pilot was still engaged. I asked the left seat Flying Pilot what had happened and they stated that we hit mountain wave turbulence. The pilot was also helping adjust the auto throttles to help the aircraft recover and maintain the airspeed as that was the most critical.  After the recovery was completed; the pilots informed me that the aircraft 100 miles ahead had hit the mountain wave turbulence and reported to ATC who told them to expect it. They also informed me that our altitude initially climbed to FL410 and the airspeed went 25 kts fast into the high speed queue for over 20 seconds. Then it went low 2;000 feet and the airspeed dropped 40 kts which was what I felt and saw as I came forward to the flight deck. They then reported it to ATC and I reported it to Dispatch. I wrote up the incident into the logbook due to the possible severe turbulence associated with the dramatic shifts in altitude and airspeed. I also looked at the Jepp FD charts to find that there was no SIGMETs in that area. What I did find were 10-15k feet mountains below us with a 200 kt jet stream crossing over them at that time. I checked on the deadheaders to make sure they were safe and everyone was OK. No further disruption to the flight.I suggest a more robust PIREP system be implemented into our companies Dispatch data and shared more readily with other aircraft in real time. The aircraft in front of us was a company plane that had just gone through it and we could have been informed by Dispatch of the incident ahead.
I was Pilot Flying. We were about 200 miles northeast of ZZZZZ on [airway] at FL400 and had been experiencing light to moderate turbulence. We then encountered a mountain wave which resulted in altitude excursions/deviations of plus and minus 1;000 ft. and airspeed deviations of plus and minus 25 kts.  We notified [ATC]. Other planes in area reported similar experiences.
B747 flight crew reported mountain wave activity due to jetstream resulting in a momentary loss of aircraft control. The reporter suggested a more robust PIREP system needs to be implemented to avoid or better prepare crews for these type of events.