B737 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Situational Awareness
Deviation - Altitude Excursion From Assigned Altitude; Deviation - Speed All Types; Inflight Event / Encounter Loss Of Aircraft Control; Inflight Event / Encounter Weather / Turbulence
Flight Crew Regained Aircraft Control
Weather
We were at FL410 in continuous light chop. The seat belt sign was on before we entered the known area of turbulence and remained on till we passed the turbulence. ATC [was] reporting light chop for the next 150 miles. I had the speed intervene window open. The N1 was set for turbulence penetration. The speed was Mach .76. Within a matter of seconds we were in severe mountain wave. The speed increased to Mach .80 and we were in the over speed band. The over speed clacker went off right away. We began losing altitude. I immediately reduced thrust which was ineffective so pulled the speed brake. The speed slowed to Mach .76. We lost 300 ft. quickly. Then the second wave hit and we climbed rapidly back to altitude then 200 ft. above the assigned altitude. The aircraft returned to FL410. The event was probably 30 to 45 seconds. During the event I had the FO (First Officer) seat the flight attendants immediately. I had the FO ask for a descent to FL390. We were given the descent. The FO reported the mountain wave and the trends to ATC. There was no traffic in the area on TCAS and none was reported by ATC. We reached FL390 with no further wave but the light turbulence continued. I checked the flight attendants in and they reported no one was up during the event. I reported the event to Maintenance as severe mountain wave and over speed. I reported the event to Dispatch as well. In my 25 years of flying I have never seen mountain wave this bad.
B737 Captain reported encountering severe mountain wave activity that resulted in altitude excursion.