B737-700
Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Communication Breakdown
Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Loss Of Aircraft Control; Inflight Event / Encounter Weather / Turbulence
Flight Crew Requested ATC Assistance / Clarification; Flight Crew Regained Aircraft Control
Environment - Non Weather Related; Weather
During climb out of SMF; everything was normal with some occasional light chop. We had asked the Flight Attendants to remain seated due to crossing the Sierra Mountains. We were climbing at approximately .76 Mach. Upon leveling at FL 370; we experienced moderate bordering on severe mountain wave. Our airspeed began decreasing rapidly and the F/O (First Officer) disconnected the autopilot; applied full thrust; and began a shallow descent to regain airspeed. The airspeed decayed to about .70 - .71. It was about a 30 knot decrease. I may be a little off on the Mach numbers. We descended about 500' and the airspeed began increasing.  I reported the encounter to ATC and we began climbing back to FL 370. The wave climbed us to FL 375; and at that time we requested a descent to FL 350. The Controller cleared us to descend to FL 350. We did not encounter any additional mountain wave and nothing more than a little light chop. We checked with the Flight Attendants to make sure everyone was alright; and the remainder of the flight was uneventful. I would like to add that the F/O did an excellent job of recognizing and reacting to the situation.
We had just leveled off at our cruise altitude of FL 370. (Filed for FL 390 but we were too heavy at that time.) During climb we had anticipated and encounter some light turbulence. Nothing more significant was encountered. WSI (Weather Service International) and Jeppesen weather showed areas of turbulence covering about the first 45 minutes of our flight. Our clearance was on the SCTWN 3 RNAV Departure. We were level at FL 370 when suddenly our airspeed had dropped from M.76 (250 KIAS) to 220 KIAS this was approximately 10-15 above the yellow maneuvering bar on the airspeed indicator and the green vector bar was indicating that the slowing was continuing and at a significant rate. I disconnected the autopilot and autothrottle and pitched down to keep airspeed. I also increased thrust to max and I was able to recover airspeed about 500' low at FL 365. In my attempt to climb back to FL 370 we started climbing at a significant rate and stopped the climb at approximately FL 376. The Captain reported to ATC that we were in MVA (mountain wave activity) and that we were unable to hold altitude. I continued to hand fly until I was sure we were passed the MVA and re-engaged the autopilot. The rest of the flight was uneventful.
B737-700 flight crew reported encountering moderate to severe mountain wave for which led to a rapid decay of their airspeed.