Date: 201203
Local time of day: 1801-2400
Location: ZZZ.ARTCC, US
Weather elements / visibility: Windshear
Light: Night
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Turbine Engine
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Confusion; Training / Qualification; Time Pressure; Distraction
Anomaly: Aircraft Equipment Problem Critical; Deviation - Altitude Excursion From Assigned Altitude; Deviation - Speed All Types; Inflight Event / Encounter Weather / Turbulence
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Aircraft Equipment Problem Dissipated; Flight Crew FLC complied w / Automation / Advisory; Flight Crew Overcame Equipment Problem
Contributing factors: Aircraft; Weather
Primary problem: Weather

Narrative: While cruising at FL400; Center advised to expect moderate mountain wave and light to moderate turbulence around the mountain. As the pilot flying; I slowed the aircraft to M0.76 in anticipation of turbulence. About 5 minutes later we encountered moderate mountain wave and associated airspeed fluctuations. The initial wave was +/- 100 FT and +/- 10 KTS in light to moderate turbulence. We encountered a subsequent wave that resulted in a loss of 300 FT in altitude and a gain of 15 KTS in airspeed. Autothrottles retarded the thrust levers to about 50% N2 to avoid an overspeed condition. As the airspeed decreased to the selected value of M.076; the autothrottles failed to advance to maintain the selected speed. As the pilot flying; I disconnected the autothrottles and manually advanced the thrust levers to attain the desired airspeed. Despite manual thrust advancement to about 97% N2; the engines remained spooled at 50% N2. I announced the loss of thrust control to the Captain. At this point; the airspeed was decaying at a fast rate and reached a minimum of 210 KTS indicated (just above the yellow band). With the thrust levers full forward and unresponsive to commanded position; I disengaged the autopilot and began a descent while asking the Captain to obtain ATC clearance to descend. At FL395 the Captain obtained an ATC clearance to descend to FL340. The engines remained at 50% N2 with the thrust levers commanding a lot more for roughly 15-20 seconds. We completed the Engine Limit/Surge/Stall Checklist; event though there were no engine exceedences. At about FL370; the engines began to spool up to the commanded thrust lever position and thrust control was recovered. After completing the checklist; we elected to reengage the autopilot and autothrottles. We contacted Dispatch and Maintenance Control. The decision was made to continue to the filed destination. The remainder of the flight was uneventful. [We experienced] loss of thrust control on both engines for 15-20 seconds during mountain wave activity.




Synopsis: After a B737-700 encountered mountain wave activity at FL400; autothrust reduced power to prevent an overspeed; but the thrust did not recover even with thrust levers full forward until the aircraft descended through FL370.