Narrative:

Cruising at FL310 noticed some clouds approaching beneath our cruising altitude. (Had been clear to ground previously.) unannounced we began to encounter moderate turbulence and mountain wave. Autoplt unable to maintain altitude and it kicked-off. Aircraft lost 1000 ft before able to regain altitude. Company had issued a turbulence plot for light/moderate turbulence and mountain wave area over great falls but the valid time for that warning had lapsed and we were already west of that area (going west) when the mountain wave action was encountered. No injuries occurred, though standing flight attendant fell to chair.

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Original NASA ASRS Text

Title: ACR MLG MAINTAINING FL310 ENCOUNTERED MODERATE TURB AND MOUNTAIN WAVE. ACFT LOST 1000 FT. COMPANY HAD FORECAST TURB AND MOUNTAIN WAVE ACTIVITY IN THE GTF AREA BUT IT WAS NOT VALID AT THE TIME OF THE INCIDENT. NO INJURIES BUT CABIN ATTENDANT DID FALL AGAINST A SEAT.

Narrative: CRUISING AT FL310 NOTICED SOME CLOUDS APCHING BENEATH OUR CRUISING ALT. (HAD BEEN CLR TO GND PREVIOUSLY.) UNANNOUNCED WE BEGAN TO ENCOUNTER MODERATE TURB AND MOUNTAIN WAVE. AUTOPLT UNABLE TO MAINTAIN ALT AND IT KICKED-OFF. ACFT LOST 1000 FT BEFORE ABLE TO REGAIN ALT. COMPANY HAD ISSUED A TURB PLOT FOR LIGHT/MODERATE TURB AND MOUNTAIN WAVE AREA OVER GREAT FALLS BUT THE VALID TIME FOR THAT WARNING HAD LAPSED AND WE WERE ALREADY W OF THAT AREA (GOING W) WHEN THE MOUNTAIN WAVE ACTION WAS ENCOUNTERED. NO INJURIES OCCURRED, THOUGH STANDING FLT ATTENDANT FELL TO CHAIR.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.