Air carrier medium large transport maintaining FL310 encountered moderate turbulence and mountain wave. Aircraft lost 1000 ft. Company had forecast turbulence 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.

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Attributes
ACN	357118
Time
Date	199612
Day	Mon
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : mlp
State Reference	ID
Altitude	msl bound lower : 30000 msl bound upper : 31000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zlc
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 180 flight time total : 14000 flight time type : 7000
ASRS Report	357118
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument
Events
Anomaly	altitude deviation : excursion from assigned altitude inflight encounter other non adherence : clearance
Independent Detector	other flight crewa
Resolutory Action	flight crew : exited adverse environment flight crew : returned to intended course or assigned course
Consequence	Other
Supplementary
Primary Problem	Weather
Air Traffic Incident	other

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.

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.