Flight crew of an medium large transport encountered mountain wave turbulence resulting in loss of airspeed and altitude before regaining control. The first officer was trying to advise ARTCC controller of their loss of altitude and need for a lower altitude, but was delayed by a congested frequency and controller workload.

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Attributes
ACN	424207
Time
Date	199812
Day	Fri
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : dbl
State Reference	CO
Altitude	msl bound lower : 37000 msl bound upper : 39000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Navigation In Use	Other
Flight Phase	cruise other
Route In Use	enroute airway : zdv
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial
Experience	flight time last 90 days : 120 flight time total : 8000 flight time type : 120
ASRS Report	424207
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Events
Anomaly	altitude deviation : excursion from assigned altitude
Independent Detector	aircraft equipment other aircraft equipment : unspecified other flight crewa
Resolutory Action	flight crew : exited adverse environment
Consequence	Other
Supplementary
Air Traffic Incident	other

Narrative:

At FL390, 50-60 mi east of dbl VOR, we encountered an extremely strong mountain wave (no turbulence just light chop). The airspeed rapidly dropped to minimum selectable range on the airspeed indicator. The captain pushed the throttles to the toga position. The airspeed continued to decrease into the alpha protection range. The captain disengaged the autoplt and started a 1000 FPM descent, still in the toga thrust position. At approximately FL375-370 the airspeed started to increase out of the alpha protection range and above the minimum selectable indication. When it was apparent we could not maintain airspeed and altitude and needed to leave FL390 to start a descent the first officer attempted to advise ATC and request a lower altitude (FL350). Due to radio congestion we were not able to contact ATC until approximately FL375. We were VMC and able to clear traffic visually and with TCASII display. ATC was advised and cleared us to FL350. Did not really have any unusual mountain wave reports. And ATC just happened to be very busy precisely when the encounter happened.

Original NASA ASRS Text

Title: FLC OF AN MLG ENCOUNTERED MOUNTAIN WAVE TURB RESULTING IN LOSS OF AIRSPD AND ALT BEFORE REGAINING CTL. THE FO WAS TRYING TO ADVISE ARTCC CTLR OF THEIR LOSS OF ALT AND NEED FOR A LOWER ALT, BUT WAS DELAYED BY A CONGESTED FREQ AND CTLR WORKLOAD.

Narrative: AT FL390, 50-60 MI E OF DBL VOR, WE ENCOUNTERED AN EXTREMELY STRONG MOUNTAIN WAVE (NO TURB JUST LIGHT CHOP). THE AIRSPD RAPIDLY DROPPED TO MINIMUM SELECTABLE RANGE ON THE AIRSPD INDICATOR. THE CAPT PUSHED THE THROTTLES TO THE TOGA POS. THE AIRSPD CONTINUED TO DECREASE INTO THE ALPHA PROTECTION RANGE. THE CAPT DISENGAGED THE AUTOPLT AND STARTED A 1000 FPM DSCNT, STILL IN THE TOGA THRUST POS. AT APPROX FL375-370 THE AIRSPD STARTED TO INCREASE OUT OF THE ALPHA PROTECTION RANGE AND ABOVE THE MINIMUM SELECTABLE INDICATION. WHEN IT WAS APPARENT WE COULD NOT MAINTAIN AIRSPD AND ALT AND NEEDED TO LEAVE FL390 TO START A DSCNT THE FO ATTEMPTED TO ADVISE ATC AND REQUEST A LOWER ALT (FL350). DUE TO RADIO CONGESTION WE WERE NOT ABLE TO CONTACT ATC UNTIL APPROX FL375. WE WERE VMC AND ABLE TO CLR TFC VISUALLY AND WITH TCASII DISPLAY. ATC WAS ADVISED AND CLRED US TO FL350. DID NOT REALLY HAVE ANY UNUSUAL MOUNTAIN WAVE RPTS. AND ATC JUST HAPPENED TO BE VERY BUSY PRECISELY WHEN THE ENCOUNTER HAPPENED.

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.