High altitude turbulence caused an medium large transport air carrier aircraft flight crew to momentarily lose control of the aircraft resulting in an excursion from assigned altitude (clear air turbulence).

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Attributes
ACN	245644
Time
Date	199307
Day	Sat
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : als
State Reference	CO
Altitude	msl bound lower : 32000 msl bound upper : 33500
Environment
Flight Conditions	Mixed
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zdv
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
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 : 220 flight time total : 9500 flight time type : 5000
ASRS Report	245644
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument pilot : private pilot : atp
Experience	flight time last 90 days : 70 flight time total : 4000 flight time type : 56
ASRS Report	246528
Events
Anomaly	altitude deviation : excursion from assigned altitude inflight encounter : weather non adherence : clearance other anomaly other anomaly other
Independent Detector	other flight crewa
Resolutory Action	flight crew : regained aircraft control flight crew : returned to intended course or assigned course
Consequence	Other
Supplementary
Primary Problem	Weather
Air Traffic Incident	Pilot Deviation

Narrative:

We saw a standing lenticular cloud ahead. We were at FL330 light chop. We slowed from .74 to .70. I made a PA advising the passenger and flight attendant that we might get a few bumps as we pass over the mountains and to remain seated with seat belts fastened. We were below the cloud in the clear. The autoplt was on in altitude hold. Suddenly, we began to climb and pitch down. I reduced power as we stopped climbing at 33,400 ft. We started to descend back down to our cruise altitude of FL330 when we encountered moderate turbulence. I reduced the power to idle as our speed picked up fast to .80 and I placed the ignition to flight, disengaged the altitude hold mode of the autoplt as we passed FL325 and we leveled at 32000 ft. We climbed back up to FL330. We were in the clear all the time. This was the worst mountain wave I have seen. We reported the mountain wave to center. Stay away from standing lenticular clouds, I will.

Original NASA ASRS Text

Title: HIGH ALT TURB CAUSED AN MLG ACR ACFT FLC TO MOMENTARILY LOSE CTL OF THE ACFT RESULTING IN AN EXCURSION FROM ASSIGNED ALT (CLR AIR TURB).

Narrative: WE SAW A STANDING LENTICULAR CLOUD AHEAD. WE WERE AT FL330 LIGHT CHOP. WE SLOWED FROM .74 TO .70. I MADE A PA ADVISING THE PAX AND FLT ATTENDANT THAT WE MIGHT GET A FEW BUMPS AS WE PASS OVER THE MOUNTAINS AND TO REMAIN SEATED WITH SEAT BELTS FASTENED. WE WERE BELOW THE CLOUD IN THE CLR. THE AUTOPLT WAS ON IN ALT HOLD. SUDDENLY, WE BEGAN TO CLB AND PITCH DOWN. I REDUCED PWR AS WE STOPPED CLBING AT 33,400 FT. WE STARTED TO DSND BACK DOWN TO OUR CRUISE ALT OF FL330 WHEN WE ENCOUNTERED MODERATE TURB. I REDUCED THE PWR TO IDLE AS OUR SPD PICKED UP FAST TO .80 AND I PLACED THE IGNITION TO FLT, DISENGAGED THE ALT HOLD MODE OF THE AUTOPLT AS WE PASSED FL325 AND WE LEVELED AT 32000 FT. WE CLBED BACK UP TO FL330. WE WERE IN THE CLR ALL THE TIME. THIS WAS THE WORST MOUNTAIN WAVE I HAVE SEEN. WE RPTED THE MOUNTAIN WAVE TO CTR. STAY AWAY FROM STANDING LENTICULAR CLOUDS, I WILL.

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.