A flight crew encountered moderate clear air turbulence. They utilized the captain's authority/authorized to change flight levels until able to receive an amended clearance near alo.

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Attributes
ACN	456566
Time
Date	199912
Day	Sat
Local Time Of Day	1201 To 1800
Place
State Reference	IA
Altitude	msl bound lower : 18000 msl bound upper : 20000
Environment
Flight Conditions	Mixed
Weather Elements	Turbulence
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zau.artcc
Operator	common carrier : air carrier
Make Model Name	Large Transport, Low Wing, 2 Turbojet Eng
Operating Under FAR Part	Part 121
Navigation In Use	other
Flight Phase	cruise : level
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 : 18000 flight time type : 180
ASRS Report	456566
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : multi engine pilot : flight engineer pilot : atp
Experience	flight time last 90 days : 150 flight time total : 8000 flight time type : 1500
ASRS Report	456712
Events
Anomaly	altitude deviation : excursion from assigned altitude inflight encounter : turbulence non adherence : clearance
Independent Detector	other flight crewa other flight crewb
Resolutory Action	controller : issued new clearance flight crew : exited adverse environment
Consequence	faa : reviewed incident with flight crew
Supplementary
Problem Areas	ATC Human Performance Weather Airspace Structure
Primary Problem	Weather
Air Traffic Incident	Pilot Deviation

Narrative:

While cruising at FL200, we encountered heavy clear air turbulence -- moderate turbulence bordering on severe. We were talking to ZAU frequency 127.3, and my first officer attempted to contact center for clearance to a lower altitude. The controller was working several frequencys and would not respond to our calls. The turbulence was very strong and I felt we had no choice but to descend to a lower altitude. We went down to FL180 and found smoother air. Finally, ZAU answered our calls and we told them what we had done and they assigned FL180 as our altitude. I feel a contributing factor in this was controller overload, working too many sectors and frequencys. It took approximately 30 seconds to 1 min to get a response from the controller.

Original NASA ASRS Text

Title: A FLC ENCOUNTERED MODERATE CLR AIR TURB. THEY UTILIZED THE CAPT'S AUTH TO CHANGE FLT LEVELS UNTIL ABLE TO RECEIVE AN AMENDED CLRNC NEAR ALO.

Narrative: WHILE CRUISING AT FL200, WE ENCOUNTERED HVY CLR AIR TURB -- MODERATE TURB BORDERING ON SEVERE. WE WERE TALKING TO ZAU FREQ 127.3, AND MY FO ATTEMPTED TO CONTACT CTR FOR CLRNC TO A LOWER ALT. THE CTLR WAS WORKING SEVERAL FREQS AND WOULD NOT RESPOND TO OUR CALLS. THE TURB WAS VERY STRONG AND I FELT WE HAD NO CHOICE BUT TO DSND TO A LOWER ALT. WE WENT DOWN TO FL180 AND FOUND SMOOTHER AIR. FINALLY, ZAU ANSWERED OUR CALLS AND WE TOLD THEM WHAT WE HAD DONE AND THEY ASSIGNED FL180 AS OUR ALT. I FEEL A CONTRIBUTING FACTOR IN THIS WAS CTLR OVERLOAD, WORKING TOO MANY SECTORS AND FREQS. IT TOOK APPROX 30 SECONDS TO 1 MIN TO GET A RESPONSE FROM THE CTLR.

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.