On March 26, 1997, about 1345 eastern standard time, a Dassault Falcon 900B, N332MC, encountered clear air turbulence, during descent, about 150 nautical miles north of Atlanta, Georgia. The airplane was operated by MCI Systemhouse Corporation under the provisions of Title 14 CFR Part 91, and instrument flight rules. Visual meteorological conditions prevailed. An instrument flight plan was filed for the positioning flight. There were no injuries to the two airline transport pilots, while the flight attendant was seriously injured. The airplane was not damaged. Origination of the flight, which was destined for Chamblee, Georgia, was Dulles International Airport, Washington, DC, about one hour prior to the accident. According to the Captain, there was light turbulence at the cruise altitude of 31,000 feet. Once a descent was initiated, moderate turbulence was encountered at 30,000 feet. As the airplane descended through 27,000 feet, there was a sudden drop of about 10 to 15 feet which resulted in the flight attendant falling to the floor in the aft lavatory. Her fall resulted in a broken ankle.