On August 17, 1993, at 0945 central daylight time, a Canadair CL-600-2B16, N392PT, experienced an uncommanded yaw while in cruise flight at FL410, near San Antonio, Texas. There were no injuries to the two airline transport rated pilots and their six passengers. Visual meteorological conditions prevailed for the executive flight. According to the captain, the airplane was in cruise flight at the assigned altitude of FL410 in VMC conditions, with the auto pilot engaged, when the crew noticed an uncommanded yaw to the right without any warning or indication in the cockpit. A normal descent was initiated as the airspeed was reduced to 250 knots. After switching to a secondary flight guidance computer and disconnecting the yaw damper and the auto pilot, the pilots discovered that rudder deflections to the right were not possible. They continued to the San Antonio International Airport and landed without incident. Examination of the airplane's rudder control system by a Federal Aviation Administration inspector and a manufacturer's representative revealed that the shaft portion of the rudder quadrant was fractured. Metallurgical evaluation of the fracture surface and associated hardware revealed that the separation of the shaft portion was the result of stress corrosion cracking. The cracking initiated from the opposite sides of the shaft and progressed inward until about 60% of the shaft cross section was cracked. The Metallurgist's Factual Report is an enclosure.