On January 16, 1994, at 1135 eastern standard time, an Air and Space America, A gyroplane, N6118S collided with the ground after the aircraft entered an uncommanded right descending turn in Atlanta, Georgia. The aerial observation flight operated under 14 CFR Part 91 with no flight plan filed. Visual weather conditions prevailed at the time of the accident. The gyroplane sustained substantial damage, and the airline transport pilot was not injured. The flight departed Jonesboro, Georgia, at 1100 hours. According to the pilot, while maneuvering about 300 feet above the ground, the aircraft entered an uncommanded right descending turn; efforts to regain control failed. The pilot selected a baseball field and attempted an emergency landing. As the aircraft approached the touchdown point, the pilot realized that continuing the final approach heading would result in a collision with the fence around the baseball field. As the pilot flared the aircraft eight feet above the ground the main rotor blades collided with the ground. Examination of the transmission and clutch engagement system failed to disclose a mechanical explanation for the inadvertent engagement of the clutch assembly. The main rotor system examination also failed to disclose a mechanical malfunction or component failure of rotor system components.