On February 1, 2003, approximately 1300 central standard time, a Gilbertson Zodiac CH601HDS experimental airplane, N474BG, struck a berm during a forced landing following a total loss of engine power near Grandfield, Oklahoma. The airplane was built, owned, and operated by the pilot under 14 Code of Federal Regulations (CFR) Part 91. The airline transport pilot, sole occupant of the airplane, was not injured, and the airplane sustained substantial damage. Visual meteorological conditions prevailed for the local flight, and a flight plan was not filed. The test flight originated from Chattanooga Sky Harbor Airport (92F) at Chattanooga, Oklahoma, approximately 1240. The 5,043 hour pilot reported to the FAA inspector that during a flight that morning, the 73 horsepower Subaru automotive engine was running rough; however, he landed the airplane without further incident. The carburetor screen was removed, cleaned of debris, and reinstalled by the pilot. On the Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1/2, the pilot reported that during the accident flight, the oil and water temperature indicated 205-210 degrees, and the pilot elected to return to the airport at an initial altitude of 700 feet agl. At an airspeed of 70 knots and an altitude of 500 feet agl, there was a total loss of engine power. The pilot performed the emergency landing procedures. The pilot was not able to turn the airplane into the wind due to an unsafe landing area east of the airport traffic pattern for runway 17. The pilot attempted a 270 degree turn to a landing area; however, due to the decreasing altitude and airspeed (55 knots) of the airplane, he was not able to compete the turn. The pilot landed the airplane in a field with a right quartering tailwind. During the landing roll, the nose landing gear struck a berm, the nose landing gear collapsed, and the airplane came to rest 110 feet from the initial impact point in the field. The pilot reported the winds were from the southwest at 25 knots, with gust to 30 knots. The FAA inspector reported that during the impact sequence, the airplane structure twisted, and the firewall and all components forward of the firewall separated from the airplane. The integrity of the fuel tank was compromised. The airplane had accumulated 27.6 hours of the required 40 hours of flight time under the operating limitations for the homebuilt airplane.