On June 29, 2011, at 1003 central daylight time, an experimental amateur-built Keesler W8 Tailwind airplane, N56PK, was substantially damaged during a forced landing at Wittman Regional Airport (KOSH), Oshkosh, Wisconsin. The pilot sustained minor injuries. The airplane was registered to and operated by the private pilot under the provisions of 14 Code of Federal Regulations Part 91. Day visual meteorological conditions prevailed for the flight, which was operated without a flight plan. The local flight was originating at the time of the accident. The pilot reported that the accident flight was the first flight in the accident airplane in over 6 months, during which time he had been modifying the airplane. The purpose of the flight was to orbit the airport while he evaluated the various airframe and engine systems. The pilot stated that during initial climb from runway 9, at an altitude of about 200 feet above the ground, the engine began to lose power. He notified the control tower of his engine emergency and began a turn to the south to remain clear of nearby buildings. The airplane landed in a grassy area to the south of runway 9, boarded by taxiways Juliet and Kilo. The forward fuselage and firewall were substantially damaged when the airplane encountered a ditch, collapsing the main landing gear. According to available documentation, the airplane was issued its airworthiness certificate on May 19, 2010, and had completed 4.0 hours of its required 40 hour phase I flight testing when the accident occurred. A conditional inspection was completed on June 27, 2011, at 3.9 hours total airframe time. The last recorded flight was completed on November 10, 2010. The engine, a Continental Motors model C-90-12F, had accumulated 5.7 hours since its last major overhaul, which was completed on January 2, 2009. The carburetor and spark plugs were replaced by the pilot/builder on September 7, 2010. The No. 2 cylinder was replaced by the pilot/builder on October 18, 2010. The last recorded engine maintenance was an oil change that was completed on June 1, 2011. A postaccident engine examination revealed that both spark plugs for the No. 4 cylinder were loosely installed into the cylinder head. The upper plug for the No. 4 cylinder was not seated on its cylinder gasket. Several spark plugs were removed and exhibited features consistent with the engine operating at a rich carburetor mixture setting. Both magnetos provided spark when rotated. Mechanical continuity was confirmed from the cockpit engine controls to their respective engine components. The carburetor heat control was in the off position. The carburetor bowl contained fuel with no evidence of water contamination. At 1011, the airport's automated surface observing system reported the following weather conditions: wind 180 degrees at 3 knots; visibility 10 miles; clear skies; temperature 21 degrees Celsius; dew point 13 degrees Celsius; altimeter setting 30.15 inches of mercury. The carburetor icing probability chart included in Federal Aviation Administration Special Airworthiness Information Bulletin No. CE-09-35, Carburetor Icing Prevention, indicated that there was a potential for serious carburetor icing while operating at glide power.