On August 7, 2012, about 1620 central daylight time a Williams T-51 Mustang airplane, N151TA, conducted a forced landing at the Wittman Regional Airport (KOSH), Oshkosh, Wisconsin. The commercial pilot was seriously injured and the airplane was substantially damaged. The airplane was registered to and operated by a private individual under the provisions of 14 Code of Federal Regulations Part 91 as a test flight. Visual meteorological conditions prevailed for the flight which operated without a visual flight rules flight plan. The local flight was originating at the time of the accident. According to the pilot, the purpose of the test flight was to trouble-shoot the lower engine power and higher than expected operating temperatures encountered during the previous test flight. After takeoff, the airplane was not performing as expected and the engine was slowly losing power. The pilot observed that the coolant temperature was 225 degrees and the airplane was only 250 to 300 feet above ground level. The pilot communicated with air traffic control that he wanted to return to the airport and started a turn. During the turn, the pilot heard and felt an explosion behind him and felt a burning sensation on his back. The pilot lost all forward visibility as the windscreen was black with smoke, but he was able to see a small amount out the left side of the windscreen. The pilot stated that he continued to try and fly the airplane, though he was limited by the inability to see outside of the airplane and the pain associated with the burning sensation on the left side of his body. The pilot extended the landing gear and conducted a forced landing in the grass, just short of the general aviation ramp. According to the Federal Aviation Administration (FAA) inspector who traveled to the accident scene, the right main landing gear collapsed during the forced landing. The airplane continued an additional 300 feet before coming to rest. The left wing and left flap were wrinkled during the forced landing. The airplane was recovered and relocated to the owner’s hangar in Ohio for further examination. The FAA inspector who provided oversight for the examination reported that there were no negative properties or operational failures observed during the testing of the cooling system. The number four spark plug was severely damaged. According to the pilot, an examination of the engine revealed no further anomalies. The pilot added that a cooling hose separated from the radiator which was located behind him. The radiator and cooling lines had not been boxed in, which would isolate the cooling system from the cockpit. Several days prior to the accident flight, the engine and cooling system operated at a higher than normal power setting and coolant temperature. After the flight, the coolant temperature reached the maximum range of 250 degrees Fahrenheit and coolant purged itself from the system. On a subsequent flight, the coolant temperature gauge did not function and shortly after takeoff the pilot observed smoke in the cockpit. As the pilot returned to the airport to land, he noticed that the engine oil temperature was high and the oil pressure was low. An examination revealed an oil leak between the cylinder head and the engine block. Following this flight, a replacement engine was installed; however, the cooling system was not replaced.