On August 3, 2009, about 1000 central daylight time, a single-engine Grumman American AA-1B airplane, N1644R, was substantially damaged during a forced landing following a loss of engine power after takeoff from the Lake Whitney State Park Airport (F50), Whitney, Texas. The airline transport pilot, the sole occupant, sustained minor injuries. The airplane was registered to and operated by the pilot. Visual meteorological conditions prevailed and no flight plan was filed for the 14 Code of Federal Regulations Part 91 personal flight. The flight was originating at the time of the accident and was destined for the Clifton Municipal Airport (7F7), near Clifton, Texas. According to the pilot, following an hour's flight including touch and go maneuvers, he landed at 7F7, added 10.9 gallons of fuel, and departed the airport. After a time of sightseeing, he landed at F50, back taxied, and performed a magneto check before departing. Shortly after takeoff the engine began to "run rough as if it had a fouled plug." While turning to downwind the engine began to "shake violently" and the RPM and altitude were decreasing. The pilot selected a field and performed a soft field landing. During the landing rollout the airplane nosed over and came to rest in an inverted position. The pilot was able to exit the airplane unassisted. Moments later the airplane was engulfed in flames. The engine, with 67 hours since major overhaul, was separated from the airframe and placed on a bench. The upper spark plugs and valve covers were removed. Investigators manually rotated the engine via the vacuum pump drive. Valve train continuity was established to each cylinder and to the oil pump. Thumb compression was developed in each cylinder. The cylinders were examined via a borescope and all of cylinder domes and piston heads exhibited "normal" deposits. Each intake and exhaust valve was found in their respective positions. Both magnetos were removed and examined. The left magneto was placed on a test bench and produced spark to all four leads. The right magneto initially produced spark to all four leads and then stopped. An examination inside the right magneto revealed thermal damage as a result of the post impact fire. The spark plugs exhibited normal wear when compared to the Champion Check-A-Plug comparison card. The exception was the number 3 lower spark plug that had sustained impact damage. In addition, the number 2 and 4 lower spark plugs were oil soaked. The carburetor exhibited impact damaged to the mounting flange. The carburetor fuel inlet screen was removed and found to be clean and unobstructed. The unit was disassembled and examined. No post accident anomalies were noted.