FTW98LA242

During the initial takeoff climb, the engine gradually lost power, and the airplane descended into trees approximately 1/4 mile from the departure end of the runway. Examination of the engine revealed evidence that the power loss resulted from an excessively rich mixture. Black carbon deposits were present on the exhaust valves and exhaust stacks of two cylinders, and the engine oil drained from all the cylinders contained aviation fuel. Examination of the carburetor revealed no evidence of any discrepancies that would have resulted in a rich mixture. The reason for the excessively rich mixture could not be determined.

On May 29, 1998, at 0930 central daylight time, a Grumman American AA-1C airplane, N9606U, was substantially damaged during a forced landing following a loss of engine power near Roanoke, Texas. The commercial pilot, owner/operator and sole occupant of the airplane, sustained minor injuries. No flight plan was filed and visual meteorological conditions prevailed for the Title 14 CFR Part 91 personal flight. When the accident occurred, the flight was originating from the Northwest Regional Airport, Roanoke, Texas, with an intended destination of Fort Worth, Texas.

According to the pilot, the pre-flight inspection and run-up were "normal" with no discrepancies noted. During the takeoff roll on runway 17, the "RPM and ground acceleration were normal" as was the acceleration to a climb speed of 80 knots. When the climb was initiated, "engine power began to diminish, and continued to do so." The 9,213 hour pilot stated that he proceeded straight ahead and "reduced airspeed to best angle of climb speed (65 KTS) to clear trees." However, "engine power continued to diminish, and [the] aircraft settled into trees approximately 1/4 mile from the departure end of [the] runway." The pilot reported that the airplane came to rest inverted and sustained structural damage to the empennage and both wings.

A witness, who observed the airplane begin its takeoff roll, reported that the "engine run up sounded normal;" however, at the start of the takeoff roll, the "engine sounded as if it was going rich (as if flooding was occurring)." Another witness, who observed the initial takeoff climb, reported that "the engine sounded weak, as if it was running very lean or very rich."

On June 25, 1998, the engine, a Lycoming O-320-D2B, S/N L-5399-39, was disassembled under the supervision of the NTSB investigator-in-charge at the facilities of Air Salvage of Dallas, Lancaster, Texas. Tan colored deposits were noted on the exhaust valves and the exhaust stacks of the #1 and #2 cylinders, and black colored deposits were noted on the exhaust valves and the exhaust stacks of the #3 and #4 cylinders. The engine oil that drained from all four cylinders was a greenish brown color and smelled like aviation fuel. The carburetor, a Marvel-Schebler MA4SPA, S/N A-5-2919, was disassembled and found to be equipped with a metal float and a one piece venturi. The needle assembly was intact and free to move. No debris was found in the fuel inlet screen or in the bottom of the carburetor bowl. On August 19, 1998, the carburetor was reassembled and a fuel supply was connected to the inlet. After 1 hour, no leaks were noted.