Flight and Engine Malfunction
After approximately 75 minutes of uneventful flying, the engine RPM began to fluctuate. The pilot attempted to adjust the throttle and mixture, but noticed that the engine RPM had dropped into the teens. Level flight could not be maintained.
Approach and Descent
Gusty winds were encountered during a circling approach to a large, flat field. During the final descending turn, the pilot recalled that the airplane's nose was diving quickly toward the ground, and the stall warning activated briefly.
Witness Observations and Impact
Witnesses reported that the aircraft was maneuvering slowly and low to the ground, then it descended steeply and crashed. Ground scars indicated that the aircraft impacted with the left wing down and at approximately 45 degrees nose down relative to the terrain.
Investigation Findings
Atmospheric conditions were not conducive for carburetor icing. Fuel samples from the aircraft and the refueling source were found to be within standards. With the exception of worn or fouled spark plugs and out-of-tolerance valve tappet clearances, no conclusive mechanical anomalies were discovered. The engine was operated to 2360 RPM in a factory test cell, but the run was terminated due to vibrations induced from impact damage.
No probable cause was determined by the investigation.