What happened
While in flight, the pilot experienced a loss of engine power associated with a decrease in engine oil pressure. Due to the power loss, the pilot performed an off-airport landing in a wheat field. During the ground rollout following the touchdown, the airplane struck a ditch that was not visible from the air. The impact caused the aircraft to receive substantial damage.
The investigation
An examination of the engine revealed that three pistons had experienced the effects of detonation. During the post-accident interview, the pilot initially informed the Safety Board that 100LL aviation gasoline was being used. However, the pilot later acknowledged that his brother had fueled the aircraft with automobile gasoline on the day of the accident. The pilot further stated that he did not detect the use of automobile gasoline during his preflight inspection.