Accident Sequence
While cruising at 9,500 feet MSL, the pilot detected vibrations accompanied by a slight loss of engine power. Shortly thereafter, a loud noise was heard, smoke was observed, and the engine lost all power. The pilot executed a forced landing, during which the airplane collided with trees in wooded, mountainous terrain.
Engine Examination
Post-accident examination of the engine revealed a 3-inch diameter hole adjacent to the #1 cylinder's attachment point on the right side of the crankcase. Debris from the #1 and #2 pistons and the #1 and #2 connecting rods was found throughout the interior of the crankcase. According to an FAA inspector's report, "Cylinders 1 and 2 evidenced a catastrophic failure similar to that caused by detonation from uneven fuel distribution." The rear crankcase journals were dry and displayed thermal damage. The interior of the crankcase and the rear section of the crankshaft showed impact damage consistent with internal engine component failures.
Maintenance Findings
Further inspection identified a portion of an air intake bypass door from the throttle body air intake lodged in the intake port of the #6 cylinder. Examination of the throttle body revealed that a replacement air intake door had been installed; however, the maintenance records did not document any service or replacement of the bypass door.