Incident Overview
The airplane climbed to 4,000 feet. After leveling off, the pilot checked the engine gauges, which were 'in the green,' set the power to 2,400 rpm and 24 inches of manifold pressure, and engaged the autopilot. The engine ran smoothly with no vibrations.
Explosion and Fire
Approximately 35 to 40 seconds later, a loud explosion occurred, and blue and yellow flames emanated from both sides of the engine compartment. The pilot turned the airplane toward a field he had seen earlier. During the turn, a second explosion occurred. The engine, which had continued to run smoothly after the first explosion, then quit. The cabin became engulfed in black smoke, and forward visibility was nil.
Forced Landing
The pilot executed a forced landing. Due to limited visibility out the side window, the airplane struck trees during the approach, stalled, and impacted the ground.
Post-Accident Examination
Investigation revealed fire damage to the accessory section of the engine, the firewall, and the interior surface of the left engine cowling. Unburned oil residue was found on the interior surface of the right engine cowling. Oil was also found on the undersides of the fuselage and wings. The oil filler tube on the right side of the engine was burned away; however, the majority of the oil dipstick remained in place in the engine. The interior surface of the left cowling was sooted and scorched. The bottom plate of the engine-driven fuel pump was burned away, and the fuel boost pump was sooted. There was a breach in the engine casing near the number 6 connecting rod. All connecting rod ends exhibited heat damage consistent with a loss of lubrication, with the number 6 rod end most severely damaged. The oil pump housing and impellers exhibited light scoring, while the main bearings were undamaged.
Pilot Statements
The pilot stated that during his preflight inspection, he checked three times that the oil cap was secure and found no oil leaks on the ramp. Another pilot with the same airframe/engine configuration reported that his oil cap had come loose during flight on several occasions; however, even when loose, there was no oil leakage.