Incident Overview
Shortly after takeoff, the pilot heard a loud bang, felt a thump, and observed the engine cowling deflect upward. In response, he reduced engine power and turned towards the departure airport. A second loud bang followed by severe vibration prompted the pilot to shut down the engine. He subsequently executed a forced landing to a field.
Wreckage Examination
Examination of the wreckage revealed two holes in the top of the engine crank case, each over 8 inches in diameter. The connecting rod bolt for the number six cylinder showed signs of catastrophic failure. The other number six connecting rod bolt was intact but did not contain a nut. The intact bolt's threads exhibited severe mechanical impact damage.
Bolt Fracture Characteristics
The broken bolt displayed three distinct fracture features. One area was consistent with failure in shear, another was consistent with a tensile overstress separation, and the majority of the third area had a rubbed appearance.
Maintenance History
The connecting rod bolts and nuts had been replaced 375 hours before the accident flight. The last annual inspection of the airplane was performed approximately 16 months prior to the accident.