Accident Summary
During cruise climb, the airplane's engine suffered a catastrophic failure. The pilot executed a forced landing, during which the aircraft impacted a tree and fence near a residential area.
Maintenance History and Engine Examination
Maintenance records indicated the engine had undergone an overhaul on August 20, 1991, and had accumulated 1,423.49 hours of operation since that overhaul. Subsequent examination of the engine showed that the number 6 connecting rod had failed in overload, but this failure was determined to be a secondary event.
Further examination focused on the connecting rod's bolt and nut assembly. The nut was found to be cut and split in the threaded area. Specifically, the first two threads of the nut, adjacent to the contact surface, were observed to be stripped. Additionally, no scuff marks were present on the copper plating in the undamaged portion of the nut's contact surface.
Findings
Investigators determined that the stripped threads were the result of the nut backing off during engine operation. The absence of scuff marks on the contact surface indicated that the nut had not been properly torqued during installation. This improper torque allowed the nut to loosen, leading to the loss of clamping force and subsequent failure of the connecting rod assembly, which ultimately caused the engine to fail.