What happened
During a cruise climb, the aircraft's engine underwent a catastrophic failure. Following the loss of engine power, the pilot attempted a forced landing. During this maneuver, the airplane impacted a tree and a fence located near a residential area.
The investigation
Maintenance records indicated that the engine had undergone an overhaul on August 20, 1991, and had been in service for 1,423.49 hours since that date.
Upon examination of the engine, investigators identified that the number 6 connecting rod had failed due to an overload; this was determined to be a secondary failure. A detailed inspection of one of the connecting rod's bolt and nut assemblies revealed that the nut was cut and split within the threaded area. The first two threads of the nut, which were adjacent to the contact surface, were found to be stripped. Furthermore, investigators noted that no scuff marks were present on the copper plating in the undamaged portion of the nut contact surface.
Findings
The investigation concluded that the two stripped threads occurred because the nut backed off during engine operation. The absence of scuff marks on the nut's contact surface indicated that the component had been improperly torqued during its installation.