What happened
Less than two hours into the flight, the engine lost power while the aircraft was in the cruise phase. During the subsequent emergency descent, the airplane struck power lines and a tree before colliding with the ground.
The investigation
Investigators examined the engine accessory case and discovered that the idler gear was fractured. Additionally, the two attachment holes in the crank case for the idler gear shaft assembly were found to be elongated. During the recovery process, investigators found three fractured teeth from the idler gear and two separated bolt heads.
The fracture surface of each gear tooth exhibited a rough surface and crack arrest marks, which are consistent with high-stress, low-cycle fatigue. The shanks located below each separated bolt head had been worn down to approximately one-third of their original diameter.
Records indicated the engine had received a zero-time equivalent overhaul on May 9, 1985, and had operated for 1,371 hours since that overhaul. While Textron Lycoming Service Instruction No. 1009AQ recommends an overhaul period of 2,000 hours, the Lycoming parts catalog specifies the required hardware for the idler gear shaft assembly: a bolt is required for one attachment hole, while a slotted nut and stud are required for the other.