What happened
The incident occurred during a night flight intended to reposition the aircraft back to its home base. While the pilot was cruising at an altitude of approximately 1000 feet above ground level (AGL), the engine suddenly lost power. Recognizing the emergency, the pilot transmitted a 'MAYDAY' call and immediately began maneuvering the aircraft to execute a forced landing on a nearby interstate highway.
As the aircraft approached the highway for landing, it struck the top wire of a power line that crossed over the roadway in that specific area. The impact with the power line caused the aircraft to flip over. It then fell onto the northbound lanes of the highway, resulting in a significant crash.
The investigation
Following the accident, investigators conducted a teardown of the engine to determine the root cause of the failure. The examination revealed that the accessory drive gear attaching bolt (Part Number: STD-847) had backed out. This disengagement allowed the drive gear to loosen, which in turn resulted in a loss of synchronization between the valve timing and ignition timing.
Further inspection uncovered a critical maintenance error: an incorrectly sized Helicoil was found installed in the crankshaft. This Helicoil had been used previously to repair the threads for the gear attaching bolt. The improper installation of this thread insert likely contributed to the bolt backing out during operation, leading to the catastrophic engine failure.
Findings
The primary factors contributing to this accident were mechanical and maintenance-related. The loss of engine power was directly caused by the accessory drive gear detaching due to a loose bolt. This mechanical failure was exacerbated by the use of an incorrect Helicoil repair in the crankshaft, which failed to secure the bolt under operational stress. The subsequent collision with power lines during the forced landing attempt prevented any possibility of a safe touchdown on the highway surface.