What happened
During a scheduled Part 135 passenger flight, the pilot experienced a sudden loud noise followed by significant engine roughness while in the cruise phase of flight. In an attempt to manage the emergency, the pilot initiated a diversion to the closest available airport. However, the engine lost all power before the aircraft could reach the runway. The aircraft subsequently struck the ground, resulting in substantial damage to the fuselage and both wings.
Findings
An investigation into the engine's condition identified that a locking screw had detached from an anti-vibration counterweight located on the crankshaft. This unsecured screw had moved far enough to score the internal back surface of the engine case. As the counterweight shifted from its intended position, it was struck by the engine's master rod, which caused the weight to shatter. The resulting debris from the destroyed counterweight interacted with other internal moving parts, leading to multiple component failures and the eventual seizure of the engine.