Accident Overview
During a Title 14 CFR Part 135 passenger flight, the airline transport pilot reported hearing a loud bang while in cruise. The engine subsequently began running rough, prompting the pilot to divert to the nearest airport. Before reaching the runway, the engine failed completely, and the aircraft was unable to land on the intended airfield. The airplane collided with terrain, sustaining substantial damage to both wings and the fuselage.
Engine Examination
A detailed post-accident examination of the engine was conducted. The inspection revealed that a locking screw had backed out from one of the anti-vibration counterweights attached to the crankshaft. This condition resulted in scoring along the interior back surface of the engine case. The unsecured counterweight shifted from its designated position within the crankshaft. It was then likely struck by the engine's master rod, causing the weight to shatter and become liberated from its normal location. The freed counterweight was subsequently struck by other internal moving parts, leading to multiple component failures. Ultimately, these events caused the engine to seize.
Probable Cause
The official probable cause is the loss of engine power due to the failure of a crankshaft component, resulting in an off airport landing.