What happened
During a visual flight rules (VFR) cross-country flight, the aircraft experienced an unexpected engine failure. The pilot executed an emergency landing at night, bringing the aircraft down into a soybean field.
The investigation
Post-accident examination of the engine revealed that a counterweight pin had become dislodged from the crankshaft counterweight. This displacement was caused by a failure of the lock ring securing the component. The loss of the pin resulted in an overload shear of the loose counterweight within the engine assembly.
Findings
The mechanical failure originated from the internal engine components, specifically the crankshaft counterweight retention system. The dislodged pin and subsequent shear damage to the counterweight led directly to the cessation of engine power.
Safety message
Regular inspection of engine lock rings and counterweight pins is critical to prevent similar failures. Pilots should be aware that internal mechanical issues can occur without prior warning signs.