What happened
During a cruise at 24,000 feet, a mechanical failure occurred involving the left propeller hub arm. This failure caused a propeller blade to detach and strike a second blade, resulting in the loss of the blade tip. The impact triggered intense engine vibrations and forced the left engine to shut down, while also causing the engine to shift downward and inboard on its mounts. This event likely led to a loss of cabin pressure.
Following the engine failure, the flight crew initiated an emergency descent. While navigating through instrument meteorological conditions, the aircraft was unable to maintain its flight altitude. The plane was directed toward Dubuque for an ILS approach but ultimately collided with a silo, crashing approximately 8 miles south of the airport. The accident resulted in eight fatalities.
Findings
Investigators determined that the failure of the left propeller hub was caused by fatigue originating from several points on the internal diameter of the pilot tube hole. The structural integrity of the hub had been compromised by several contributing factors, including corrosion, residual stresses, and a mixed microstructure. Additionally, the presence of machining marks and decarburization further reduced the fatigue resistance of the component.