​​​On April 19, 1993, at 1552 central daylight time, a Mitsubishi MU-2B-60, registered in the United States as N86SD and operated by the South Dakota Department of Transportation, as a public use airplane, collided with a silo on a farm near Zwingle, Iowa, while attempting an approach to an emergency landing at Dubuque Regional Airport, Dubuque, Iowa. The airplane was destroyed in the collision and postcrash fire. The captain, first officer, and the six passengers aboard were fatally injured. Instrument meteorological conditions existed at the time. The flight originated from Cincinnati, Ohio, at 1406, on an instrument flight rules flight plan.

​We determined that the probable cause of this accident was the fatigue cracking and fracture of the propeller hub arm. The resultant separation of the hub arm and the propeller blade damaged the engine, nacelle, wing, and fuselage, thereby causing significant degradation to aircraft performance and control that made a successful landing problematic. 

The cause of the propeller hub arm fracture was a reduction in the fatigue strength of the material because of manufacturing and time-related factors (decarburization, residual stress, corrosion, mixed microstructure, and machining/scoring marks) that reduced the fatigue resistance of the material, probably combined with exposure to higher-than-normal cyclic loads during operation of the propeller at a critical vibration frequency (reactionless mode), which was not appropriately considered during the airplanelpropeller certification process. 

​As a ,result of this investigation, we made the following recommendations: 

To the Federal Aviation Administration: 

• Determine whether any 4-bladed Hartzell propeller hubs have ever been installed on MU-2B airplanes and are now installed on other model airplanes, and issue the necessary airworthiness directives to inspect the hubs for fatigue damage. (A-93-153)
• Identify airplanes that can, through a combination of the resonant RPM, the ground idle RPM range, and repair limits at the blade tip, produce the reactionless mode in the normal operating range. For those airplanes containing Hartzell hubs at risk for reduced fatigue properties (manufactured prior to April 1984), require inspection for cracks in the pilot tube hole. (A-93-154)
• Perform a designated safety inspection for cracking in the pilot tube hole on high time Hartzell 3-, 4-, and 5-bladed propeller hubs that are found to have high operating stress and that were manufactured with the pilot tube holes fmished machined prior to heat treatment. (A-93-155)
• Increase the mhimum ground idle RPM speed of the HC-B4 propeller on the MU-2B airplane to provide a greater margin between the resonant frequency of the reactionless mode and the ground idle speed. (A-93-156) 
• Revise maintenance and repair limits for propeller blades on HC-B4 hubs on MU-2B aircraft to reduce the length of the uncontrolled area at the blade tip to minimize the in-sewice increase in the reactionless mode frequency. (A-93-157) 
• Enhance the Emergency Assistance section of Air Traffic Control handbook 71 10.65 to fully address the issue of selecting the best possible diversion airport for an aircraft in an emergency status. (A-93-158) 
• Provide all ARTCC sector positions of operation with the capability of displaying several hourly weather sequence reports at once. This display should be updated automatically, and displayed at a11 times. (A-93-159) 
• Provide expanded emergency procedures training for air traffic controllers. The general capabilities of airplanes in various emergency scenarios involving air traffic control should be a focal point of this training, and past air traffic control-related accident reports should be used. (A-93-160)