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| Title: | In-flight loss of propeller blade and uncontrolled collision with terrain, Mitsubishi Mu-2B-60, N86SD, Zwingle, Iowa, April 19, 1993 |
| Micro summary: | This Mu-2B-60 experienced a catastrophic loss of a propeller, leading to a crash. |
| Event Time: | 1993-04-19 at 1552 CDT |
| File Name: | 1993-04-19-US.pdf |
| Publishing Agency: | National Transportation Safety Board (NTSB) |
| Publishing Country: | USA |
| Report number: | NTSB-AAR-93-08 |
| Pages: | 131 |
| Site of event: | Near Zwingle, Iowa |
| Latitude/Longitude: | N42°15'21.6" W90°41'20.4" |
| Departure: | Lunken Field, Cincinnati, Ohio, USA |
| Destination: | Joe Foss Field Airport, Sioux Falls, South Dakota, USA |
| Airplane Type(s): | Mitsubishi Mu-2B-60 |
| Flight Phase: | Cruise |
| Registration(s): | N86SD |
| Operator(s): | South Dakota Department of Transportation |
| Type of flight: | Revenue |
| Occupants: | 8 |
| Fatalities: | 0 |
| Serious Injuries: | 0 |
| Minor/Non-Injured: | 8 |
| Other Injuries: | 0 |
| Executive Summary: | 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. The National Transportation Safety Board determines 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. The safety issues in this report include the propeller hub design, certification and continuing airworthiness, and air traffic control training. Safety recommendations concerning these issues were addressed to the Federal Aviation Administration. |
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