Summary
On April 19, 1993, a Mitsubishi MU-2B-60 (N86SD) was involved in an accident near Zwingle, IA. The accident resulted in 8 fatal injuries. The aircraft was destroyed.
The National Transportation Safety Board determined the probable cause of this accident to be: 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 AIRPLANE/PROPELLER CERTIFICATION PROCESS.
This accident is documented in NTSB report DCA93GA042. AviatorDB cross-references NTSB investigation data with FAA registry records to provide comprehensive safety information for aircraft N86SD.
Accident Details
Probable Cause and Findings
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 AIRPLANE/PROPELLER CERTIFICATION PROCESS.
Aircraft Information
Registered Owner (Historical)
Data Source
Data provided by the National Transportation Safety Board (NTSB). For more information on this event, visit the NTSB Records Search website. NTSB# DCA93GA042