ZWINGLE, IA, USA
N86SD
MITSUBISHI MU-2B-60
WHILE CRUISING AT FL 240, A PROPELLER (PROP) HUB ARM ON THE LEFT PROP FAILED, RELEASING THE PROP BLADE, WHICH STRUCK A 2ND BLADE, BREAKING OFF ITS TIP. THIS RESULTED IN A SEVERE ENGINE VIBRATION & SHUTDOWN OF THE LEFT ENGINE. THE LEFT ENGINE WAS FORCED DOWNWARD & INBOARD ON ITS MOUNTS. THE CABIN DEPRESSURIZED, POSSIBLY FROM BLADE CONTACT. THE FLIGHT CREW MADE AN EMERGENCY DESCENT & RECEIVED A VECTOR TO DIVERT FOR AN ILS APPROACH TO DUBUQUE. THE AIRPLANE WAS INCAPABLE OF MAINTAINING ALTITUDE & DESCENDED IN INSTRUMENT CONDITIONS. SUBSEQUENTLY, IT COLLIDED WITH A SILO & CRASHED ABOUT 8 MILES SOUTH OF DUBUQUE. AN INVESTIGATION REVEALED THE LEFT PROP HUB FAILED FROM FATIGUE THAT INITIATED FROM MULTIPLE INITIATION SITES ON THE INSIDE DIAMETER SURFACE OF THE HOLE FOR THE PILOT TUBE. THERE WAS EVIDENCE THAT THE FATIGUE PROPERTIES OF THE HUB WERE REDUCED BY A COMBINATION OF FACTORS, INCLUDING MACHINING MARKS OR SCRATCHES, MIXED MICRO- STRUCTURE, CORROSION, DECARBURIZATION, AND RESIDUAL STRESSES. (FOR FURTHER INFO, SEE BLUE COVER RPRT: NTSB/ARR-93/08.)
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.
Source: NTSB Aviation Accident Database
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