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NTSB Report
April 19, 1993 · Mitsubishi MU-2B-60 · N86SD
Zwingle, IA
Overview
Report Status
Probable Cause
Event Type
Accident
Date
April 19, 1993
Injury Outcomes
8 injuries (8 fatalities)
Location
Zwingle, IA
Aircraft
Mitsubishi MU-2B-60
Tail Number
N86SD
Injuries
Fatal
Aircraft Damage
Destroyed
NTSB Number
DCA93GA042
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 and shutdown of the left engine. The left engine was forced downward and inboard on its mounts. The cabin depressurized, possibly from blade contact. The flight crew made an emergency descent and received a vector to divert for an ILS approach to Dubuque. The airplane was incapable of maintaining altitude and descended in instrument conditions. Subsequently, it collided with a silo and 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 report: NTSB/ARR-93/08.)
NTSB Probable Cause
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.
Full Narrative
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.)
Flight
Event TypeAccident
Event Time15:52 CDT
Nearest AirportDubuque Regional (8nm S)
Runway31
Runway Length6498 ft
Runway Width150 ft
Flight Plan FiledIFR
Aircraft FireGrd
Aircraft ExplosionNo
Aircraft
AircraftMitsubishi MU-2B-60
Tail NumberN86SD
Aircraft CategoryAir
DamageDestroyed
Engines2
Engine TypeTurboprop
Engine DetailsGarrett · TPE-331-10 · Turboprop · 940
Seats10
People & Injuries
Injury Summary8 injuries (8 fatalities)
Fatal8
Serious0
Minor0
Uninjured0
Total8
Crew 1Age 52 · Second Class
Conditions
ConditionsIMC
LightDaylight
Wind40° @ 16 kt
Visibility1 sm
CeilingOvercast 200 ft
Temperature / Dew Point46 / 45 C
Altimeter29.39 inHg
Observation Time20:55
Weather SourceWFAC