# NTSB DCA93GA042: Mitsubishi MU-2B-60, N86SD, Zwingle, IA, April 19, 1993

- Type: NTSB accident investigation
- Classification: Accident · Fatal · Class 1 · Final report
- Date: April 19, 1993, 15:52 local (20:52 UTC)
- Place: Zwingle, IA
- Location: 42.2864° N, 90.7177° W: Placed by OpenAirDocs from NTSB’s distance and direction from DUBUQUE RGNL (DBQ), within about 2 miles (OpenAirDocs)
- Nearby events: https://openairdocs.org/map.md?lat=42.2864&lon=-90.7177&mi=10
- Phase of flight: [Enroute](https://openairdocs.org/ntsb-phases/400) (OpenAirDocs' grouping)
- Highest injury: Fatal
- Injuries: 8 fatal
- Status: Completed, report May 10, 1994
- Report: Board-adopted at Board Meeting
- Investigation class: Class 1
- NTSB office: Major Investigations Division
- Safety recommendations: Yes
- NTSB case key (mKey): 15962
- NTSB case page: https://www.ntsb.gov/investigations/Pages/DCA93GA042.aspx
- NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/15962/pdf
- Board report AAR9308 (PDF): https://www.ntsb.gov/investigations/AccidentReports/Reports/AAR9308.pdf
- Page: https://openairdocs.org/ntsb/DCA93GA042

## 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.

## Sequence of events

In NTSB's coding before 2008 (https://www.ntsb.gov/GILS/Documents/codman.pdf).

- Airframe/component/system failure/malfunction, cruise - normal
  - propeller system/accessories,hub - fatigue (cause)
  - propeller system/accessories,blade - separation
  - powerplant - vibration
- Loss of engine power(total) - mech failure/malf, cruise - normal
  - 1 engine
- Decompression, cruise
- Forced landing, descent - emergency
- In flight collision with object, approach
  - weather condition - low ceiling (contributing factor)
  - weather condition - fog (contributing factor)
  - weather condition - rain (contributing factor)

## Analysis

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.)

## Aircraft

- Registration: [N86SD](https://openairdocs.org/deregistered/N86SD-1994-07-14), registration cancelled 1994-07-14, S/N 765SA (OpenAirDocs)
- Reported make and model: MITSUBISHI MU-2B-60
- Registry model: [Mitsubishi MU-2B-60](https://openairdocs.org/model/mitsubishi-mu-2b-60) (registry code 5780460) (OpenAirDocs)
- Category: Airplane
- Serial number: 765 S.A.
- Airframe hours: 4580
- Engines: GARRETT TPE-331-10 940 horsepower turbo prop
- Damage: Destroyed
- Fire: On-Ground
- Flight: Public aircraft, Public use
- Registered owner: State Of South Dakota
- Airworthiness certificate: Standard, Normal
- Last inspection: Continuous airworthiness, April 12, 1993, 10 hours before
- Departed: LUK, CINCINNATI, OH
- Destination: PIR, PIERRE, SD
- Flight plan: IFR
- Airport: DBQ, Dubuque Regional, runway 31, 6,498 × 150 ft, asphalt

## Flight crew

As NTSB recorded them. NTSB leaves out names.

- Pilot; age 52; airline transport, flight instructor; airplane ratings: single-engine land, multi-engine land; instrument ratings: airplane; medical: class 2, valid medical--w/ waivers/lim; seat: left; toxicology performed; flight hours: 10,607 total, 1,922 in this make and model, 26 in the last 30 days, 6 in the last 24 hours

## Weather

- Conditions: Instrument (IMC), Day
- Wind: 40° at 16 knots
- Visibility: 1 statute miles
- Lowest ceiling: 200 ft overcast
- Temperature: 46 °F, dew point 45 °F
- Altimeter: 29.39 inHg
- Observed at: DBQ, 9 nm away

From the NTSB's public API. Aircraft are matched to registry aircraft by N-number and event date, checked against the reported serial number, make and model; the registry's history has gaps, so older matches can be wrong. Compare the reported make and model.

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