Mitsubishi MU-2B-60 accident near Zwingle, Iowa, April 19, 1993
On April 19, 1993 at about 8:52 pm local time, a Mitsubishi MU-2B-60, registered N86SD, was destroyed in an accident near Zwingle, Iowa (Dubuque Regional airport). It was a public-use flight under public-use (government) rules. 8 people were killed. The weather was instrument conditions (cloud, fog or low visibility).
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The NTSB's probable cause their words, unchanged
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, copy made October 4, 2026. Docket and reports at the NTSB.
What the record shows
- Date
- April 19, 1993 · about 8:52 pm local time
- Place
- Zwingle, Iowa · Dubuque Regional
- Type
- Accident
- Injuries
- 8 people were killed.
- Weather
- instrument conditions (cloud, fog or low visibility)
- Aircraft
- Mitsubishi MU-2B-60 · all MU-2B-60s on the register
- Registration
- N86SD · registry record · serial 765 S.A.
- Damage
- Destroyed
- Flight
- Public-use flight · public-use (government) rules
The NTSB's narrative final · quoted from the NTSB record
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 complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.
The factual record from the NTSB's investigation tables, in plain English
Pilot
- Certificate: airline transport pilot, flight instructor
- Ratings: multi-engine land; single-engine land; instructor: airplane multi-engine; instructor: airplane single-engine; instructor: instrument airplane; instrument: airplane
- Flight time: 10,607 hours in all; 1,922 in this make and model; 26 in the last 30 days
- Medical certificate: Class 2 (valid medical--w/ waivers/lim.)
- Seat: left
The aircraft
- Airframe total time: 4,580 hours
- Last inspection: continuous airworthiness programme, April 12, 1993; 10 hours since
- Maximum gross weight: 11,575 lb
- Seats: 10
- Landing gear: retractable
- Engine: Garrett TPE-331-10 (turboprop); 0 hours total
- Fire on the ground
The flight
- Departed from: LUK Cincinnati OH at 6:00 pm
- Destination: PIR Pierre SD
- Flight plan: IFR
- Runway 31, 6,498 ft by 150 ft
- A second pilot was aboard
Weather at the time
- Light: daylight
- Wind: from 040° at 16 knots
- Visibility: 1 statute miles
- Sky: overcast at 200 ft; not recorded at 200 ft
- Temperature: 46°F (8°C), dew point 45°F (7°C)
- Altimeter: 29.39 inHg
- Observation at 8:55 pm from DBQ, 9 miles away
Injuries
| Fatal | Serious | Minor | None | |
|---|---|---|---|---|
| Crew | 2 | |||
| Passengers | 6 |
About this page
Everything above comes from the NTSB's public accident database: the narrative, probable cause and findings are the NTSB's own words, and the coded tables behind its report (pilot, aircraft, flight, weather, injuries, sequence of events) are written out in plain English. Pilots' ages, home towns and medical details are left out on purpose. The docket at the NTSB holds the report as a PDF and the investigation's photographs and documents. This site never names pilots, passengers or anyone else involved. A preliminary report describes what is known soon after the event and can change; the final report, with the probable cause, usually follows one to two years later and this page is refreshed when it does. Case number DCA93GA042.