- Date
- April 19, 1993, 15:52 local (20:52 UTC)
- Place
- Zwingle, IA
- Injuries
- 8 fatal
- Damage
- Destroyed
- Phase of flight
- Enroute
- Aircraft
- N86SDMitsubishi MU-2B-60
- Weather
- Instrument (IMC) · Day
- Status
- Completed · report May 10, 1994
- Report
- Board-adopted at Board Meeting
- Investigation class
- Class 1
- NTSB office
- Major Investigations Division
- Safety recommendations
- Yes
- Case key
- 15962
From the NTSB’s public API. By law, NTSB reports can’t be used as evidence in lawsuits over the accident.
On this page
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
NTSB’s coding before 2008: each occurrence and phase of flight, and the causes and factors under it. The codes are defined in NTSB’s coding manual.
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
- Reported as
- Mitsubishi MU-2B-60
- Category
- Airplane
- Serial number
- 765 S.A.
- Airframe hours
- 4,580
- Engine
- 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
Matched to the registry by N-number and the event date; the registry’s history has gaps, so older matches can be wrong. Compare the reported make and model.
Flight crew
As NTSB recorded them. NTSB leaves out names.
- Role
- Pilot
- Age
- 52
- Certificates
- 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