Probable cause
A quality control failure in the Sungear, Inc. manufacturing and quality control process allowed sungears with surface finishes of the root fillet radius of the spline that did not conform to the drawing specifications to be released. The surface finish defect caused fatigue cracks to initiate early in the part life.
NTSB analysis
The 1st stage planetary gear assembly was examined by the NTSB laboratory in Washington, DC, and a Materials Laboratory Factual Report, No. 12-100A was issued. A summary of the NTSB laboratory report findings is as follows:
The average surface roughness of the root fillet radius surface of the sungear measured 22.68 micro-inches, which is coarser than 8 micro- inches, maximum, specified in the sungear engineering drawing. The fractures of the sungear was the result of a fatigue crack that emanated from the coarse surface finish within the root fillet radius portion of the sungear.
The gears were not manufactured in accordance with the engineering drawing.
A quality control failure in the Sungear, Inc. manufacturing and quality control process allowed sungears with incorrect surface finishes of the root fillet radius of the spline to be released.
Text from the NTSB. Open the NTSB docket for ENG11IA032 for the full record.
Event
- Date
- May 21, 2011
- Type
- Incident
- Place
- Peach Springs, Arizona
- Nearest airport
- Boulder City Muni (BVU)
- Highest injury
- None
- Weather
- Visual (VMC)
- Light
- Day
Dehavilland DHC-6-300
- N-number
- N178GC
- Category
- Airplane
- Operating rule
- Part 135 (charter and commuter)
From the NTSB aviation accident database: N-registered aircraft, events since 2008, data through Oct 28, 2026. The file is published monthly, so the latest weeks are incomplete and recent records can change. The official source is the NTSB aviation investigation search. CheckAero is independent and not affiliated with the NTSB.