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Aviation Accident

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NTSB Identification: CHI05CA032
The docket is stored in the Docket Management System (DMS). Please contact Records Management Division
14 CFR Part 91: General Aviation
Accident occurred Sunday, November 21, 2004 in Oconomowoc, WI
Probable Cause Approval Date: 02/24/2005
Aircraft: Olson Sonex, registration: N478SX
Injuries: 1 Serious.

NTSB investigators used data provided by various entities, including, but not limited to, the Federal Aviation Administration and/or the operator and did not travel in support of this investigation to prepare this aircraft accident report.

The amateur-built airplane was damaged during a forced landing following a loss of engine power during cruise flight. The propeller and hub separated in-flight as result of a crankshaft failure. The airplane impacted a small ditch during the forced landing that resulted in the airplane nosing-over. The experimental airplane was powered by a non-certified Volkswagen 2180cc engine that was modified for aircraft use. The engine crankshaft was fractured between the oil-slinger and the retaining snap-ring groves. The fracture initiated at the aft edge of the forward keyway. The fracture features were consistent with a fatigue-type failure. The propeller and hub were not recovered during the investigation. The pilot/builder reported that he had mistakenly installed the number four (aft-most) bearing at the number two position during initial assembly of the engine. He had to remove the shrink-fit propeller hub, drive gears and spacer in order to reinstall the bearings in the correct order. The pilot/builder reported that he did not check the dimensions of the shrink-fit components prior to reassembling the engine, nor did he consult with the engine kit manufacturer on how to disassemble, inspect, and reassemble the engine components. The engine kit manufacturer did not provide the builder with engine disassembly or overhaul instructions with delivery of the engine kit. The engine kit manufacturer did not instruct builders to verify the dimensions of the shrink-fit components prior to their installation. The experimental airframe and engine did not have to comply with federal regulations required for a standard category airplane. The engine and airframe had accumulated 45.3 hours since new.

The National Transportation Safety Board determines the probable cause(s) of this accident as follows:
  • The loss of engine power due to a fatigue fracture of the crankshaft. An additional cause to the accident was the pilot/builder's failure to verify the dimensions of the shrink-fit crankshaft components prior reassembling the engine. A factor to the accident was the ditch encountered during the forced landing.