Amateur-built Sonex accident near Oconomowoc, Wisconsin, November 21, 2004
On November 21, 2004 at about 8:30 pm local time, a amateur-built Sonex, registered N478SX, was substantially damaged in an accident near Oconomowoc, Wisconsin. It was a personal flight under general aviation rules (Part 91). 1 person was seriously injured. The weather was visual conditions (good weather).
The NTSB's probable cause their words, unchanged
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.
Source: NTSB aviation accident database, copy made October 5, 2026. Docket and reports at the NTSB.
What the record shows
- Date
- November 21, 2004 · about 8:30 pm local time
- Place
- Oconomowoc, Wisconsin · map
- Type
- Accident
- Injuries
- 1 person was seriously injured.
- Weather
- visual conditions (good weather)
- Aircraft
- Amateur-built Sonex
- Registration
- N478SX · registry record · serial 478SX
- Damage
- Substantial damage
- Flight
- Personal flight · general aviation rules (Part 91)
The NTSB's narrative final · quoted from the NTSB record
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 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: private
- Ratings: single-engine land
- Flight time: 582 hours in all; 45 in this make and model; 35 in the last 90 days; 14 in the last 30 days
- Last flight review: July 28, 2003
- Medical certificate: Class 3 (valid medical--w/ waivers/lim.)
- Seat: left
- Injury: serious injuries
The aircraft
- Airframe total time: 45.3 hours
- Last inspection: annual inspection, May 12, 2004; 45.3 hours since
- Maximum gross weight: 1,100 lb
- Seats: 2
- Landing gear: fixed
- Engine: Volkswagen 2180 (piston); 0 hours total
The flight
- Departed from: RYV Watertown WI at 8:00 pm
- Destination: RYV
- Flight plan: none
Weather at the time
- Light: daylight
- Wind: from 240° at 6 knots
- Visibility: 10 statute miles
- Sky: scat at 8,500 ft
- Temperature: 43°F (6°C), dew point 27°F (-3°C)
- Altimeter: 30.29 inHg
- Observation at 8:35 pm from RYV, 11 miles away
Injuries
| Fatal | Serious | Minor | None | |
|---|---|---|---|---|
| Crew | 1 |
Documents from the investigation the NTSB's docket: the evidence folder behind the report
The docket, the folder of records gathered during an investigation (maintenance records, photographs, witness statements, examinations), has not been read from the NTSB for this case yet; the list appears here once it has. Open the docket at the NTSB.
Everything on this page comes from the NTSB's public records. The narrative, probable cause and findings are the NTSB's own words; the coded tables behind the report are written out in plain English, with pilots' ages, home towns and medical details left out. The documents and photographs are the NTSB's docket, shown as the NTSB released them. This site's own text never names anyone involved. A preliminary report can change; the final report usually follows one to two years later, and the page is refreshed when it does.