Amateur-built SONEX accident near Garfield, Washington, June 3, 2017
On June 3, 2017 at about 6:30 pm local time, a 2016 amateur-built SONEX, registered N255BZ, was substantially damaged in an accident during enroute (cruise) near Garfield, Washington. It was a personal flight under general aviation rules (Part 91). No one was hurt; 1 person was on board or involved. The weather was visual conditions (good weather).
The NTSB's probable cause their words, unchanged
The pilot’s improper torque of screws that attached the flywheel mounting flange to the crankshaft during engine assembly, which resulted in heavy vibrations and the subsequent in-flight loss of the propeller.
Source: NTSB aviation accident database, copy made October 5, 2026. Docket and reports at the NTSB.
What the record shows
- Date
- June 3, 2017 · about 6:30 pm local time
- Place
- Garfield, Washington · map
- Type
- Accident
- Injuries
- No one was hurt; 1 person was on board or involved.
- Weather
- visual conditions (good weather)
- Aircraft
- Amateur-built SONEX UNDESIGNAT, built 2016
- Registration
- N255BZ · no longer on the register · serial 0255
- Damage
- Substantial damage
- Flight
- Personal flight · general aviation rules (Part 91)
The NTSB's narrative final · quoted from the NTSB record
The pilot reported that, during cruise flight, the airplane began vibrating, followed by a loud bang. The pilot saw the propeller separate from the airplane and he subsequently performed a forced landing to a road. During the landing roll, the left wing impacted a guy wire, resulting in substantial damage to the left wing. Postaccident examination revealed that the crankshaft separated at the forward main bearing journal. The area of separation exhibited beach marks propagating from the woodruff key area throughout most of the separated surface area. The flywheel mounting flange attached to the crankshaft and the flywheel mating surface showed material deformation at several screw holes and two fractured screw shafts that remained in their holes in the mount. A 1/4-inch wide area of fretting was noted around the outer circumference of the mounting flange and flywheel mating area. The trigger shaft, which was attached to the flywheel, displayed scuffing and material deformation at the locations of the flywheel mounting flange screw heads. The pilot/builder reported that he found no torque values in the manufacturer’s assembly manual during assembly of the flywheel and he was uncertain of the torque that he applied to the screws, although he thought that it was sufficient. It is likely that he did not adequately torque the screws during installation of the flywheel, which allowed the screws to loosen over time, resulting in elongation of the screw holes and fretting on the flywheel mounting flange and flywheel mating surface. The loose flywheel assembly would have increased vibration throughout the engine, along with torsional loads that ultimately resulted in the separation of the forward crankshaft and propeller assembly. A Federal Aviation Administration (FAA) Advisory Circular shows the recommended torque to be used for a specified fastener. Builders can also contact the manufacturer, who can supply them with a general torque table. The maintenance history of the airplane was not obtained during the investigation.
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
What happened, in order
- Powerplant sys/comp malf/fail during enroute (cruise)
- Collision during takeoff/land during landing (landing roll)
- Part(s) separation from AC during enroute (cruise) defining event
The NTSB's findings
- Aircraft › Aircraft power plant › Engine (reciprocating) › Recip eng rear section › Incorrect service/maintenance
- Personnel issues › Action/decision › Action › Incorrect action performance › Pilot
Pilot
- Certificate: private
- Ratings: single-engine land
- Flight time: 269 hours in all; 93 in this make and model; 38 in the last 90 days; 18 in the last 30 days; 269 as pilot in command
- Last flight review: August 16, 2015
- Medical certificate: None
- Seat: left
- Injury: no injuries
The aircraft
- Airframe total time: 92.4 hours
- Last inspection: annual inspection; 17.4 hours since
- Maximum gross weight: 3,400 lb
- Seats: 2
- Landing gear: fixed
- Engine: Aerovee 2180 (piston); 92 hours total
The flight
- Departed from: SFF Spokane WA at 5:25 pm
- Destination: PUW Pullman/Moscow WA
- Flight plan: none
Weather at the time
- Light: daylight
- Wind: from 180° at 8 knots
- Visibility: 10 statute miles
- Sky: clear
- Temperature: 73°F (23°C), dew point 50°F (10°C)
- Altimeter: 29.95 inHg
- Observation at 5:53 pm from KPUW, 15 miles away
Injuries
| Fatal | Serious | Minor | None | |
|---|---|---|---|---|
| Flight crew | 1 |
Documents from the investigation the NTSB's docket: the evidence folder behind the report
5 documents, released by the NTSB on January 5, 2021. View them here, or download them; the NTSB redacts some personal details before release, and this site shows the NTSB's own titles rather than its file names.
| # | Document | What it is | |
|---|---|---|---|
| 1 | NTSB Engine Examination Summary | PDF, 9 pages | View Download |
| 2 | Aircraft Builder Notes and Photos | PDF, 13 pages | View Download |
| 3 | FAA Advisory Circular Ac 43.13-1B Excerpt | PDF, 1 page | View Download |
| 4 | Statement of Party Representatives to NTSB Investigation-sonex Aircraft | PDF, 1 page | View Download |
| 5 | Photo Array-accident Site | PDF, 4 pages | View Download |
The same docket at the NTSB · documents without a copy here are fetched from the NTSB when you open them.
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.