Weatherly Aviation CO INC 620B accident near Lewistown, Montana, April 10, 2017
On April 10, 2017 at about 11:00 pm local time, a 1995 Weatherly Aviation CO INC 620B, registered N9004M, was substantially damaged in an accident during maneuvering (low-alt flying) near Lewistown, Montana (Lewistown Muni airport). It was an aerial application (crop spraying) flight under agricultural flying rules (Part 137). No one was hurt; 1 person was on board or involved. The weather was visual conditions (good weather).
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The NTSB's probable cause their words, unchanged
The pilot’s failure to maintain altitude during low-altitude maneuvers in flat light conditions.
Source: NTSB aviation accident database, copy made October 4, 2026. Docket and reports at the NTSB.
What the record shows
- Date
- April 10, 2017 · about 11:00 pm local time
- Place
- Lewistown, Montana · Lewistown Muni · map
- Type
- Accident
- Injuries
- No one was hurt; 1 person was on board or involved.
- Weather
- visual conditions (good weather)
- Aircraft
- Weatherly Aviation CO INC 620B UNDESIGNAT, built 1995 · all 620Bs on the register
- Registration
- N9004M · no longer on the register · serial 1604
- Damage
- Substantial damage
- Flight
- Aerial application (crop spraying) flight · agricultural flying rules (Part 137)
The NTSB's narrative final · quoted from the NTSB record
The pilot reported that, during a low-altitude, practice agricultural application flight, he lost depth perception between the snow-covered ground and overcast sky. He added that the airplane impacted the ground and came to rest after sliding about 500 ft. The airplane sustained substantial damage to the right wing and fuselage. The pilot reported that there were no preaccident mechanical failures or malfunctions with the airplane that would have precluded normal operation. The Federal Aviation Administration's pamphlet, "Flying in Flat Light and White Out Conditions," states the following: Flat light is an optical illusion, also known as "sector or partial white out." It is not as severe as "white out" but the condition causes pilots to lose their depth-of-field and contrast in vision. Flat light conditions are usually accompanied by overcast skies inhibiting any good visual clues. Such conditions can occur anywhere in the world, primarily in snow covered areas but can occur in dust, sand, mud flats, or on glassy water. Flat light can completely obscure features of the terrain, creating an inability to distinguish distances and closure rates. As a result of this reflected light, it can give pilots the illusion of ascending or descending when actually flying level. However, with good judgment and proper training and planning, it is possible to safely operate an aircraft in flat light conditions.
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
- Loss of visual reference during maneuvering (low-alt flying) defining event
- Collision with terrain or object (not controlled flight into terrain) during maneuvering (low-alt flying)
The NTSB's findings
- cause Personnel issues › Task performance › Use of equip/info › Aircraft control › Pilot
- cause Aircraft › Aircraft oper/perf/capability › Performance/control parameters › Altitude › Not attained/maintained
- cause Environmental issues › Conditions/weather/phenomena › Light condition › Flat light › Effect on personnel
Pilot
- Certificate: commercial pilot
- Ratings: multi-engine land; single-engine land; instrument: airplane
- Flight time: 2,601 hours in all; 557 in this make and model; 90 in the last 90 days; 6.1 in the last 30 days; 2,403 as pilot in command
- Last flight review: August 2, 2016
- Medical certificate: Class 2 (with waivers/limitations)
- Seat: ctr
- Injury: no injuries
The aircraft
- Airframe total time: 2,534.5 hours
- Last inspection: annual inspection, March 23, 2017
- Maximum gross weight: 4,000 lb
- Seats: 1
- Landing gear: fixed
- Engine: P&W R-985/AN14B (piston); 0 hours total
- Operator: Skyline Aviation LLC
The flight
- Departed from: LWT Lewistown MT at 10:45 pm
- Destination: LWT Lewistown MT
- Flight plan: none
Weather at the time
- Light: daylight
- Wind: from 220° at 13 knots, gusting 26
- Visibility: 10 statute miles
- Sky: broken clouds at 8,000 ft
- Temperature: 39°F (4°C), dew point 27°F (-3°C)
- Altimeter: 29.95 inHg
- Observation at 10:54 pm from KLWT, 1 miles away
Injuries
| Fatal | Serious | Minor | None | |
|---|---|---|---|---|
| Flig | 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.
About this page
Everything above comes from the NTSB's public accident database: the narrative, probable cause and findings are the NTSB's own words, and the coded tables behind its report (pilot, aircraft, flight, weather, injuries, sequence of events) are written out in plain English. Pilots' ages, home towns and medical details are left out on purpose. The documents and photographs are the NTSB's docket, the folder of records gathered during the investigation, shown as the NTSB released them (the NTSB redacts some personal details first) and listed under the NTSB's own titles. This site's own text never names pilots, passengers or anyone else involved. A preliminary report describes what is known soon after the event and can change; the final report, with the probable cause, usually follows one to two years later and this page is refreshed when it does. Case number GAA17CA225.