Weatherly Aviation Co Inc 620B N9004M accident near Lewistown, Montana

Probable cause

The pilot’s failure to maintain altitude during low-altitude maneuvers in flat light conditions.

NTSB analysis

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.

Text from the NTSB. Open the NTSB docket for GAA17CA225 for the full record.