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31st of January 2001

A Douglas DC-6B Cargo Aircraft was operating a Cargo flight from Fairbanks International Airport (Alaska, USA) to Donlin Creek Airstrip (Alaska, USA), a remote mining site, delivering 4800 gallons of fuel oil. Runway 11 at Donlin Creek is 4500 feet (~1370 meters) long and 150 feet (45 meters) wide and features and features a 7% uphill gradient. It is a gravel runway that was covered in dry snow.

N4390F after coming to a stop with the left-wing torn off (Source: www.baaa-acro.com)


During the later part of the approach, light snow showers were present and they contributed to a condition called 'Flat Light'.

The FAA us the following definition for Flat Light;

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, mudflats, 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.


An example of a flat light situation, with the actual horizon indicated by the added dashed line (Source; https://www.boldmethod.com/)


With a visibility of around 2 miles. The crew stated that it was very difficult to see the runway surface in the present light conditions. The touchdown was firm but not excessively heavy and, according to the captain to be within acceptable tolerances. However, just after touch down the left wing broke off at the wing to fuselage attachments. The aircraft veered to the left and went off the side of the runway coming to a rest against a snowbank. The calculated landing weight was just over 92.250 pounds (~41840 kilograms)

The aircraft in June 1998 (© Danny Grew)


An investigation into the incident was launched by the National Transportation Safety Board, the determined the cause of the accident to be: "A misjudged flare while landing. Factors associated with the accident were flat light conditions, snow-covered terrain, and an uphill runway grade."


The NTSB report is accessible by clicking here.

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