Cessna 180 N66036 accident near White Mountain, Alaska

Probable cause

The pilot’s inadvertent flight into flat light/whiteout conditions over snow-covered terrain.

NTSB analysis

The airline transport pilot was in cruise flight at 2,500 feet msl over snow-covered mountainous terrain. He said the weather conditions were reported as having a ceiling of about 3,000 feet, and a visibility of 10 miles. The route crossed an area of low hills, and the pilot said the visibility began to deteriorate. He descended to about 2,000 feet msl, and suddenly flew into whiteout conditions. The airplane collided with a mountain ridge and overturned. The airplane received structural damage to the wings, landing gear, and fuselage. A pilot who flew to the site shortly after the accident stated that he had difficulty seeing the ridge line. He stated that “visibility was good, about 5 miles, but I couldn’t make out the ridge. The ridge line was relatively horizontal, without any contrasting features, making it difficult to identify against the sky.”

The Federal Aviation Administration’s Aeronautical Information Manual (AIM), Section 7-5-13, “Potential Flight Hazards, Flying in Flat Light and Whiteout Conditions,” notes that whiteout “occurs when a person becomes engulfed in a uniformly white glow…a result of being surrounded by blowing snow, dust, sand, mud or water.” The AIM states that flat light “is an optical illusion, also known as ‘sector or partial whiteout.’ Flat light conditions are usually accompanied by overcast skies inhibiting visual cues. Flat light can completely obscure features of the terrain, creating an inability to distinguish distances and closure rates.” According to the AIM, “flat light conditions can lead to a whiteout environment quite rapidly, and both atmospheric conditions are insidious; they sneak up on you as your visual references slowly begin to disappear.”

Text from the NTSB. Open the NTSB docket for ANC08LA042 for the full record.