NTSB Probable Cause

THE PILOT-IN-COMMAND'S FAILURE TO MAINTAIN AN ADEQUATE TERRAIN AVOIDANCE ALTITUDE. FACTORS WHICH CONTRIBUTED TO THE ACCIDENT WERE THE DARK NIGHT AND FEATURELESS SNOW COVERED TERRAIN, BOTH WHICH CREATED A 'FEATURELESS TERRAIN ILLUSION' WHICH PROBABLY LED THE PILOT TO VISUALLY OVERESTIMATE HIS ALTITUDE.