The aircraft departed Fairbanks around 11:30 to deliver about 4,800 gallons of fuel oil to the remote mining site 12 miles north of Crooked Creek, AK. The 5,400 feet long by 100 feet wide airstrip is situated within hilly, snow-covered terrain and has a 7 percent uphill grade. Flat light conditions existed at the airstrip, and light snow showers were present, with visibility reported at 2 miles. The captain stated that during final approach, as the airplane passed over the airstrip threshold, flat light conditions made it very difficult to discern where the airstrip surface was. He said that the initial touch down was “firm”, but was thought to be within acceptable tolerances. Just after touchdown, the left wing broke free from the airplane at the wing to fuselage attach point. The airplane veered to the left, and off the left side of the runway. PROBABLE CAUSE: “The flight crew’s misjudged flare while landing. Factors associated with the accident were flat light conditions, snow-covered terrain, and an uphill runway grade.”