What happened
The flight departed from Boeing Field in Seattle, Washington, with an intended stop in Cheyenne, Wyoming. Before takeoff, the crew completed a pre-flight inspection and loaded the aircraft with 1,020 gallons of fuel and 60 gallons of oil. At the time of departure, the aircraft's gross weight was 45,401 pounds, which exceeded the maximum authorized passenger weight of 45,000 pounds by 401 pounds, though the center of gravity remained within limits.
The crew filed an instrument flight rules plan for a cruising altitude of 13,000 feet. Weather reports provided prior to departure indicated potential icing and cloud cover above 6,000 feet along the route. After departing at 0050, the aircraft made routine position reports, including passing over Malad City at 13:00 altitude at 0358. However, following these reports, all radio communication ceased.
A large-scale search operation led by United States Air Force rescue units was initiated. Five days later, a reconnaissance pilot located the wreckage approximately eight miles west of Fish Haven. The accident resulted in 40 fatalities.
Findings
Investigation of the wreckage showed that all major components were intact and there was no evidence of fire or mechanical failure prior to the crash. Both engines were still producing power at the moment of impact.
The investigation concluded that the primary cause was an inadvertent descent into severe turbulence and icing. While the crew had been briefed on potential weather hazards, the aircraft encountered moderate to heavy icing and significant turbulence at lower altitudes, which prevented the crew from maintaining a safe altitude.