Incident Overview
During climbout, the airplane entered an area of freezing rain, which led to rapid ice accumulation on the airframe and a subsequent loss of climb capability. The pilot informed air traffic control (ATC) that he was unable to maintain altitude and requested, and received, clearance to return to Great Falls, the departure airport.
Radar and Flight Path
ATC radar data showed that the airplane then began a right turn over mountainous terrain with peaks extending up to 8,309 feet. Notably, lower and relatively flat terrain, descending to less than 5,000 feet, was located to the left of the aircraft's track. Over the last minute of radar contact, the aircraft was in a right turn at a descent rate of approximately 400 feet per minute. It passed less than half a mile from the summit of the 8,309-foot mountain at an altitude between 8,400 and 8,500 feet, just before radar contact was lost.
Crash Site and Wreckage
The aircraft crashed on the southwest flank of the 8,309-foot mountain, about half a mile south of the last recorded radar position. Wreckage and impact signatures at the crash site were indicative of an inverted, steep-angle, relatively low-speed, downhill impact with the terrain.
Investigation Findings
The investigation revealed no evidence of any aircraft mechanical problems.
Probable Cause
The failure of the pilot-in-command to ensure adequate airspeed for flight during a forced descent due to airframe icing, resulting in a stall. Factors included: freezing rain conditions, airframe icing, an improper decision by the pilot-in-command to turn toward mountainous terrain (where a turn toward lower and level terrain was a viable option), mountainous terrain, and insufficient altitude available for stall recovery.