What happened
The pilot received a weather briefing before filing an instrument flight rules plan to Idaho Falls, Idaho. The aircraft departed at 1804 PST. Approximately ten minutes into the flight, while climbing toward 13,000 feet, the pilot reported encountering light turbulence and light rime ice. Four minutes later, he reported experiencing heavy vibrations and requested permission to return to Portland and descend to 7,000 feet.
Shortly after receiving clearance, the aircraft entered a tight descending turn that the pilot stated was difficult to arrest. He regained directional control at 6,000 feet but noted the aircraft had accumulated a heavy load of ice. The aircraft subsequently crashed into mountains at an elevation of 3,130 feet.
The investigation
Due to damage from the impact and the process of retrieving the wreckage, it was not possible to verify the pre-impact condition of all deicing components. However, the aircraft type was not certified for flight in known icing conditions. Weather forecasts indicated moderate mixed icing and moderate turbulence, and the pilot had been briefed on reports of moderate icing throughout the area.
Findings
The primary factors contributing to this accident were the pilot's decision to continue flight into known icing conditions despite the aircraft not being certified for such operations. This led to significant ice accumulation, which resulted in a loss of control and subsequent crash.