What happened
On the day of the accident, the pilot initiated an IFR flight toward Idaho Falls, ID, with a planned cruising altitude of 13,000 feet. Following a weather briefing, the aircraft departed at 1804 PST. During the climb, the pilot reported encountering light rime ice and light turbulence at 1814 PST. Shortly thereafter, the flight experienced heavy vibrations, prompting the pilot to request and receive permission to descend to 7,000 feet and return to Portland.
During this descent, the aircraft entered an uncontrolled, tight descending turn that the pilot struggled to stabilize. Although directional control was eventually regained at 6,000 feet, the pilot noted a significant accumulation of ice on the airframe. The flight ended when the aircraft struck mountainous terrain at an elevation of 3,130 feet. All five occupants were killed in the crash.
Findings
The investigation identified several contributing factors related to the weather and the aircraft's performance. The area was forecast for moderate turbulence and moderate mixed icing, with reports of moderate icing throughout the region. The aircraft was not certified for flight operations in known icing conditions. During the descent, the accumulation of ice on the wings and stabilizer led to a deterioration in aircraft performance and an uncontrolled spiral. Because of the severity of the impact and the nature of the wreckage recovery, investigators were unable to verify the operational status of the deicing equipment prior to the crash.