Flight History
On the day of the accident, the pilot obtained a weather briefing and filed an IFR flight plan to Idaho Falls, Idaho, with a requested altitude of 13,000 feet. The flight departed at 1804 Pacific Standard Time. At 1814, shortly before reporting level at 13,000 feet, the pilot indicated encountering light turbulence and light rime ice. Approximately four minutes later, the pilot reported heavy vibrations and requested clearance to return to Portland, Oregon, with a descent to 7,000 feet. The clearance was granted.
Accident Sequence
Shortly after beginning the descent, the aircraft entered a tight descending turn that the pilot reported difficulty arresting. The pilot stated that directional control was regained at 6,000 feet and that the aircraft had accumulated a heavy load of ice. Soon thereafter, the aircraft collided with terrain in a mountainous area at an elevation of approximately 3,130 feet.
Wreckage and Impact
The impact and subsequent wreckage retrieval prevented verification of the preimpact condition of all deicing components. The aircraft was not certified for flight in known icing conditions.
Weather and Icing Conditions
Forecast conditions included moderate mixed icing and moderate turbulence. The pilot was briefed that reports of moderate icing existed throughout the area.
Probable Cause
The official probable cause findings identify that the pilot continued flight into known adverse weather, resulting in ice accumulation on the wing and stabilizer. This degraded aircraft performance and led to an uncontrolled spiral, culminating in a collision with mountainous terrain.