What happened
A pilot, who was not instrument rated, filed an IFR flight plan without first obtaining a weather briefing. The forecast for the area included light to moderate rime icing between 5,000 and 16,000 feet.
At 15:41, the ARTCC controller observed that the Mode C altitude report was descending and 1,300 feet lower than the assigned altitude of 10,000 feet. When questioned by the controller, the pilot stated, "I cannot maintain altitude." Radio and radar contact with the aircraft were lost at 15:43.
At the time of the accident, a ground witness reported an overhead thunderstorm characterized by lightning, thunder, hail, and moderate rain and snow showers. The witness also reported hearing a loud sound described as a "winding up engine sound" followed by a large bang. Additionally, an airline flight operating in the vicinity of the accident site reported encountering moderate rime ice.
Findings
The last recorded test and calibration of the pitot-static system occurred on July 8, 1986. During the first six months of 1986, the pilot had completed approximately 44 hours of dual instrument training.