Weather Briefing and Icing Conditions
The pilot's weather briefing included a flight precaution for occasional moderate icing conditions. Later, the pilot received a report from another aircraft indicating moderate rime icing along the intended route and at the proposed flight altitude. The airplane was equipped with a heated pitot tube and a heated propeller.
Flight Sequence
Approximately one hour after departure, the airplane began an uncontrolled descent from its assigned altitude of 9,000 feet. The pilot reported that he was unable to maintain altitude due to ice after the airplane had descended more than 1,000 feet. Air traffic control cleared the pilot first to 6,000 feet, then to 4,000 feet, but he was unable to maintain either altitude. The airplane was last observed on radar descending through 2,400 feet.
Witness Observation
A witness on the ground saw the airplane in a nose-high attitude with the wings rocking from side to side. The witness stated that the nose suddenly pitched down and the airplane descended into trees, from which a fireball was observed to emanate.
Radar Data
Radar data showed an approximate 15-knot reduction in ground speed during the five minutes preceding the descent.
Probable Cause
The investigation did not state a probable cause.