What happened
While en route, the pilot encountered freezing rain conditions. To remain above the precipitation, he climbed to approximately 3,500 feet. Upon reducing throttle power, the engine began surging and failed to produce sufficient thrust for level flight. The pilot was aware that ground elevation in the area ranged from 1,700 to 1,900 feet MSL.
As he descended through 2,500 feet, he placed two mailbags on his lap to free his hands. He dimmed the cockpit lights, activated the taxi lights, and applied windshield alcohol in an attempt to improve visibility. Despite these efforts, he did not see the ground until the moment of impact.
The investigation
The pilot believed that the automatic feature of the alternate air door had malfunctioned or failed. He reported that he and another pilot had previously experienced a similar problem when encountering the same type of weather. It was noted that at one time, the aircraft had been equipped with a manual control to operate the door, but this feature had been removed.
During a functional check after the accident, the engine operated normally. However, the alternate air door parts were too badly damaged from the impact to be operationally checked.
Findings
The pilot's inability to see the ground despite using available lighting and de-icing aids contributed to the accident. The suspected failure of the automatic alternate air door during freezing rain conditions likely caused the engine surge and loss of power.