What happened
During a period of favorable weather conditions, a Twin Otter was performing a descent toward Appleton Airport after departing from Sheboygan. Simultaneously, a Convair CV-580, registered N90858, was operating a flight from Green Bay to Oshkosh with three crew members and two passengers on board. Both aircraft were operating under visual flight rules within uncontrolled airspace.
At an altitude of approximately 2,500 feet, the two planes collided. The impact caused both aircraft to crash into Lake Winnebago, roughly five miles southeast of the Appleton Airport. Search efforts located wreckage floating on the water surface. Both aircraft were completely destroyed in the accident, and there were 13 fatalities among the occupants of both flights.
Findings
Investigators determined that the primary cause of the collision was the failure of both flight crews to visually identify the other aircraft early enough to perform evasive maneuvers. While the specific reason for the lack of detection remained undetermined, officials noted that atmospheric conditions and inherent human visual limitations likely hindered the crews' ability to spot the oncoming aircraft in time to prevent the impact.