What happened
On a flight traveling from Green Bay to Oshkosh, an unidentified aircraft was cruising at 2,500 feet during favorable weather conditions when it struck an Air Wisconsin De Havilland DHC-6 Twin Otter 200, registration N4043B. The Twin Otter was operating a flight from Sheboygan to Appleton with two pilots and six passengers on board and was in the process of descending toward Appleton Airport.
At the time of the incident, both aircraft were navigating through uncontrolled airspace under visual flight rules (VFR). The impact between the two planes caused both to descend into Lake Winnebago, approximately five miles southeast of the Appleton airfield. The collision resulted in the total destruction of both aircraft and 13 fatalities.
Findings
Investigators concluded that the primary reason for the accident was that neither flight crew successfully identified the other aircraft early enough to perform necessary maneuvers to avoid the impact. While the specific reasons for the lack of detection remained unclear, officials noted that atmospheric conditions and inherent human visual limitations likely hindered the crews' ability to spot the oncoming plane in time to prevent the collision.