Investigation Summary
The accident occurred in an area where both aircraft were transitioning toward their respective destinations: North Central Flight 290 was approaching Oshkosh from Green Bay, while Air Wisconsin Flight 671 was en route from Sheboygan to Appleton. North Central Flight 290 had originally been scheduled to fly a multi-leg route starting in Houghton, Michigan, but bad weather in northern Michigan forced the cancellation of the early legs. Consequently, the flight originated in Green Bay using a replacement crew flown in from Chicago. Although both flights had portions of their journeys planned under Instrument Flight Rules (IFR), both crews elected to fly the accident legs under VFR. This decision meant neither aircraft was receiving air traffic control separation services at the time of the collision. The NTSB investigation highlighted that the North Central crew was flying toward the sun, which likely created a glare that hindered their ability to spot the Twin Otter. Furthermore, the high closure rate of 688 feet per second (approximately 469 mph) left very little time for recognition and reaction once the aircraft were within visual range. The wreckage was recovered from the bottom of Lake Winnebago, scattered over an area of approximately one mile by one-half mile.
Final Conclusions
The National Transportation Safety Board released its final report, NTSB-AAR-73-09, on April 25, 1973. The board concluded that the probable cause was the failure of both flight crews to detect the other aircraft visually in sufficient time to initiate evasive action. The report noted that the ability to see and avoid was reduced by atmospheric conditions and human visual limitations. It also emphasized that had either flight requested in-flight advisories or remained on an IFR plan, the collision would likely have been prevented by ATC intervention.
Video Analysis
No video analysis linked for this case file yet.
Airframe & Maintenance
North Central Airlines North Central 290 — Convair CV-580 · N90858
Air Wisconsin Air Wisconsin 671 — de Havilland Canada DHC-6 Twin Otter · N4043B
Systems & Failure Modes
Visual Flight Rules (VFR)
A set of regulations under which a pilot operates an aircraft in weather conditions generally clear enough to allow the pilot to see where the aircraft is going. The primary responsibility for collision avoidance rests with the pilot's ability to 'see and avoid'.
Air Traffic Control (ATC) Advisories
Services provided by ATC to inform pilots of other traffic in their vicinity. Because both flights were VFR and had not requested these services, they were not being actively separated by controllers.
Interesting Facts
- 01The rate of closure during the final five seconds before the collision was 688 feet per second.
- 02Both aircraft were operating under Visual Flight Rules (VFR) at the time of the accident.
- 03The collision resulted in an explosion that tore off the Twin Otter's wing and a quarter of the Convair's left wing.
- 04The North Central crew was flying toward the sun, which likely impeded their vision.
- 05Neither captain had requested in-flight advisories from air traffic control for the VFR segments.
Safety Actions & Advisories
Visual Search Training
The NTSB recommended that the FAA create a standardized method for training and grading flight crews in visual search techniques and time-sharing between instrument checks and visual searches.
Anticollision System Development
The NTSB recommended that the FAA expedite the development of airborne anticollision systems to provide a backup to the 'see and avoid' concept.