Convair 580 accident at Milwaukee, Wisconsin, 4 Aug 1968

Accident narrative

At 0948 c.d.t. on August 4, 1968, North Central Airlines Flight 261, a Convair 580, and a privately rented Cessna 150 collided approximately 11.5 miles southwest of General Mitchell Airport in Milwaukee, Wisconsin. The collision occurred at an altitude of 2,700 feet. The Cessna was destroyed, and its three occupants were killed. The Convair sustained extensive damage, but the captain maintained controlled flight and landed successfully at Mitchell Field. The Convair’s first officer sustained serious injuries; the captain, stewardess, additional crewmember, and eight passengers were uninjured.

**The flights**

The Cessna 150, rented from Home Airmotive, Inc., departed Mitchell Field in Lombard, Illinois, at about 0900 for a flight to Sheboygan, Wisconsin. The 19-year-old pilot, who had 183 total flying hours and three hours in the Cessna 150, was flying under Visual Flight Rules (VFR) with two passengers. No flight plan was filed, and there was no known radio contact with any ground station during the flight.

North Central Airlines Flight 261 departed O'Hare International Airport in Chicago at 0934 on an Instrument Flight Rules (IFR) flight plan to Manitowoc, Wisconsin, with an en route stop in Milwaukee. The captain, who had 12,163 flying hours, was handling radio transmissions. The first officer, who had 2,400 total hours and 25 hours in the Convair 580, was operating the controls.

**Sequence of events**

During climbout from Chicago, the Convair encountered an unusually heavy accumulation of insect strikes on the windshield. The captain compared the situation to a "snow shower" in which ice was accumulating at an enormous rate. The flight requested and was granted a climb to 7,000 feet, where the captain stated the insect accumulation continued at a lesser rate.

Control of Flight 261 was transferred to Milwaukee Approach Control. At 0943:58, Approach Control cleared the flight to fly a heading of 350 degrees and descend to 2,600 feet for a vector to the Instrument Landing System (ILS) approach for Runway 7R.

During the two minutes prior to the collision, the approach controller issued three radar traffic advisories regarding a target ahead of the Convair: * At 0946:24: "traffic at twelve thirty four miles northbound slow moving" * At 0947:02: "three miles northbound" * At 0947:35: "the northbound target is at one o'clock and a mile and a half north northwest bound."

The Convair crew acknowledged the advisories but reported they could not see the target. At 0947:44, the captain transmitted, "Got so many bugs on the window we can hardly see out of here."

The Convair pilots stated they searched intently for the reported traffic but were unable to sight it until immediately prior to impact. The first officer reported seeing the other aircraft at the 1:30 position, closing on an intercept angle of approximately 30 degrees, at a distance of a matter of yards. He stated the proximity precluded any evasive action.

At impact, the Convair was at 2,700 feet on a heading of 356 degrees at an indicated airspeed of 190 knots. The Cessna was calculated to be on a heading of 314 degrees at 80 knots. Neither aircraft attempted an evasive maneuver.

The force of the impact embedded a portion of the Cessna cabin, containing its three occupants, and the main landing gear into the upper forward baggage compartment on the right side of the Convair. The remainder of the Cessna fell to the ground.

The right side of the Convair cockpit was crushed inward, trapping the first officer and limiting elevator down travel. The right engine and propeller were extensively damaged, and electrical power was lost. The captain took control, shut down the right engine, and turned toward the airport. He manually extended the landing gear, blew out the gear up latches with the emergency compressed air charge, and accomplished a no-flap landing at Mitchell Field at about 0954.

**What the investigation found**

The Board determined that both aircraft were properly certificated and airworthy, and both flightcrews were properly certificated.

The Cessna’s NAV-COM radio was recovered and found tuned to 116.4 MHz, the frequency of the Milwaukee VOR station, with a selected course of 332 degrees. The Board noted this indicated the Cessna was tracking inbound toward the Milwaukee VOR along Victor Airway 479. As the aircraft being overtaken and converging from the left, the Cessna had the right of way under Federal Aviation Regulations. The Convair was required to give way by seeing and avoiding the Cessna, weather conditions permitting.

A cockpit visibility study determined that the Convair would not have appeared in any clear glass area from the pilot's seat of the Cessna during the final two minutes of flight. The Board concluded it would have been physically impossible for the Cessna pilot to have detected the Convair visually prior to the collision.

For the Convair crew, the study showed the Cessna target would have appeared in the captain's front windshield, partially obstructed by the center post, and in the first officer's direct vision window. However, the Board found that several factors substantially impaired the crew's ability to see the smaller aircraft.

Weather observations at Mitchell Field reported visibility of 3 miles in haze and smoke. The Board estimated flight visibility in the collision area was 3 to 5 miles. Sun glare, reflecting off smoke and moisture particles, further restricted visibility to the right of the Convair's nose. The Cessna’s predominately white color scheme and its lack of relative motion in the Convair's windshield also reduced its conspicuity.

The most significant restriction to visibility was the insect accumulation. Examination of the Convair captain's windshield revealed an average of 12 insect strikes per square inch, covering roughly 20 percent of the windshield, with the greatest concentration adjacent to the center post. The Board noted that even with a concerted effort to search beyond the windshield, the pilots' eyes would have tended to focus on the insect smears.

The Board concluded that visual detection by the Convair crew in time to avoid the collision was possible, but not probable, and that their inability to do so was more a product of these limitations than a lack of vigilance.

However, the Board noted that the Convair crew was aware their forward visibility was severely reduced, and that the reported slow-moving traffic could be a VFR aircraft operating legally at their altitude. The Board concluded that if the Convair crew had requested an avoidance vector shortly after receiving the third advisory, a vector could probably have been issued and effectuated in sufficient time to avoid the collision.

The radar controller stated he did not recall what happened to the Cessna target after it turned north-northwest. The Board noted the target's disappearance probably resulted from a known radar limitation called "tangential effect," which causes primary targets to drop off the radarscope when an aircraft flies a course tangential to the radar antenna.

**Probable cause**

The Board determined that the probable cause of this accident was "the inability of the Convair 580 flightcrew to detect the Cessna 150 visually in sufficient time to take evasive action, despite having been provided with three radar traffic advisories concerning the latter aircraft. Visual detection capabilities were substantially reduced by the heavy accumulation of insect smears on the forward windshield and direct vision windows of the Convair. Visibility was further reduced by haze, smoke and sun glare, and by the inconspicuous color and lack of relative motion of the Cessna. Under these circumstances, the crew of the Convair should have requested a radar avoidance vector."