WASHINGTON -- At rush hour Thursday evening in the sky near Chicago, a commercial jetliner came within 50 feet of colliding with a small plane, aviation officials said yesterday.
The close call occurred in some of the world's busiest airspace, an area that has been identified by the National Transportation Safety Board as having "a high collision potential" because of the way big jets and smaller planes mingle on the outskirts of the Chicago air traffic control zone.
A month ago, the safety board called on the Federal Aviation Administration to alter air traffic control rules in the area to address the problem. Yesterday, the safety board requested copies of the air traffic control communications and radar data collected Thursday so that it could investigate the incident.
On Thursday, Southwest Airlines Flight 758, a Boeing 737 carrying 57 passengers and a crew of five, was descending at about 5,600 feet, pre paring for a landing at Midway Airport, when an unidentified small plane flashed by from the opposite direction. Only at the last moment did the commercial jet's pilots see the approaching plane and take evasive action, they told the aviation agency.
The FAA is trying to identify the pilot of the smaller plane, who has not yet filed the report that is called for after any such incident.
The episode, which aviation experts said represented an extremely unusual close call, illustrates the problem that has been worrying the safety board since 144 people died in a 1978 collision between a Boeing 727 and a Cessna 172 at San Diego.
The problem of small and largeplanes mingling in the same airspace becomes most acute when small airports grow, attracting jet service. In the case of Chicago, the growth has been at Midway, which had no jet service before 1978.
The other Chicago airport, O'Hare International, is surrounded by aring of controlled airspace that pilots of small planes typically avoid, since they do not have the sophisticated equipment to avoid collisions and do not want to fly under the guidance of air traffic controllers.
But avoiding O'Hare puts the small planes in the path of jets approaching Midway. The solution, the safety board suggested, is to expand the controlled airspace to include the main approaches to Midway.
In Thursday's episode, the smaller plane was not carrying a kind of radio called a Mode C transponder, which sends out a repetitive signal that shows controllers the plane's position and identity to help avoid collisions. Neither was its pilot in radio contact with controllers.
Had the smaller plane been about 5 miles closer to O'Hare, which is at the center of the region's controlled airspace, it would have been required to carry the special equipment and be under the control of the tower.