NTSB: Planes at Chicago's Midway came within 300 feet of each other January 11, 2012 -- Updated 2218 GMT (0618 HKT) The Southwest crew tells the tower, "The thing went right over our head" The incident happened on December 1 A preliminary NTSB report does not assign blame WASHINGTON (CNN) -- A Learjet departing Chicago's Midway airport last month came within about two plane-lengths of striking a taxiing Southwest Airlines Boeing 737 that had been cleared to cross the same runway, according to a preliminary report by the National Transportation Safety Board. The airborne Learjet passed 62 feet above -- and within 287 feet laterally -- of the Southwest passenger plane, according to the NTSB, which is investigating the incident. According to the NTSB, the co-pilot of the taxiing Southwest jet saw the Learjet and shouted "Stop" to the plane's captain, who brought the plane to a halt near the edge of the runway. Air traffic control audio tapes, released by LiveATC.net, a website for aviation enthusiasts, captured conversations between the Southwest crew and controllers in the minutes immediately afterward. You just cleared us onto a runway (where) there was a plane taking off," one of the pilots said to controllers. "What technology clears us on the runway when a plane was taking off?" a pilot added, sounding exasperated. The Southwest crew then radioed another controller. "We had to hit our brakes and the thing went right over our head," the crew said. According to the NTSB's preliminary report, the incident occurred about 9 a.m. CT on December 1, when a Learjet 45 was instructed to line up and wait on runway 31 right. Southwest Flight 844 had landed on a parallel runway -- 31 center. As the Southwest plane was decelerating on the center runway, the Learjet was cleared for takeoff on the right. When an air traffic controller gave the Southwest plane clearance to turn right on a taxiway that crossed the right runway, the two planes came into potential conflict. Kevin Hiatt, a former 737 pilot now with the non-profit Flight Safety Foundation, said it is still unclear what caused the mishap, but credited the Southwest co-pilot for averting potential disaster. "Fortunately, the first officer was 'heads up' in that cockpit and basically saw the Learjet coming, and told the captain to stop, which he did just prior to entering into the active runway," Hiatt said. "If that Learjet had had any kind of a problem and not lifted off when it did -- they prevented a very serious accident," Hiatt said. Hiatt said Midway airport is one of 35 airports equipped with an surface movement detection system known as ASDE-X to detect potential runway conflicts. "The investigation will probably look into whether ASDE-X was in operation at the time or (whether) the controllers using it or not using it," he said. The NTSB said the report is preliminary, and no blame has been assigned in the matter. No one was injured in the event. CNN's Mary Snow contributed to this report.