Incident Details
Midway Flight 172, a McDonnell Douglas DC-9 registered as N1065T, was conducting an IFR departure from Chicago Midway Airport. Simultaneously, a Beech 200, registration N981LL, was on an IFR arrival to Merrill C. Meigs Airport, performing a visual approach. The aircraft were operating in visual meteorological conditions (VMC).
Communication Misunderstanding
Earlier, O'Hare Terminal Radar Approach Control (TRACON) had imposed a restriction on easterly departures from Midway, requiring not less than 20 miles in-trail separation. Subsequently, the handoff controller at TRACON instructed the controllers at Midway Tower to "cancel the restriction," meaning to cancel the in-trail restriction. However, the Midway Tower controllers interpreted this as a cancellation of a climb restriction on the Canal Three SID (Standard Instrument Departure), which Flight 172 was following.
Outcome
As a result of the misinterpretation, Flight 172 climbed to 3,000 feet while the Beech 200 descended to 2,000 feet. Radar data showed the aircraft passed each other with less than the required vertical and horizontal separation. No further details about injuries or damage were reported in the source.