On March 21, 1993, at 0806 central standard time, minimum air traffic control separation was lost between N374MQ, a DeHavilland SHD-6, operated as Simmons Flight 4085, and N361UA, a Boeing 737, operated as United Airlines Flight 556. Both airplanes had departed on Runway 22L at the O'Hare International Airport, Chicago, Illinois, just prior to the loss of separation. Visual meteorological conditions prevailed and IFR flight plans were filed. There were no injuries or damage reported as a result of this incident. Both airplanes had departed runway 22L which was being controlled by the South Local Controller (SLC). This controller was responsible for arrivals on runway 27L and depatures from runway 22L and 32L T-1 intersection. The events which resulted in the loss of separation occurred as follows: At 0802, Simmons 4085 was cleared for takeoff and instructed to fly the runway heading. At 0803, Simmons 4085 was instructed to fly a heading of 200 degrees. At 0804, United 556 was cleared for takeoff and instructed to fly a heading of 140 degrees. United 556 was instructed not to delay it's takeoff roll. At 0805, Simmons 4085 was instructed to contact departure control. At 0806, United 556 is instructed once again to fly a heading of 140 degrees. Twelve seconds later, United 556 is instructed to maintain 4,000 feet. Ten seconds later, the flight is instructed to contact departure control. (The controller stated she stopped United's climb at 4,000 feet because without turning to 140 degrees, it would have over taken the slower Simmons flight ahead of it. However, by this time the loss of separation had already occorred.) Air Traffic Control Handbook 7110.65G, paragraph 5-72 states, "Separate aircraft by the following minima: a.1. when less than 40 miles from the antenna-3 miles." According to radar data, lateral separation between Simmons 4085 and United 556 decreased to 2.25 miles with 400 feet of vertical separation. See Air Traffic Control/Human Performance Group Chairman's Factual Report and Attachments for CHI-93-I-A115.