An American Airlines jetliner leaving O'Hare International Airport in Chicago last Friday passed within a quarter mile of an incoming American flight after air traffic controllers confused the departing jetliner with a United Airlines plane that had taken off moments earlier.

The incident occurred just before 8 p.m. -- at the height of the usual Friday night rush hour at O'Hare, and about an hour after a supervisor at the nearby Chicago control center asked that air traffic be thinned out because the area was overcrowded with planes.

"A mess," the supervisor wrote into the log book that night. He said the O'Hare supervisor described the situation as "the worst I've ever seen. Aircraft were squawking the wrong beacon code . . . . Some climbing, some not. A real screw-up."

"This isn't a safe operation," the supervisor concluded, as he described the distances maintained between planes traversing the skies over Chicago.

The incident was the second near-collision in five days and drew the attention of National Transportation Safety Board investigators, who said yesterday the pair of near collisions provides further evidence of the need to act on the board's recommendation that the Federal Aviation Administration reduce air traffic in the most congested areas during the peak summer travel season.

"The broader implications of this investigation are an indication to us of the problems we tried to bring to the FAA's attention May 13," said Alan Pollock, a spokesman for the board. Board chairman Jim Burnett, who has sharply criticized the FAA for running the air traffic control system "up to the red line," declined to comment.

Safety board investigators are also probing a report that a controller's error on June 2 led to a near-collision between a Northwest Airlines jetliner and a military plane over South Dakota. The pilot of the Northwest plane, a Boeing 727, told investigators that if he had not taken evasive action, he would have collided with the military plane.

In the Chicago incident, no evasive action was taken. The two American jetliners passed within a quarter-mile horizontally and 500 feet vertically, NTSB investigators said.

Controllers told investigators that Chicago air traffic was "heavy and complex" at the time of the incident. According to investigators, the Friday night rush was building when at about 6:40 p.m. the supervisor at the Chicago Air Route Traffic Control Center asked a traffic management specialist to reduce the number of planes departing to the west from O'Hare.

Traffic was not reduced, however, and about an hour later, a United Airlines jet and American Airlines Flight 637 to Las Vegas prepared to take off. The United plane took off first and was mistakenly assigned the American jet's flight number by an air traffic assistant. The information went into the computer and showed on the radar scope as the blip that represented the United flight.

About three minutes later, the real American Flight 637 took off, but because its flight information was assigned to the United flight, the American plane appeared on the radar scope as an unspecified plane.

The controller working the departure saw the United plane move across his radar and, thinking it was American Flight 637, told the pilot of that plane to turn left and climb to 23,000 feet.

The United pilot, not knowing the controller was actually speaking to him, ignored the instructions. The American pilot did not.

When the American pilot followed the instruction, and because he wasn't where the controller thought he was, he passed within 500 feet of incoming American Flight 393.

In a letter dated June 6, FAA chief Donald D. Engen told the NTSB he had asked that "immediate steps" be taken to correct control practices that led to the misunderstanding between the two control facilities and caused the near-collision.

"Has what happened caused us to feel that we should reduce or constain traffic flow in the Chicago area?" Engen asked in the letter. "We have determined that flow management procedures, when properly applied, do maintain a safe level of operation in the Chicago area."

Engen added that traffic patterns around Chicago will be reviewed, and two supervisors from the Chicago center have been moved to the control tower at O'Hare to monitor all departure sectors.