An American Airlines jetliner flew within one mile of three other planes at 31,000 feet over northwestern Illinois on Friday after the American pilot shifted the plane's course without notifying ground controllers, according to the results of a preliminary investigation.

The American flight, a DC10 flying from Chicago's O'Hare International Airport to the West Coast, shifted out of airspace monitored by Kansas City controllers at about 3:45 p.m. CDT and flew into an area monitored by Chicago controllers, possibly in order to avoid a patch of bad weather in its original path, the Federal Aviation Administration report said.

The near-collision occurred about 13 minutes later, 30 miles southwest of a navigational installation at Bradford, Ill., which is on the border between regions controlled by Kansas City and Chicago.

FAA officials from both regions are continuing their investigation and said they would disclose more details today, said Sandra Campbell, acting public affairs officer for the FAA's central region.

Investigators are examining transcripts of tapes that monitored the conversations of the flight crew and ground controllers in both regions around the time of the incident, Campbell said yesterday.

She said results of inquiries completed as of last night indicated there was "less than standard distance" between the DC10 and the other planes -- a Continental Airlines DC9, an American 727 and an Air Force F111, all of which were in airspace monitored by Chicago controllers.

The distance controllers are required to leave between planes varies with the size and the power of the aircraft involved, Campbell said.

She said she did not know how many passengers were aboard any of the four jets, exactly how close they came to colliding or the cities of origin and departure of the planes involved. She would not speculate on why the pilot did not inform ground control of his deviation or ask permission to change course.

The incident was the second within two weeks involving planes flying in airspace monitored by Chicago ground controllers.

On June 5, an American jetliner leaving O'Hare flew within a quarter of a mile of an incoming American flight after air traffic controllers confused the American plane with a United Airlines jet that had taken off moments earlier.

Earlier this month, National Transportation Safety Board investigators said the FAA should reduce air traffic in the most congested areas during the peak summer travel season.