Federal aviation investigators are probing two weekend incidents that were caused by air traffic controllers and nearly resulted in collisions of commercial jetliners over Chicago and New York.

In the New York incident, an air controller transposed flight numbers for two Trans World Airlines planes and erroneously allowed a TWA jumbo jet carrying 200 passengers to fly into the path of a Pan American Airways jet preparing to land at LaGuardia Airport.

The Pan Am plane, a Boeing 727 flying a Boston-to-New York shuttle, was carrying 81 passengers. The incident occurred at 3:15 p.m. Sunday about 12 miles southwest of LaGuardia, Federal Aviation Administration spokeswoman Kathleen Bergen said.

Bergen said the controller gave landing instructions for TWA Flight 843, bound from Milan to John F. Kennedy International Airport, to TWA Flight 834, bound from St. Louis to Kennedy. Consequently, she said, Flight 834 landed at Kennedy.

But Flight 843 continued into LaGuardia airspace and into the path of the Pan Am flight being directed by another controller. When both controllers saw the collision course, they directed the planes to climb to 4,000 feet, she said.

Soon after, according to a Pan Am spokesman, the flight engineer of the Pan Am plane saw the TWA plane and called out, "Down, down, down." The Pan Am plane dove, passing under the TWA flight at about 3,200 feet. The Pan Am pilot estimated that the planes missed each other by 600 feet.

Both planes were being directed by controllers in the New York Terminal Radar Approach Control, a control facility in Westbury, N.Y., that handles flights for the three New York-area airports. The controllers were working different sectors of airspace, Bergen said.

The controller working the two TWA flights is a 25-year FAA employe and has worked that area, known as the Kennedy sector, for 20 years. Until Sunday, Bergen said, he had never made a controller error, meaning he had always maintained the required separation between jetliners.

The controller was working five aircraft at the time of the mixup and was handling both TWA flights. He had been on duty for 75 minutes when the error occurred, she said.

After the incident, the controller was removed from his position and decertified, a routine procedure involving controllers who make errors. He must undergo retraining and be retested before being reassigned to his post, Bergen said.

The Chicago incident occurred about 2:25 p.m. Saturday and involved planes taking off from O'Hare International, the world's busiest commercial airport, and the nation's busiest general aviation airport nearby.

An Air Canada Boeing 727 jet bound for Calgary and a twin-engine Cessna en route to South Bend, Ind., passed within about 300 feet of the other after the jet left O'Hare and the Cessna took off from Palwaukee Airport 10 miles northeast of O'Hare, FAA spokesman Mort Edelstein said. Both planes were being directed by controllers at the Chicago Terminal Radar Approach Control facility at O'Hare.

The Air Canada jet, carrying 126 passengers, departed on a northeasterly course and had climbed to 5,000 feet when the Cessna passed beneath him, Edelstein said. The Cessna, owned by the Quaker Oats Corp., had also been directed to climb to 5,000 feet.

The FAA also reported that in Detroit over the weekend, a controller mixed up numbers and gave instructions to Northwest Airlines Flight 577 rather than Flight 557.

The incidents follow a string of mistakes committed by pilots and may refocus air-safety debate on human performance in the high-technology aviation world.

The FAA is in the midst of a study to determine whether changes in controller training would help prevent such mistakes.

Harold W. Becker, manager of the FAA's safety programs division, said the research may conclude that airlines should use widely different numbers on flights approaching or leaving an airport within minutes of each other.