New regulations took effect this week aimed at reducing the chance of midair collisions between jets and small planes, but federal aviation officials find themselves investigating incidents that fall just outside the boundaries of the rules.

New regulations require that small planes flying near 14 additional major airports be equipped with devices that help air traffic controllers pinpoint their altitude.

The rule previously applied to small planes flying in congested skies around nine major cities. The 14 airports added Dec. 1 are Cleveland, Denver, Detroit, Honolulu, Houston, Kansas City, Las Vegas, Minneapolis, New Orleans, Philadelphia, Pittsurgh, St. Louis, San Diego and Seattle.

Previously, transponders were required on small planes flying near Atlanta, Boston, Chicago, Dallas, Los Angeles, Miami, Newark, San Francisco, Washington's National Airport and New York's LaGuardia and John F. Kennedy airports.

Small planes are involved in the majority of near-collisions reported by pilots. As of Oct. 31, 946 near-collisions had been reported to the Federal Aviation Administration, compared to 705 for the same period last year.

The airlines have long pushed for a rule requiring that small planes carry altitude-reporting devices, which are standard equipment on commercial jets.

Specifically, the new rule was prompted by the collision between an Aeromexico DC9 and Piper Archer over Cerritos, Calif., Aug. 31, 1986 in which 82 persons were killed. The Piper was not equipped with a Mode C transponder, which transmits an altitude signal to controllers, and was flying inside the boundary of the controlled airspace that requires all planes to be equipped with the devices.

Transponders were required equipment in airspace around Los Angeles at the time of the accident, and the FAA is considering additional measures to guard against collision, including expanding the boundaries of restricted airspace. Los Angeles has the most congested skies in the country and the highest rate of near-collisions -- numbering more than two a week. As of Aug. 1, the FAA had received reports of 87 near-collisions from pilots.

The most recent incident occurred Sunday between a United Airlines jet and a single-engine Cessna just outside restricted airspace. The Cessna was not required to have a transponder.

The United pilot reported to the FAA that he was forced to descend while climbing after takeoff to avoid hitting the Cessna.

The FAA has not yet located the pilot of the Cessna, which did not transmit altitude information to controllers and was flying under visual flight rules.

The United jet, a Boeing 767, carrying 108 passengers and eight crew, came within 100 feet of the Cessna as it passed underneath, the United pilot reported. The United aircraft was climbing after taking off from Long Beach Municipal Airport on a flight to Chicago.

The encounter occurred about about five miles southwest of the airport at 3,500 feet, said Elly Brekke, an FAA spokeswoman in Los Angeles.