Some aviation safety experts and the airlines say a new requirement aimed at easing the threat of collisions near airports still leaves commercial jets exposed to danger from small planes.

The Federal Aviation Administration rule announced last week calls for sharply expanding the airspace in which small, private planes must have equipment that tells controllers their altitude. FAA officials predict it will result in a significant increase in air safety.But critics of the final regulation accuse the FAA of scaling back their proposed requirements too far by requiring the new equipment only if planes fly within 10 miles of airports in most parts of the country. Only at the busiest facilities will the equipment be required for 30 miles around the airports.

"We thought (he FAA rule) was going to go much further," Jim Burnett, chairman of the National Transportation Safety Board, said in an interview. He called the aspect of the regulation that applies to 116 airports with moderately busy air traffic "clearly inadequate" to protect jetliners from small planes.

The Air Line Pilots Association, which represents 40,000 pilots, and the Air Transport Association, the trade group for the airlines, joined in the criticism, although FAA officials called the rule a significant safety improvement.

"It brought some improvement, but didn't go as far as it should have," said John Mazor, a spokesman for the pilots union.

The ATA issued a statement saying the new requirement for small planes, all of which does not go into effect until after 1990, "clearly does not go far enough. The potential for conflicting incidents (etween aircraft) remains."

The FAA rule, announced last Thursday, was an outgrowth of the 1986 collision of a small Cessna and an Mexican airliner over Cerritos, Calif., in which 82 people were killed. The small plane had no device which would show controllers its altitude on the radar screen.

After the accident, aviation safety advocates and members of Congress began calling for private planes flying in the same airspace with commercial jets to have so-called Mode-C transponders that provide controllers with altitude information.

Last February the FAA suggested a sweeping rule that would have required the equipment on all planes flying within 40 miles of 254 airports and above 6,000 feet.

The suggestion unleashed a storm of protest, including 80,000 letters to Congress and the FAA from private pilots, who argued their freedom to fly was being threatened. The devices cost from $600 to $2,000 per aircraft.

The FAA's final rule rolled back the number of airports covered to 143 and the airspace to a 30-mile radius around the busiest airports and a 10-mile radius around 116 midsize airports. In other airspace it lowered the floor for such equipment from 12,500 feet to 10,000 feet, but not to the 6,000 feet previously suggested.

The critics said they are not alarmed by the FAA narrowing its new requirements to more than 100 fewer airports. But they are critical of the FAA limiting the airspace in which small planes must have the Mode-C equipment to a 10-mile radius around 116 airports with moderately heavy air traffic.

Salt Lake International Airport is among the 23 terminal control areas where planes must have the Mode-C transponders after July 1, 1989, if flying within 30 miles of the airport.