Pilots and other aviation experts are satisfied that the Federal Aviation Administration has kept flying safe since the air traffic controller strike started nine days ago, but there have been problems.

Pilot sources monitoring the system said yesterday there have been four confirmed near-collisions over the United States since the strike began Aug. 3. While that sounds terrifying to the average airline passenger, it is a lower figure than usual and not one that concerns pilots.

The impression of safety varies in its conviction from individual to individual, but there is general agreement among experts that the Federal Aviation Administration has been highly successful in handling air traffic despite the firing of 12,000 striking controllers.

"As long as the FAA is willing to restrict traffic, there's no problem," a knowledgeable pilot said. "But if they start to cave in to demands for more system access and begin to push the capacity, then safety could be in trouble."

The FAA has been reducing flights by as much as 50 percent in some areas and has been maintaining a systemwide average of about 75 percent of the commercial airline schedule.

However, private planes and business jets that normally require radar assistance from FAA controllers are being denied clearances in some cases and encouraged not to fly. Preference is going to the scheduled airliners.

The question of near collisions and how frequently they occur is one that bedevils the FAA in the best of times. Asked for statistics yesterday, the FAA said that in the first five days of the strike it received nine unconfirmed reports of near midair collisions, but had confirmed none of them. There were 10 confirmed near midair collisions in the same week last year, when all controllers were working.

A near midair collision is defined by the FAA as occurring when two planes unexpectedly come within 500 feet of each other or when either aircraft has to take "evasive action."

Another indicator of airspace system performance is a document called UCR, for "unsatisfactory condition report." It is filed by air traffic controllers when a potentially dangerous situation occurs. Such situations often involve problems with the computerized radar system as well as difficulties in flight.

In the first week of the strike, the FAA said, 30 UCRs were written by supervisors pressed into duty as controllers. During the same period a year earlier, with everyone at work, there there 61 UCRs.

The key to the FAA's master plan for handling traffic during the strike has been to put no more planes in the air than can be handled at one time. While holding patterns have been virtually eliminated, there have been delays on the ground.

"I have never seen everything go so smooth," said Larry Kinsey, a pilot for Eastern Airlines who regularly flies the shuttle between Washington National and LaGuardia in New York. "I feel the system is now working the way it's designed to work; there are no hassles with controllers."

His optimism was not shared by a pilot for another major airline who asked not to be identified. "I think there is a clear strain on the system," he said. "On a couple of occasions I could hear the strain in radio communications with controllers . My feeling is there is some level of risk above the normal."

However, this senior captain said, he is still flying and sees no reason not to continue.

Commuter airline, business and general aviation interests all expressed confidence in the system. However, some small-plane operations are having trouble getting clearances from the FAA's flow control center, according to the Aircraft Owners and Pilots Association.

For commercial airline operations, delays on the ground have been particularly lengthy at Kennedy and LaGuardia in New York City, which have heavy traffic loads.

This situation, in New York and elsewhere, has resulted in pressure on the supervisors manning the control towers, which is one of the long-term concerns of pilot groups and the FAA leadership.

Supervisors are working 10-hour days and six-day weeks without the overtime pay the controllers they once supervised received. Sooner or later they are going to get tired.

However, senior FAA officials said yesterday, more and more military controllers are qualifying on various radar "positions." As their numbers increase, the pressure on the supervisors will decrease.

The Aviation Consumer Action Project, a Ralph Nader group, asked the National Transportation Safety Board yesterday to investigate the safety of the air traffic control system. "We believe serious questions exist about the safety of operating the air traffic control system with fatigued supervisors and inexperienced military personnel," ACAP said. The board has the request under study, a spokesman said.