Nine radar scopes emit an eerie, green glow in the cramped, darkened room under the tower at Washington National Airport. They usually are manned by 10 air traffic controllers and three supervisors. Yesterday about noon, three supervisors had the room and the radar to themselves.

No problem. Federal Aviation Administration restrictions necessitated by the air traffic controllers' strike had slowed the pace at Washington National to a gentlemanly average of one flight about every 3 1/2 minutes, and that is child's play for the radar room at the world's 16th busiest airport.

"We're goint to meter traffic in proportion to our ability to handle it," said Harry Hubbard, National's tower chief. "The system is absolutely safe."

Up in the glass-enclosed tower cab, three people were at work where seven usually toil. The cab is where the radioed command "Cleared for Takeoff" launches 600 flights on a normal day. Some hours at National -- those when as many as 90 flights are handled on three runways, each of which intersects the other two -- one airliner starts rolling for takeoff just as another lifts off the far end of the same runway while a third passes behind the first on an intersecting strip.

Not yesterday. There was plenty of time between flights and none of that sleight of hand the National controllers -- judged the best in the nation last year by the FAA's own statistics -- perform so often.

At National and at control towers and en-route air traffic centers nationwide yesterday supervisors and controllers who ignored the strike were "combining positions" -- handling traffic in more than one sector at a time. Such a practice is common during slow traffic periods.

"Some of our supervisors are saying they can handle more traffic," said Ray Alvarez, deputy director of the FAA's air traffic service. "We'll start adding as we're sure the total system can handle it."

The central safety question, in the view of FAA experts, is how long supervisors can continue to work before they "burn out." The longer a strike continues, one experienced FAA official said yesterday, the less safe the system becomes because of fatigue among those controllers working.

The FAA is planning, if necessary, to bring back some retired controllers and borrow controllers from the military and train other people with controller experience. That is a long-term prospect, however, because it usually takes about four years to train a new controller and another year to bring him up to speed at a high-tension facility like National.

"We can shorten that substantially if we have to," Alvarez said.

PATCO president Robert Poli has questioned the safety of letting supervisors do the work, but that did not appear to trouble the Air Line Pilots Association (ALPA) of the Air Transport Association of America, the airline trade organization.

ALPA set up a task force to debrief pilots after their flights and determine whether they felt safety was threatened. "We have had no reports of major problems," said ALPA spokesman John Mazor.

"We feel the plan is safe," said Daniel Henkin, spokesman for the Air Transport Association.

The situation is that the Federal Aviation Administration is better prepared to deal with a controllers' strike or slowdown situation than it ever has been in the past -- as long as the airline industry and the flying public are willing to accept substantial lost revenues and inconvenience of a cut in flights.

For more than 18 months, the FAA has been anticipating a major strike or job action by the Professional Air Traffic Controllers Organization. Langhorne Bond, the Carter administration's FAA chief, planned extensively for it and the results of his planning were showing up yesterday. FAA line supervisors, all former controllers, have been "working the positions" on the radar scopes to maintain and improve proficiency.

In meetings with the airlines, the FAA worked out schedule cuts that were designed to "eliminate the peaks." In other words, not everybody was permitted to take off at 8 a.m. and land at 5 p.m. That kind of scheduling, which the airlines always want, is what puts the pressure on the air traffic system. If planes arrive and the depart several minutes apart, the pressure -- and the need for controllers -- is substantially reduced.

One so-called "near-miss" was reported in the national airspace system yesterday. That happened about noon when a New York Air flight from LaGuardia to National and an Air Canada flight from Kennedy to Toronto pased within a quarter-mile of each other. No evasive action was taken by either pilot.

According to FAA officials, the New York Air crew "either mis-read or mis-heard" its altitude assignment and there was no controller fault.