The Federal Aviation Administration placed an air traffic controller on administrative leave yesterday after a preliminary investigation indicated she gave permission for a helicopter to take off from National Airport on a course that would have placed it in the path of an Eastern Air Lines shuttle jet.

FAA spokesman Fred Farrar said that barring the controller from her post in the tower when she reported for work at 3 p.m. was "standard action and doesn't imply guilt" on her part. But Farrar said he could not recall the last time a controller at National was removed from the job.

Farrar said the woman, who was not named, is rated a "developmental" controller, which means she was not fully qualified in all aspects of the job. But he emphasized that she was "fully qualified" for her assignment as helicopter controller in the tower.

The FAA credited quick action by both pilots in averting a collision Tuesday between the New York-bound Boeing 727, which carried a full load of 175 passengers and a crew of seven, and the helicopter, with two persons aboard.

The helicopter pilot swerved left and upward when he saw the jet. The Eastern pilot slammed on the brakes as the jet was lifting off the runway and steered the aircraft to a safe landing on a newly constructed dirt overrun beyond the end of the runway, about 50 yards from the Potomac River. No one was injured.

G.H. Patrick Bursley, a member of the National Transportation Safety Board, which investigates all such incidents, said the near collision here was the latest in a series of so-called "runway incursions" in which two planes were assigned the same flight patterns by air traffic controllers operating on different radio frequencies.

The NTSB is in the midst of a nationwide investigation of such incidents. But Bursley said the report had been delayed by a record number of airline fatalities -- the latest a single death in a mail plane crash in West Virginia yesterday that raised this year's total to 1,445 -- and is not expected to be completed before next year.

The increasing frequency of runway incursions -- 54 in the first six months of this year, compared with 41 in the first half of 1984 -- prompted FAA Administrator Donald D. Engen to ask controllers to increase their vigilance.

The closest call occurred in Minneapolis in March, when two Northwest Airlines DC10s came within 50 feet of a collision after one of them was cleared for takeoff on a runway that the other was told it could cross.

A congressional report released earlier this month found that the margin of safety had diminished in air travel because of fatigue and stress on the part of controllers, most of whom were hired after President Reagan ordered the firing of 11,400 striking members of the Professional Air Traffic Controllers Organization four years ago.

The helicopter controller has worked as a controller for about five years, according to Farrar, but her tenure was interrupted by the 1981 strike. She was among about 500 of the strikers subsequently rehired.

Bursley said it is "too early to categorize" the cause of the incident at National, but he said that in addition to the possibility of controller error, "there might not be adequate procedures."

The helicopter pilot in Tuesday's near collision, Jesse Hadaway Jr., told the FAA he had stopped at National to be refueled after flying a television crew to the Shenandoah Valley Airport near Harrisonburg, Va., to report on a plane crash near there that killed 14 persons on Monday.

An NTSB team wrapped up its investigation yesterday of that crash of a Henson Airlines commuter plane but would not comment on the cause of the crash into the side of a 2,700-foot mountain in the George Washington National Forest.

Bursley said the helicopter controller told airport tower chief Harry Hubbard "she thought she had given clearance for a northwest takeoff" that would have taken the helicopter away from the jet's path. But Bursley said an examination of the helicopter's cockpit voice recorder indicated that the controller gave the pilot permission for a takeoff that would take it across the main north-south runway.

Bursley said Hadaway asked for a clearance for a takeoff on Helicopter Route One to Greenbelt, a standard route that would have allowed him to fly due east from the helicopter pad at the north end of the airport.

The controller told Hadaway he was "cleared for takeoff," Bursley said. "He should have been given instructions in detail" that would have put him on a course away from the path of the jet, "but we can't find that on a preliminary read-out of the tape."

NTSB investigators had not talked to the Eastern pilot, J.C. Goachee, or to the local controller monitoring his takeoff, as of late yesterday, but Bursley said he had "not seen any suggestion that either pilot or the other controller did anything other than what they were supposed to."

"It's critical that the controllers talk to each other," said Bursley, a retired admiral who is one of three members of the NTSB. Bursley emphasized that "the runway belongs to the local controller."

The two controllers were part of a small team that was working the 3 p.m.-11 p.m. shift in the main tower at National. Bursley said they likely were using headsets with long cords that free them from sitting at a desk, and allow them to "wander about" the tower.

Although the controllers talk to the pilots by radio, they must rely on "sight to a great degree" in directing takeoffs and landings, Bursley said. "Had she wanted to send the helicopter across the runway, she was obligated to clear it with the local controller. The runway is his domain."

A controller is assigned exclusively to helicopter traffic only at peak periods, where there is too much traffic on the local frequency, Bursley said.

Bursley said both controllers and pilots are used to operating on multifrequencies, as pilots are handed off to different controllers along their flight paths.

Bursley said Hadaway, chief pilot for Whirlybird Inc., based at Glenn L. Martin State Airport in Baltimore, told FAA officials he had taken off from National 75 to 100 times, and that on at least 50 of those occasions he had been cleared for takeoff across the runway. But Bursley said he doubted permission was granted often "at that particular time of day," about 5 p.m., when commercial airline traffic is near its peak.

Bursley said, however, there was nothing wrong in Hadaway asking for that takeoff route because "he had asked many times and it had been given many times."

One of the passengers on the Eastern shuttle, television personality David Hartman, told viewers on his "Good Morning America" show yesterday that "we had a millisecond literally before we were off the ground or we had to stop."

Hartman praised the crew, saying, "Those three guys did it . . . and they did it right."