A crowded Eastern Air Lines shuttle that apparently had reached takeoff speed narrowly averted a collision with a helicopter at National Airport yesterday when the Eastern pilot slammed on the brakes and brought the jetliner to a halt only 40 yards from the Potomac River.

Witnesses said the Boeing 727, filled with 175 passengers plus seven crew members bound for New York, swerved onto grass after crossing the end of the main runway. No injuries were reported.

The helicopter pilot, Jesse Hadaway Jr., 42, said last night that he and the Eastern pilot had been "cleared for takeoff by separate controllers on different radio frequencies" and that his course took him into the path of the Eastern jet. "I looked up and saw him about the same time he saw me, and we both did what we had to do."

Hadaway, chief pilot of Whirlybird Inc., which operates out of Martin State Airport in Baltimore, said that he and the Eastern pilot "have been completely exonerated" by investigating officials.

Ira Furman, an National Transportation Safety Board spokesman, said a full investigation is being conducted and that although an individual investigator may have told the pilots that they did nothing wrong, "We don't exonerate because we don't accuse."

He said the investigation will go beyond flight crew interviews and include analysis of air traffic control tapes. "You can get instructions on more than one frequency," he said, adding that such mixups have occurred before.

Among the passengers on the jetliner were several media personalities, including David Hartman, host of ABC's "Good Morning America," Terrence Smith of "CBS Morning News" and executives of the New York Daily News.

Passengers on the 5 p.m. shuttle to La Guardia Airport said the pilot, his voice trembling, announced that he had aborted the takeoff because a helicopter crossed "too close" in front of the jet. Eastern spokesman Glenn Parsons identified the pilot as J.C. Goachee.

After the near-collision, federal sources said the control tower requested that the helicopter return to the airport so the pilot could be interviewed.

The pilot, Hadaway, told a reporter last night that "at no time did we actually cross each other's paths," and that the closest the two aircraft came was "about 100 yards."

He said the seven-seat Bell Jet Longranger helicopter had stopped for refueling at National after flying a charter from the Shenandoah Valley Airport near Harrisonburg, Va., near the site of an airline accident that killed 14 people on Monday.

He said the helicopter was cleared for takeoff on "Helicopter Rte. 1 to Greenbelt," a course due east from National, and that the Eastern jet was cleared for takeoff on runway 36, which is due north.

Yesterday's near-collision and the commuter airliner crash in the Blue Ridge Mountains are likely to add to mounting concern about aircraft safety. This year already is the worst in aviation history, with 1,444 fatalities in crashes worldwide, including 11 serious accidents involving U.S. commercial airlines.

It was learned that the nose wheel of the Eastern jet had just lifted off when the pilot elected to abort the takeoff. Aviation specialists say nose wheel liftoff, known as rotation, usually occurs when the aircraft is beyond the normal speed at which the pilot decides whether to abort. At such a point, a 727 would likely be traveling in excess of 160 mph, which is beyond the speed calculated to guarantee a halt on the runway.

Federal Aviation Administration spokesman Stephen Hays said Flight 1500 stopped on a section of runway apron completed in December to give pilots more room to stop in the event of an emergency.

The jet sat on the grassy plot for about half an hour, surrounded by emergency vehicles with flashing lights, as tow trucks attempted to pull it onto the runway. When that failed, the passengers were taken off the plane by the rear stairs and returned to the gate in buses. Later, the crew managed to get the plane back on the runway under its own power.

The passengers were given the choice of remaining in Washington or taking another plane to New York, and nearly all of the 175 opted to go on to La Guardia.

One passenger, Linda Monroe of Mamaroneck, N.Y., was taken to National Hospital for Orthopaedics and Rehabilitation in Arlington, where she was treated for a headache and released.

"We were just about to take off and just about to rotate," said passenger R. Peter Strauss. "The pilot cut the engines and slammed on brakes . . . . We ended up 40 yards from the river."

Strauss, chairman of the board of New York radio station WMCA, and his wife, Ellen Sulzberger Strauss, chairman of the board of Strauss Communications, who live in Southwest Washington, said they were en route to New York to spend Yom Kippur with their parents but decided not to go on because the delay had pushed the time past sunset and into the start of the Jewish holiday.

"Good Morning America" host Hartman told ABC'a local affiliate, WJLA-TV Channel 7, last night that it was "like putting on the brakes very fast, very fast." He said that "people took a lot of deep breaths. Was I scared when I saw the water coming up? You bet." He said the passengers "erupted into applause" when the plane came to a halt.

He added that he was going on to New York. "I love to fly -- it's safer than getting into the bathtub."

Scott Williams, 28, Washington producer for "Good Morning America," was on the plane sitting across an aisle from Hartman. "All of a sudden it was an abort," Williams said. "The plane swayed back and forth; the pilot made an S turn. I'm told this was to help slow it down. It felt like a car driving on a gravelly road and then there were a couple of jolts.

"There was no panic whatsoever. The pilot, his voice sounded nervous, said there was a helicopter taking off and that this was something that posed a danger to the plane. I never felt fear. I just thought the plane is going into the water and I'm going with it and when it stops I'll get out."

The $7 million runway "overrun" project, authorized in late 1983, added seven acres at the northern end of the main runway. The fill, standing 10 feet above the high-tide mark, lengthened from 200 feet to 750 feet the overrun area awaiting any plane skidding off the north edge of the pavement.

The project smoothed out the southern end of the main runway as well, creating a 1,000-foot overrun there. The addition was designed to raise safety margins at an airport designed and built for propeller planes of the 1940s.

Two other overrun accidents occurred at National, both involving Eastern 727s. In October 1976, a jet went off the north end during a landing and stopped 15 feet from the water. The next May, one aborted takeoff at the north end and stopped about same point. No one was injured either time.