On a Sunday afternoon in 1977, Washington lawyer Eric Bernthal began his long obsession with National Airport.

He had gone to inspect a brick and wood house being built for his family in a shady corner of Cabin John. Contemplating the scene from a seat on an unfinished porch, he looked up to see a jetliner screaming low overhead. Another followed momentarily, then another and another--a dozen or more in 40 minutes.

"I buried my head in my hands and said, 'My God, what have I gotten myself into?' " says Bernthal, 37. " . . I suddenly realized we were sitting right in the middle of the flight path from National Airport."

Bernthal still owns the house in Cabin John but says he and his family have never grown accustomed to the intrusion from the sky--as many as 275 jets can pass over in a day. He can't play ball outside with his three sons without being annoyed, he says. He does not open windows in the summer.

"Airplane" was the first word spoken by one of his sons, Bernthal says. "You have to remember that where I live, there's always an airplane overhead," he says.

Since 1977, Bernthal has devoted countless hours to directing a campaign to restrict traffic at National and shift it to Dulles or Baltimore-Washington International airports. At the same time, he has pursued an active practice as a high-priced communications lawyer downtown.

As president of the Coalition on Airport Problems, an umbrella group that claims about 100 citizen associations and other organizations as members, he is a familiar face at hearings, congressional offices and news conferences--anywhere where people are willing to hear arguments that National Airport is overcrowded and unsafe.

But he is viewed by some officials in the aviation industry as a meddling amateur who does not understand modern aircraft nor represent the public and is really after the television publicity he has gained.

"Off the wall" is how many of his ideas on safety were viewed at the Federal Aviation Administration, said one former official there.

He is criticized by some in his own organization as a secretive operator who rarely consults his members and has insisted on pushing a controversial plan, opposed by many coalition members, to test new flight paths that are intended to spread the airport's noise more evenly across the area.

But most everyone agrees that with his Ivy League good looks (he graduated from Columbia University) and energy, Bernthal has been an effective advocate for the anti-National lobby. Largely self-taught in aviation, he can hold his own with FAA engineers talking thrust settings or with airline executives talking scheduling esoterica.

"The key to his success is that he knows how to work with the community, the government and the press, and that is a pretty devastating combination," says Stephanie Bolick, an assistant to one of the coalition's allies, Rep. Frank R. Wolf (R-Va.).

"He's not a wild-haired bomb thrower. He's a very smooth and intelligent and easy to work with man," one former FAA official says about Bernthal. "He is listened to . . . . He is Brooks Brothers to the hilt."

Bernthal helped organize the coalition in 1979 to give a single voice to the airport's foes and worked closely with the Transportation Department when a new policy to govern noise and reduce traffic was implemented at National in 1981. He is now part of a transportation department task force studying the airport's future and says the key to change is educating Congress and the public.

Bernthal says that despite long years of battling the airport, he harbors a love for aviation. He reads the technical journals, wrote a study of airport noise in law school at George Washington University and keeps a half dozen model jets in his Connecticut Avenue office at the firm of Arent, Fox, Kintner, Plotkin & Kahn, where he is a partner.

But he doesn't want planes so close to home, he says. "People cannot begin to appreciate the scope of the problem until they have lived with it," Bernthal says.

He concedes that another factor in his personal commitment is a fascination with observing and manipulating the confluence of forces--Congress, the FAA, the airlines, the community groups and the traveling public--that fashion aviation policy at National, one of the most intensely politicized airports in the country.

Both his opponents and supporters credit much of his punch to his handling of the media.

When national news agencies needed a spokesman for anti-airport sentiment after the crash of an Air Florida jet at the 14th Street bridge last year, they called Bernthal. He was almost always available. "In the four days following the crash, I did 27 live interviews, including ABC Nightline and Good Morning America," Bernthal says.

When American Airlines began late-night flights two weeks ago, Bernthal was on hand at Hains Point and outside the airport terminal for TV interviews. He did not get home until nearly midnight.

Though much of the uproar over the airport has focused on noise, Bernthal argues vehemently that the real issue is safety problems--shorter runways than the ones at Dulles or BWI, short "overrun" areas for aborted takeoffs, curved flight paths and general congestion. The FAA has responded that National meets federal safety standards.

The coalition maintained general unity until earlier this year, when discussion began heating up over the so-called "scatter plan," a means of spreading airport noise more evenly across the area by shifting departure paths. Under the plan, planes leaving National would turn off the Potomac River shortly after takeoff rather than proceeding about 10 miles up or down the river, as they are now supposed to do.

Bernthal lives in a neighborhood that stands to benefit greatly from such a change. He has thrown his support behind the scatter plan test, which the FAA has announced it will conduct sometime after Sept. 15 for up to 90 days.

But the governments of Alexandria, Arlington and Fairfax County and D.C., as well as many citizens groups there, have opposed the scatter plan on the grounds it will ruin the environment in now-quiet neighborhoods. Bitterness has arisen in the organization over what they see as betrayal of their interests by Bernthal.

"His treatment of the river is pretty selfish, I think," says Alexandria Mayor Charles Beatley. "It's strictly whatever affects the Palisades up there north of Georgetown (where Bernthal lives) . . . His motivation may be more parochial than it is regional."

Bernthal responds by pointing out the test was requested by the Metropolitan Washington Council of Governments, which represents area governments, and is only a test. If it proves to be unfair or ineffective, he says, it can be canceled and the community will have learned something.

He is also faulted by some of the coalition members for operating secretively, meeting with airline and FAA officials constantly with only rare consultations with the member organizations. He concedes he often acts independently and sometimes without a master plan, but says events have moved so quickly that they demand immediate responses.

Bernthal says--not totally in jest--that in the best of all worlds National Airport would be bulldozed and a giant amusement park put in its place. As thousands of people flocked to this new place, profits would be generated that would go to build a rail line to Dulles, which along with BWI would be handling all commercial air traffic in the region.

But he says he is not expecting that anytime soon. For the moment he would be willing to settle for further reductions in flights there.

The coalition, Bernthal says, "has always worked to try to accomplish the politically possible."