Dulles and Baltimore-Washington International Airports are slowly dispelling old reputations as great airports built in the wrong place.

Long in the shadow of National Airport, the two suburban terminals have seen growing numbers of passengers and planes in the past year. Both are benefiting from two factors: the suburbs' gain in population and jobs in the past decade and air traffic restrictions at National.

In April, 243,000 travelers passed through Dulles' majestic, glass-walled terminal, about 17 percent more than in April 1982. Takeoffs and landings increased by 14 percent in that period. At BWI's expansive, high-tech terminal, passenger traffic was up 21 percent in March compared to March 1982, totaling 458,000 people.

Aircraft operations grew 12 percent and are expected to go up again this month when Piedmont Airlines inaugurates 22 new daily flights from BWI.

"We are at a takeoff point," says T. James Truby, head of the Maryland State Aviation Administration, which operates BWI.

Both airports have launched promotional campaigns to maintain the momentum, which also has brought major gains in air freight. But, as the scramble for business continues, many airlines remain firmly--and their critics say irrationally--wedded to National Airport. Crowded into 680 acres on the Potomac River, its runways alive with jets and private planes 15 hours a day, National continues to serve twice as many passengers as the other two combined.

Airline executives acknowledge there is new demand at Dulles and BWI, but their loyalties are with the old but crowded corridors of National.

"There is no question that the best place to be and the place that you can run the highest load factors is Washington National," says W.G. Kaldahl, American Airlines' senior vice president for planning.

Dulles has ridden booms and busts since it opened in 1962, but it has yet to catch up to its last peak in 1979, leading to concern that the current growth may be temporary. BWI, with an undisputed pool of passengers from the Baltimore area, has competed less directly with National and enjoyed a more stable growth.

The work of master architect Eero Saarinen, Dulles was the world's first commercial airport designed for the jet age. From its soaring terminal, passengers ride in climate-controlled comfort on mobile lounges to jets parked a half mile away.

Planners predicted the 10,000-acre facility, which straddles the border of Fairfax and Loudoun counties about 22 miles west of downtown Washington, would serve 9 million people by 1975, overtaking National, which would peak at 6 million passengers a year.

BWI opened in 1950 as Friendship International Airport, owned by the City of Baltimore. It stands about 27 miles from downtown D.C. In 1972, the state of Maryland took it over, renamed it BWI and began rebuilding the terminal with the goal of establishing it as an important regional center and as a competitor with National.

Optimism at both Dulles and BWI soon gave way to the reality of vacant lounges and unused capacity, much of which was blamed on National.

In the mid-1960s, manufacturers introduced smaller, short-range jets that could use National's relatively short runways. That helped keep the heavy--and profitable--domestic traffic there, while Dulles was left with transcontinental and overseas flights and prestige aircraft like the supersonic Concorde.

Location was a major handicap for the two suburban airports from the start. Dulles can be a 45-minute or more car ride from downtown Washington, a $25 cab fare. It has no rail service. BWI had its captive clientele in Baltimore, but it fell short on convenience for many Washington area residents.

In recent years, it has become clear that Dulles has another major shortcoming--in its layout. Saarinen's futuristic design was ill-suited to today's demands for airports at which passengers can board quickly and make easy switches to connecting flights.

"The lounge system simply is not well equipped to handle that," says James Wilding, director of Dulles and National airports. Airlines agree. Time spent on the half-mile trip to and from the jets' parking area is acceptable to the passenger on a six-hour flight but not to someone hopping up to New York.

For this reason, much of Dulles' recent growth has come at gates located beneath the control tower, where as many as six small planes can pull up directly to the terminal and load with old-fashioned stairs. Originally intended just for propeller-powered planes, they were opened to smaller jets as airlines shunned the cumbersome mobile lounges.

Probably the most important factor in the two airports' promising traffic statistics is the emergence of the Virginia and Maryland suburbs as job and population centers in their own right. New offices and subdivisions filled with potential travelers have taken over much of the open land around the two airports.

Fairfax County's population surged 31 percent to almost 600,000 residents during the 1970s. Commercial and industrial centers grew up in Tysons Corner and Springfield. The population of Loudoun County grew 55 percent to 57,000 and Prince William went up 30 percent to 145,000 residents.

"Eighty-two percent of the growth in the Washington area in that 10-year period occurred in . . . the Dulles service area," says Thomas Morr, president of the Washington Dulles Task Force, a promotional group established one year ago with government and private money.

Around the Capital Beltway in Maryland, similar but slower growth has added to both Dulles' and BWI's potential passenger pool. Montgomery and Prince George's expanded as bedroom and employment communities. In Baltimore the inner-city population plummeted but in nearby Howard County, the population grew by 90 percent and in Anne Arundel by 24 percent.

The Dulles task force contends that more than 37 percent of Washington area passengers now can get to Dulles quicker than they can to National or BWI. The Dulles camp cites a Council of Governments study that indicated 20 percent of the people using National would prefer Dulles if the service were there.

BWI sees itself as the best airport for much of Montgomery and Prince George's counties, as well as sections of Northwest Washington--areas its promotional staff calls the "battle zone" with National.

But many airline executives continue to view Dulles and BWI as consolation prizes for the place they really want: National. They have spent $20 million on terminal facilities there and see no point in backing away from a winner, where ticket counters and gates are always busy, in favor of a potential loser.

Airlines argue that their bread and butter in the Washington market continues to be the business traveler headed downtown--the Chicago lawyer here for the day for a court appearance, the Capitol Hill aide returning from a meeting with constituents in Miami.

"They don't want to end up 26 miles from their destination if there is an airport 4 miles from where they want to go," says Edwin I. Colodny, president of USAir.

But there are two reasons why airlines cannot count on expanding forever at National.

In 1981, after years of noisy debate and litigation involving civic groups, Congress and the airline industry over traffic at National, the FAA cut airliner landings and takeoffs there from 40 to 37 an hour. It ruled that passenger volume would be allowed to rise to 16 million a year (it is now at about 13.5 million) but no further.

In addition, National, one of the country's smallest "big" airports, is crowded on the ground. Its 42 airliner gates are heavily used at peak hours and the FAA will not allow more to be built. Terminals, roads and parking lots are crowded, a condition that will be eased but not alleviated by FAA plans to modernize them.

"Certainly the congestion at National Airport is working in our favor," says Truby of the Maryland Aviation Administration.

In addition to limiting National, the FAA has worked to make Dulles more accessible. In September, a 3-mile, $25 million section of highway linking the Dulles Access Road to Interstate 66 will open, moving Dulles an estimated 20 to 25 minutes from the Roosevelt Bridge. The agency is also looking at buying 14 new buses to shuttle passengers between downtown and Dulles.

To help attract flights, the FAA has tried to such measures as waiving fees that airlines pay for use of the runways and mobile lounges.

The FAA is reworking the airport's master plan, giving serious thought to correcting the design complaints. This could take the form of more gates below the tower, Wilding says, or construction of an all-new terminal with at least 12 gates and reached by underground passageways that might have automated "people movers."

Wilding says planners are leaning toward the new terminal approach. That is partly due to bureaucratic concerns--it would leave the face of Saarinen's terminal unchanged, making the plan more palatable to the various commissions that would have to approve it.

"You can't touch a brick," complains one industry official about the architect's creation, which is a registered historic landmark.

Dulles' recent upswing has seen United and American airlines add six return flights to Chicago and People Express and Western Air Lines open their first service. USAir plans to start service to Pittsburgh next fall. Federal Aviation Administration officials hope the package of incentives and pressure they have devised will bring more news like that.

BWI traditionally has faced challenges similar to Dulles' in location and lack of service. To help meet them, Maryland has spent about $100 million on new facilities and brought in Amtrak service.

It was in mid-December that BWI learned officially that Piedmont was scouting for a new hub in the northeast. By Jan. 6, state aviation chief Truby had lined up the approval of Gov. Harry Hughes and legislative leaders to commit $20 million in state funds to build the 12 new gates Piedmont wanted.

"My staff and I literally worked around the clock all during the Christmas holidays," Truby recalls. The deal was closed by the end of January, and construction quickly began on a six-day-a-week schedule. Such speed would be unthinkable at Dulles.

On July 15, with seven of the gates finished, Piedmont plans to begin 22 new daily departures. After the remaining gates are done, the airline will add further to its schedule at BWI.

"We just see this great untapped, underserved market there," says Piedmont spokesman Don McGuire.