At 3 p.m. on Sept. 14, National Airport ran out of parking.

Each of the 4,826 spaces at the airport's seven public lots was taken, hundreds more vehicles were parked in unmarked spaces and about 300 "overflow" spaces in employe lots were filled. Airport officials say that for the first time since 1978, parking managers turned cars away.

Three hours later that Wednesday, the lots were reopened. Spaces became available as returning travelers moved their vehicles.

But officials predict the overflow parking conditions at the airport will be repeated in coming months.

"Squeezing some more cars into the nooks and crannies has been the way here for many years," says airport director James Wilding, "but one senses that we have pretty much used up all those opportunities."

The Federal Aviation Administration, which owns National, wants to modernize the airport's roads and parking. But those plans face both tight federal budgets and critics who oppose spending tax dollars to make National more convenient when Dulles International remains underused.

In the meantime, about 20,000 people park at the airport each week when they take flights from the facility.

Some travelers complain that they must spend too much time searching for a free parking space, and that when they do find a space it is usually a long distance from their departure gate. "If you don't get here early in the morning, you aren't going to park," said a Maryland labor union official last Friday as he headed for his car.

"Most travelers do not allow enough time to park and get to the terminal," says Michele Hagans of D & H Parking Systems, which runs the lots on contract to the airport.

Others, however, say the system is adequate for their needs. "I can walk to the airport from my car in 10 minutes," says Terry Reinhart, a water-bed company executive who flies from National two or three times a month, usually on flights that depart at around 7 a.m.

"Because I get here so early it's not a problem for me," he says.

One main cause of the crunch at National is the lack of land on which to build additional parking. Lots have been built just about everywhere there aren't buildings, runways or airplanes--even under Metrorail's elevated tracks and in the once-grassy circle outside the main terminal. There are about 47 acres devoted to parking, out of a total of 680 acres at the facility.

In addition to the public spaces, there are about 3,000 spaces for airport employes, who pay $28 a year to park. A total of 111 spaces are reserved for members of Congress, diplomats and Supreme Court justices, who pay nothing to park.

But officials say the main reason for the congestion is that about 15 percent of the airport's passengers each year park their cars in airport lots.

Officials say that the lots near capacity on Tuesdays and Wednesdays because business travelers who leave town early in the week leave their cars in the lots for several days. By Thursdays and Fridays, as these people return from their trips, parking spaces are freed up.

The lots are rarely crowded during weekends.

To keep close tabs on the parking situation, lot supervisors call in a car-count each morning.

Problems are likely on any day that begins with fewer than 400 vacant spaces, according to David Hess, an airport spokesman.

As the lots fill to capacity, vehicles are sometimes directed to use unmarked spaces. When all the possible spaces are filled, drivers are issued free passes that allow them to park in lots reserved for employes.

As the situation worsens, airport officials notify news services and radio stations to advise travelers. A recorded message about parking conditions is continuously broadcast over a low-power AM radio transmitter on 530, and can be picked up near the airport.

Some messages suggest travelers try Dulles next time.

That's the same message some community groups