The federal government is subsidizing cheap parking for thousands of private employes at National Airport, one of the area's most difficult places to park.

For $15 a year -- $5 more than it would cost some travelers to park at the airport for one day alone -- the Federal Aviation Administration allows airport employes to park in 3,500 coveted spaces.

There are only 4,000 parking spaces available to the public at the airport and those are priced from $4,50 to $10 a day. Parking for the private employes averages about 6 cents a day.

"It's damn outrage," said Fred Caponiti, an administrator in the D.C Department of Transportation, an agency that consistently has opposed the federal government offering cheap parking to civil service workers in the area.

The cheap National Airport parking affects not only the approximately 750 government employes who work there but thousands of airline and privately employed support personel as well. More than 300 of the spaces, for example, are granted to the Marriott Corp., which prepares airline meals there.

Officials of the Federal Aviation Administration, which operates National Airport and Dulles International, concede that the parking is a bargain.

It has to be, says Ben Castellano, an FAA official. The agency is prohibited from making a profit, he said.

That's just fine with the airport workers. "I cherish it," said Margaret Collier, a Trans World Airline secretary. "But $15 is quite high compared to what it used to be."

Until 1976, it was $7.50 a year and once jumped to $16 a year, FAA officials say.

That infuriates D.C.'s Caponiti. "The city is opposed to all subsidezed parking, especially in a congested area like the airport," he said yesterday. "It's bad enough to pay $15 a month, but $15 a year? That's outrageous," he said. "I hate it."

Castellano said that the airport parking plan "could be abused" by workers who purchase more than one decal and give the second decal to a friend. Workers are allowed to purchase two automobile decals and one for a motorcycle each year "because some people have two cars," he said.

The second decal costs $3 a year and nearly 4,000 workers at the airport have registered more than one car at the airport -- creating a situation that Castellano says is almost impossible to police.

"We try to regulate the decals by having the employe's supervisor check the car's registration," he said. "... We have spot checks and have causgt several of them (nonairport workers) that way. But we don't get around to it very much."

The same discounted parking rates are offered employes at Dulles airport, where the FAA has reserved 3,500 of about 4,700 spaces for them. Dulles Airport manaer Dexter Davis said yesterday he does not like to think of the parking here as cheap. "Responsible is a better word," he said.

Other government agency officials say they are surprised by the extent of the airport parking benefits. Jay Cohen, a transportation specialist for the General Services Administration, which controls 16,000 parking spaces for 150,000 government workers in downtown Washington, said government workers typically pay beween $60 and $190 a year for their parking. At some agencies the workers must agree to carpool to obtain parking benefits, he said.