The Federal Aviation Administration warned against putting a proposed garbage landfill in the suburb of Potomac yesterday for fear that scavenger birds attracted to the dump might collide with jet airplanes taking off from National Airport.

In an unusual twist to an otherwise political battle, the agency declared the area an "unacceptable bird fly-way" where planes flying north might "coincide" with the birds heading towards the landfill from their large roosting spot on the Potomac River below the Woodrow Wilson bridge.

Also, the agency said in a report issued to county officials considering the landfill location, presidential helicopters fly over the Potomac site en route to Camp David.

The FAA stated that airplanes had encounted gulls over the present Gude-Southlawn landfill in Montgomery County at an altitude of 1,500 feet.

The report from William A. Whittle, chief of the Washington Airports District Office, said that the jet airplane traffic near the other landfill sites the county has proposed, in Laytonsville, also presented bird hazards but they are notas significiant as over Potomac. The FAA said the Laytonsville site should only be picked if the landfill could somehow be made less attractive to scavenger birds.

"There are no two ways around it," said David Sobers, the director of the Office of Environmental Planning for the county. "Bird strikes are a serious problem. The birds can intersect with planes." But the birds just do not fly as high as airplanes most of the time, said Sobers.

"Most of the reported bird activity around landfills is a few hundred feet above the landfill," Sobers said. "I can't see why the birds would have the propensity to come in at the height of an airplane (about 3,000 feet at lowest) and then drop vertically into the landfill. The only birds I know of with that pattern are duckhawks. They hover at a considerable altitude and then drop vertically on their prey."

Soaring birds become hazards when they fly into the engines of jet air-planes causing engine failure, and in some cases, disastrous crashes. But they are not new to the area.

The same birds that might fly north to the Potomas site currently are flying north to the Fairfax landfill on I-66 near traffic from Dulles Airport, according to Sobers.

"Our consultants is coming out with a 100-page study on bird hazards and landfill," said Sobers, "which talks about this as a common problem to the area. At this time there has been no decision to delete the potomac site from our study for landfill. The FAA report will become an additional piece of information."