More flights at National Airport and increasing noise problems along the Potomac River could result from aviation legislation approved in the final hours of the congressional session.

As part of the legislative package, Congress has called for a review by the Department of Transportation of National Airport's flight scheduling system that may mean eight to 12 more flights a day, airport officials and federal sources said.

"There's no doubt that whatever happens, there will be more noise for residents near the airport to deal with," civic activist Sherwin Landfield said. "They're talking about more flights, and more flights mean more noise."

In the last week, those fighting noise at the airport have won important battles in court and in Congress, but many activists such as Landfield fear that a noisier National may be inevitable.

Congress's request for the review came less than two days after anti-noise activists trying to delay National's $735 million expansion program won the latest round in a two-year-old court battle against the airport authority, which operates Dulles International and National airports.

A federal appeals court in the District ruled last week that legislation transferring control of the airports to the local authority four years ago was flawed. The ruling makes it likely that Congress will have to change the law before the authority begins any new renovation projects at both airports.

Although several proposals that would have dramatically increased noise and air traffic problems at National were rejected by Congress, the package approved after weeks of frantic negotiating and lobbying by airlines, the airport authority and local lawmakers raises the specter of more flights and more noise.

The DOT review could result in a proposal to Congress next year to increase flights. Any new flights would be allocated to smaller carriers. Many have had difficulty breaking into the airport's market because of a federally mandated "slot" system, designed to prevent delays and safety problems at National by limiting the airport to 37 scheduled flights an hour between 7 a.m. and 10 p.m.

Also included in Congress's aviation package were plans to phase out most of the nation's old, noisy airliners by the turn of the century. National, which already planned to phase out the noisier aircraft by 1998, was allowed to keep its stricter policy.

Federal sources said this week that because Congress told DOT to come up with ways for more flights at National without exceeding the 37-an-hour scheduling limit, the review is likely to focus on how to allocate landing and takeoff times not being used at National.

The review was seen as a compromise between lawmakers such as Sen. John McCain (R-Ariz.), who wants to smooth the way for smaller carriers at National, and others such as Sen. Charles S. Robb (D-Va.), who say that raising the hourly limit on flights could create chaos there.

On a given day, there are now eight or 10 unused slots at National, most of them in off-peak, mid-afternoon hours, said James A. Wilding, general manager of the airport authority.

"We place an enormous value on this slot system maintaining workable conditions at National, and we'll oppose any challenge to it," Wilding said.

Under one scenario envisioned by federal officials, room for more daily flights at National could be created by relaxing the scheduling limit to allow, for example, 39 flights during popular morning and evening periods and only 35 flights during other hours. An average of 37 flights an hour would be maintained.

Wilding and other airport officials are cool to that idea, as are anti-noise activists.

Another way to add flights at National would be to schedule more flights in certain hours, on the assumption that inevitable delays and mechanical problems would create enough openings to allow the airport to maintain an average of 37 flights an hour.

"There may well be some way" to add more flights, Wilding said. "But I can just close my eyes and think of the long-term implications if the slot system is weakened . . . and it's not something we would want."