Flights at National Airport will be cut 21 percent on Dec. 1 under the new Federal Aviation Administration plan to scale back the already curtailed air traffic control system at 20 of the nation's major airports.

The new schedules will be in effect for the Christmas season, when airlines normally add extra flights to accommodate holiday travelers.

A spokesman for Eastern Airlines said the airline, the largest operator at National, does not expect its shuttle to be affected by the cutbacks.

Flights at Dulles and Baltimore/Washington airports are not affected by the new cutbacks since neither is on the flight-restricted list.

Both may gain some services as airlines seek to use nonrestricted airports. Already, People Express Airlines, which will have to reduce flights at Newark, its major center, has announced new flights from BWI to Sarasota, another nonrestricted airport.

FAA officials said the cutbacks are designed to reduce flight delays and give the understaffed air control system added flexibility for dealing with adverse winter weather.

The reductions also will allow the FAA to give controllers, now working 48 hours every other week, a reduced workweek and more time off.

The flight cutbacks vary at the affected airports but are most severe in the Northeast, particularly in the New York area hardest hit by the controllers' walkout. Flights at New York's LaGuardia Airport, for example, will be reduced by 26 percent on Dec. 1.

The FAA's plan is to reduce airline activity nationwide to about 78 percent of the operating level before the strike Aug. 3 by the Professional Air Traffic Controllers Organization.

Although initial flight reductions at the nation's 22 major airports originally reduced the system nationwide to 75 percent of the prestrike level, the FAA routinely has allowed airlines to add additional flights, and airline activity now is approaching 84 percent of prestrike flight levels.

Two of the 22 major airports --Miami and Kansas City--are not required to make the new cuts on Dec. 1.

Of the remaining 20, some airports, such as Las Vegas and Fort Lauderdale, will be affected little, with flight reductions occurring one or two hours a day. Others, including National, LaGuardia, Pittsburgh and Chicago's O'Hare, will suffer reductions during most of their operating hours.

Generally, the airlines operating at the affected airports have been told they must eliminate all flights added since their original Sept. 1 schedules were set, according to airline and government sources. Additionally, they have been told to reduce remaining flights at certain hours by specific percentages.

Some of these reductions are sharp--airlines operating at LaGuardia, for example, must reduce arrivals between 5 p.m. and 6 p.m. by 43 percent, arrivals between 6 p.m. and 7 p.m. by 38 percent and arrivals between 7 p.m. and 8 p.m. by 41 percent. Carriers operating one flight an hour do not have to make any reductions.

Although airline officials say the FAA is considering requests for changes, it appears that newer airlines offering lower fares may be hurt most by the cutbacks.

In part, that is because the FAA is using Sept. 1 schedules as its base. New York Air, for instance, had a schedule on file Sept. 3 that increased its services at LaGuardia. New York Air officials are worried that the FAA order would totally eliminate six round-trip flights that began Sept. 20, with FAA approval, between New York and Detroit at fares 70 percent below those of its competitors.

Air Florida, another carrier offering lower fares, began flights Sept. 17 between Newark and Florida, picking up service suspended by Pan American. "It's a computer telling us what to do," an Air Florida official said. "If it's not in the Sept. 1 base, you're told you can't do it Dec. 1."