A ceiling on air passenger traffic at National Airport and a small decrease in the number of flights -- especially at night -- are the key elements of a new policy the Federal Aviation Administration is proposing for the airport.

The FAA's proposals will be open to public discussion at several hearings, the first of which is scheduled for tonight at 7:30 at the Key Bridge Marriott Hotel, Arlington.

A second hearing is set for 7 p.m. Tuesday at the Dulles (Airport) Marriott. The third is scheduled for 7:30 March 27 at the Key Bridge Marriott.

The FAA policy would set a limit of 18 million passengers a year at the airport by 1985. That is 3 million more passengers than used the airport last year. The airport presently has no limit on passengers, and use of the airport has been growing by about 1 million passengers a year.

The increase in passengers and decrease in flights would be accomplished, in part, by permitting wide-bodied jets to land at National. The jumbo jets can carry up to 300 passengers, twice the capacity of most two and three-engine jets now using the airport. They also are quieter than most of the big jets.

The FAA is proposing to halt big-jet flights at 9:30 p.m. instead of 10 p.m. All but emergency flights would be prohibited after 10:30. This ban would also include propeller-driven aircraft, which now may operate all night at the airport.

The airlines presently schedule or "stack" 20 or more flights for exactly 10 p.m., which means the big jets frequently don't land or take off until 11 p.m., and are a cause of frequent noise complaints from people who live near the Potomac, in the airport's flight path.

The FAA proposal would permit more long-distance, nonstop flights out of National by extending the present 650-mile limit to 1,000 miles. Established in 1966 when jets first began using the airport, the 650-mile limit was designed to keep National a short-haul airport and encourage long-distance flights to use Dulles International Airport.

The 1966 agreement permits nonstop flights to only seven cities that are between 650 and 1,000 miles from Washington. These flights were in operation when the agreement was signed. The proposed 1,000-mile limit would allow nonstop service to cities such as New Orleans, Birmingham, Ft. Lauderdale and Kansas City.

The FAA estimates a 20 percent reduction in big-jet flights under its proposed policy. National permits only 60 scheduled flights or "slots" an hour between 7 a.m. and 10 p.m., to allow sufficient space between planes in bad weather. Major airlines are allotted 40 of those slots; 12 go to general aviation (private and business planes) and eight are for commuter airlines.

The proposed policy would give the major airlines 36 slots an hour instead of 40, increase commuter airline slots to 12 or 15 and either keep general aviation at 12 or decrease it to nine slots.

While the slot system strictly limits the number of major airline and commuter planes that use National, general-aviation flights there now far exceed their number of official "slots." When the weather is good, air traffic controllers can safely handle as many as 100 flights an hour at National; they then squeeze in as many private and business planes as request to use the airport.

An average of about 1,000 planes a day take off from or land at the airport, about 30 percent of them general aviation. Major airlines also are permitted to fly "extra sections" of their shuttle flights, which are not counted as part of their slots. About 3,800 extra sections were flown last year, an average of 10 a day.

The flight path along the Potomac, and where jets turn away from the river north of Washington, will be the subject of a public meeting next Wednesday at noon at the Metropolitan Washington Council of Governments (COG), 1875 I St. NW. COG is proposing that the FAA test alternative flight paths to distribute jet noise more equitably over the Washington area. The FAA tested an alternative flight pattern south of National last fall, when planes flew 10 miles south of the airport -- instead of five miles -- before turning toward their destinations.

Meanwhile, the FAA is preparing plans for an extension of the "over-run" spaces at the end of National's 6,870-foot main runway, as a safety measure.

The cost of that project is estimated at between $4 and $17 million.

Two airliners overshot the runway during the past three years, narrowly avoiding the Potomac River. No one was injured and neither plane was severely damaged.

The FAA proposals call for creating up to 1,000-foot "extended safety over-runs" at both ends of the main north-south runway. The runway now has 1,000 feet of grassy overrun space to the south but only 200 feet of overrun to the north, where both planes skidded to rest near the water.

One FAA proposal calls for filling in 800 feet of the Potomac-Four Mile Run estuary by the Washington Sailing Marina south of the airport, and centering the runway farther south, a $17 million project.

The preferred FAA proposal is to fill in about 550 feet of the small Roach's Run-Potomac estuary north of the airport, where the National Park Service has a boat-launching ramp. This would cost about $7 million and would leave a channel of about 150 feet.

A third proposal is to do no filling in the river but to center the runway farther south, creating a 600-foot overrun at either end, a $4 million project. FAA officials say the agency recommends minimum 200-foot overruns at U.S. airports.