The last passenger jets now land at National Airport around 10:30 p.m. But that does not mark the close of business there.

Throughout the night, National's three runways are used by a string of prop-powered commuter planes, executive jets and tiny single-engine planes owned by sport flyers. About 50 of these planes arrive or depart between 10 p.m. and 7 a.m. on a typical night, their engines meeting nighttime noise standards that bar other, larger, aircraft.

National Airport is a 24-hour operation, a fact often overlooked in debate over traffic and noise levels there.

American Airlines' surprise announcement last week that it plans the first late-night passenger jet flights into the airport, using a loophole in the noise rules, could be the start of major expansion in this after-hours traffic. The flights, some fear, may hamper federal goals of bringing more traffic to Dulles International Airport.

Airline industry analysts say that travelers' desires for later flights and the operational advantages such flights bring mean that American's competitors are sure to follow its lead. TWA already has said that it probably will move a 9:59 p.m. flight, which is chronically late, to sometime after 10 p.m.

Still, the growth in late-night flights at the federally owned airport is likely to be gradual, these analysts say. The McDonnell Douglas DC9 Super-80, the only passenger jet quiet enough to qualify for late-night arrivals at National, still is rare in the carriers' fleets. And although it is allowed to land, it is rated as too noisy to take off until after 7 a.m., when the noise rules are lifted.

Area politicians who have fought for reduced traffic at National have called on Transportation Secretary Elizabeth Dole to block American's late flights, calling them a "violation of the spirit and intent" of the airport's 1981 noise plan, which they said was meant to function as a curfew against all air carrier operations.

Both the FAA and the airline dispute this reasoning and American insists that it knows of no reason to halt its first late-night landing, planned for 10:55 p.m. on Wednesday.

The FAA, for its part, traditionally has opposed total curfews as too disruptive to national aviation. It has allowed tight noise standards, which can function in the same way, and says that if a plane meets National's noise standard--which supposedly was drafted to assure that a minimum number of people are disturbed--there is no reason why it should not be allowed in.

Some industry officials were surprised at the timing of American's controversial move because it comes in the midst of another heated dispute with airport activists over a proposal by Dole to lower the number of passengers allowed to use National each year.

The move is the latest of American's aggressive marketing efforts that effectively short-circuit restrictions at National. In 1981 it scheduled flights between Dallas and National in violation of the FAA's rules on the length of direct flights from the airport. American backed down on orders from the Transportation Department, but then began "hopping" its Dallas flights from National to Dulles, where no such restrictions apply.

Experience shows that where airlines can operate late at night, they will. Of 46 takeoffs and landings at National last week between 10 p.m. Tuesday and 7 a.m Wednesday, for example, 23 were commuter airline flights, according to the FAA.

New York's LaGuardia Airport, which like National is a busy East Coast center for domestic traffic, handles about 40 of its 700 daily passenger jet takeoffs and landings between 10 p.m. and 7 a.m. (National has about 550 such take-offs and landings per day.)

Most of those late flights to New York are arrivals. Eastern, for instance, has a flight to LaGuardia from Miami that gets in at 11:33 p.m. and another from Atlanta at 11:57 p.m. Often, airlines will fly largely empty planes to an airport late at night to position them for profitable flights out the next morning.

Twenty domestic passenger jets fly into Baltimore-Washington International Airport between 10 p.m. and 7 a.m. and another 11 take off.

"There is a market for those late-night arrivals," says airport planner Nelson Ormsby.

The flights at LaGuardia and BWI tend to cluster between 10 and 11 p.m. and again between 6 and 7 a.m. and could be expected to do so at National, too.

Estimates of how many late-night arrivals eventually may be scheduled there run from half a dozen to 24. Whatever the number, one FAA official says, it could draw some growth away from underused Dulles, where traffic has been picking up in recent months, apparently due in part to difficulties in getting into National.

Airlines long have complained that the 10 p.m. deadline at National, even with its 30-minute grace period for late arrivals, forces jetliners to depart from their cities of origin earlier than airline schedulers would like. This is a particular problem in cities to the west.

The two-hour flight to Washington from Chicago, for instance, requires leaving by 7 p.m. Central time, although there probably are many travelers who would prefer to leave later. Not surprisingly, one of the two flights that American plans is from Chicago, departing at 8:05 and arriving here at 10:55 p.m. Some analysts predict that the Chicago route will figure importantly in late-night arrivals.

National's system of requiring airlines to obtain a "slot," or rights for one takeoff or landing, creates another incentive for carriers to come in late.

Between 7 a.m. and 10 p.m., National has 37 such slots an hour and all of them are taken. Until last week, all of the airport's slots after 10 p.m. have been unclaimed. Thus both American jets will arrive without using any of the airline's valuable daytime slots.

The DC9-80's special status at National already has led to friendly ribbing among the airline executives who have the new jetliner and those who do not. Among National's tenants, TWA, American and Republic have these planes, although only TWA now uses them there. Two other National carriers, New York Air and Ozark, soon will take delivery of their first DC9-80s.

Other airlines do not have any orders to buy these planes. The result ultimately could be complaints or legal challenges. This is what happened at John Wayne Airport, 40 miles south of Los Angeles, where officials wrote noise regulations with the specific intent of barring noisier jets and making the airlines switch to DC9-80s.

"It is the quietest machine that is being manufactured," says W. J. Martin, noise control official at the Orange County airport. Despite the court challenges, the airport has proceded with the plan. Carriers that get new slots must use DC9-80s or not come at all. Progress is being watched closely as a potential precedent for stiff noise rules elsewhere.

In 1981, Federal Aviation Administration planners proposed a similar plan for National. Stiff noise regulations were to be introduced in 1986 to force the airlines to use the newer generation of quieter jets. But that proposal was withdrawn after the airlines complained that they could not switch to the new jets by that date.

American's new flights could generate complaints from McDonnell Douglas' major U.S. competitor, the Boeing Co. The Seattle-based aircraft maker has two models that compete directly with the DC9-80: the 757 and the soon-to-be-introduced 737-300. Both of these aircraft, however, miss National's noise limits by a hair's breadth.

Since service at National Airport, a high-density air center, is a major consideration for airlines shopping for planes, Boeing is placed at a competitive disadvantage. The company has declined to comment on whether it plans to protest. other than to say that it is "watching developments with a great deal of interest."