American Airlines has discovered a loophole in National Airport's 10 p.m. curfew large enough to fly two 142-passenger jets through.

The airline said yesterday that it will begin two daily late-night flights into the airport next week, using a new-model DC9 jet that is so quiet that the curfew--actually a restriction based on the noise commercial jetliners make--doesn't prohibit its landings there.

Some airport industry officials expressed a belief that since a half-dozen other airlines serving National also have the DC9 Super 80 jets in their fleets, they, too, are likely to seek permission to make late-night flights into National, something the noise standard effectively prohibits with other commercial jets.

National Airport director James Wilding said yesterday that while the American planes meet noise standards for landings, they do not meet National's takeoff noise standards. The American planes therefore will have to remain overnight at National, awaiting the facility's 7 a.m. lifting of the noise rule.

The president of the Coalition on Airport Problems, an umbrella organization of area groups campaigning to reduce traffic and noise at National, condemned American's plan and said it violated the spirit of the airport policy, if not the letter.

"We were told flat out that no existing turbojet aircraft would come in after 10:30," said coalition president Eric Bernthal. " . . . If there's any legal basis to challenge it, we will."

American Airlines spokesman Art Jackson defended the flights. "We feel it's a service for the community," he said. "And we're not violating rules. There's no reason not to do it."

American's first late-night flight will arrive at 10:55 p.m., Wednesday, Aug. 31, from Chicago and its second at 12:19 a.m., Thursday, Sept. 1. That flight originates at Dallas/Fort Worth and will come to National after a stop at Baltimore-Washington International.

Wilding said the Federal Aviation Administration, which operates National, believes airlines are free to schedule arrivals at any time of the night because of the quietness of the new DC9s.

Airline officials said TWA is only other airline currently flying the DC9 Super 80 into National, but competitive pressures may prompt other carriers to plan late-night arrivals there. Under the current noise standard, the FAA cannot refuse them landing rights.

Wilding said the noise policy, which bars planes emitting more than 85 decibels on landing, was intended from its adoption in 1981 to exempt the DC9 Super 80. McDonnell Douglas Corp., the plane's manufacturer, called it "the quietest jetliner in major airline service" when it was introduced to commercial service in October 1980.

FAA tests show that the Super 80, the latest version of the DC9 series that McDonnell Douglas introduced in the mid-1960s, emits maximum noise of 83.9 decibels when landing, Wilding said.

National policy provides that no plane that emits more than 85 decibels as it reaches a certain distance from the runway may land between 10 p.m. and 7 a.m. (Arriving planes, running late, have an added 30-minute grace period, giving flights until 10:30 to land.) For takeoffs, the noise limit is 72 decibels.

Prop planes and some private jets meet these noise levels and routinely operate at National at night. The noise level effectively has functioned as a curfew for big jets because they cannot meet the noise standards, FAA officials say, and until now no airline has scheduled Super 80s between those hours.

The 1981 policy originally would have imposed daytime noise standards that by 1986 would pressure carriers to switch to new generation of low-noise planes such as the Super 80. Those rules were removed from the final policy on the grounds that the airlines could not be expected to change their fleets so quickly.

Airlines long have wanted freedom to schedule National flights later at night because they feel there is demand for them. American's move comes as the airlines are fighting a proposal from Transportation Secretary Elizabeth Dole to lower the number of passengers allowed to use National Airport each year.

Northern Virginia politicians have argued for such a cap on National traffic as a way of bolstering flights at Dulles International Airport and as a way of easing complaints by area residents troubled by the noise of National's many flights.