Two American Airlines jetliners touched down at National Airport late last night and early today, becoming the first flights to capitalize on a loophole in noise regulations that ban most passenger operations at the airport from 10:30 p.m. to 7 a.m.

Federal Aviation Administration observers on the ground beneath the approach path tried to measure the sound of the twin-jet DC9-Super 80s, but in each case the meter reading was declared invalid because the flights passed about a quarter-mile to the side of the listeners' position.

Nevertheless, the observers said the readings--68.65 decibels for the first plane and 67.6 for the second--seemed to indicate the aircraft are no noisier than predicted.

FAA regulations ban operations at National after 10:30 p.m. that produce more than 85 decibels of noise--louder, according to the Encyclopedia Britannica, than "industrial noise," but quieter than a "food blender"--a specified distance from the runway.

The DC9-Super 80, with specially designed engines, is the only jetliner in service that meets the requirement, and the FAA rated it in previous tests as producing 83.9 decibels.

Each flight--the first from Chicago that landed at 11 p.m. yesterday and the second from Dallas/Fort Worth with an intermediate stop at Baltimore-Washington International Airport that landed at 12:21 a.m. today--carried a fraction of the planes' 142-passenger capacity. The first had 42 and the second 28.

Last night's noise metering--the first of a series the FAA plans to assess the late-night landings--was taken by a mobile team on the grounds of Bolling Air Force Base, which is beneath the southern approach to National.

Earlier in the day, the FAA turned aside local politicians' objections to the late landings, and the Metropolitan Washington Council of Governments abandoned plans to seek a last-minute injunction to bar them.

The FAA rejected suggestions from Rep. Frank Wolf (R-Va.) that the flights might be louder than expected and compromise safety.

But the agency reversed its earlier position that noise monitoring would be unnecessary and issued a statement saying: "We respect and understand the community's concerns over this issue, and we will take those concerns into consideration as we monitor the impact of these flights."

The Council of Governments' legal staff decided there were inadequate legal grounds to ask a judge to stop the initial late flights. A COG spokesman said the organization will try to bar the flights by changing the airport's noise policy.

American Airlines spokesman Al Becker said objections to the flights should be addressed to the FAA, not the airline.

"All we're doing is living within the regulation. The regulation permits operations by airplanes that emit fewer than 85 decibels, and the Super 80 does just that," he said.

Those who have objected to American's plans since they were announced last week say that when the policy was being devised in 1981, they were assured the noise standards would be strict enough to exclude all passenger jet traffic from 10:30 p.m. to 7 a.m.

FAA officials have confirmed they gave those assurances but say later test data showed one jet, the DC9-Super 80, was in fact quiet enough to land at night, although it was still too noisy to be allowed to take off. Because the standards were set to assure minimum disruption to the public, officials say, the plane should be allowed in.

On Monday, COG president H.R. Crawford said the organization might take legal action against the flights on grounds they violated an intent of the noise policy to completely stop air carrier traffic at night.

COG staff yesterday polled their board members and found a majority favored legal action, a spokesman said. However, the council's attorneys later concluded the flights did meet the letter of the noise policy and would be difficult to challenge on short order in court.

COG participates in airport planning and would be in a position to lobby the FAA to further tighten noise rules so as to exclude all passenger jet traffic, a spokesman said.

Eric Bernthal, president of the Coalition on Airport Problems, said the group will continue its campaign against the flights by asking Transportation Secretary Elizabeth Dole to tighten the rules when she returns from vacation next week.

The impact of American's two night flights would not be serious in itself, he said. "It's the longer-term implications of this that are so serious," he said.

Bernthal and other opponents say American's competitors may follow its lead, eventually scheduling large numbers of late-night landings at National as the DC9-Super 80, which was introduced in 1980 and can seat as many as 172 passengers (the American Airlines flights are configured to seat only 142), becomes more common in airline fleets.

Trans World Airlines has said it may reschedule its single DC9-80 flight at National, a chronically late 9:59 p.m. flight from St. Louis, to arrive shortly after 10 p.m.

Republic Airlines, another National tenant, has the planes but does not now use them at National. New York Air and Ozark Air Lines have them on order.