The airlines, employing a new generation of quieter aircraft, have scheduled 24 flights a day between 10 p.m. and 7 a.m. at National Airport, where noise rules once virtually prohibited nighttime operations.

The sounds those flights make and the fact they are certain to increase in number have provided a new rallying point for antinoise activists, who seek a nighttime ban on commercial jet flights at National. The airport's central location means Potomac River waterfront residents in some of the area's wealthiest neighborhoods cannot escape the noise.

The flights in question include 11 by full-size commercial jets and 13 by smaller commuter planes, according to the Metropolitan Washington Airports Authority, which operates National and Dulles airports.

"The ambient noise on the ground is so quiet in the still of the night that any jet aircraft is a disturbance," said Helen H. Popenoe, vice president of Citizens for the Abatement of Aircraft Noise Inc., a regional coalition of antinoise groups. "We want an absolute curfew."

Many area residents mistakenly believe that National already has such an absolute curfew because, until recently, few commercial airplanes were quiet enough to meet the regulations in effect between 10 p.m. and 7 a.m.

During those hours, the airport prohibits flights by types of airplanes that, when they are certified, exceed 72 decibels on takeoff or 85 decibels on landing. That rule effectively prohibits night operations at National by 72 percent of the civil aircraft in the country, including the widely used McDonnell Douglas DC9 and the Boeing 727. A decibel is a unit of noise; 85 decibels is comparable to the sound of a garbage truck 200 feet away.

However, advances in aircraft technology have created newer, quieter, more fuel-efficient jet engines that can operate within those limits during certification tests. These aircraft, which include the McDonnell-Douglas MD80 and Boeing 757, have been labeled "Stage 3" according to the Federal Aviation Administration's complex formula.

The actual decibel level an individual flight makes varies widely from flight to flight and depends on a multitude of variables from the weight of the plane as determined by passenger and fuel loads, whether the temperature outside is warm or cold and whether it is humid or dry. A plane that recorded 71 decibels on takeoff when it was certified might make considerably more (or less) noise during an actual flight, but it is still regarded as operating within the airport's rule.

Popenoe said sound readings of actual flights would be more accurate and should be the basis for penalties.

Airport officials decided against using readings of actual flights when they established the noise rules in 1982, however, out of concern that pilots would "play the beat-the-meter game" and simply fly around noise monitors, said Sue F. Silverman, the airports authority's community relations director. "We felt that by using the stringent FAA certification criteria, we were much better assured that pilots would be more consistent and conscientious in their compliance with the rules," she said.

Airport officials at National, Baltimore-Washington International Airport and elsewhere across the country have pinned their hopes for noise reduction on the airlines' gradual replacement of their fleets with Stage 3 aircraft. These aircraft account for 28 percent of the U.S. civil fleet, according to the Airport Operators Council International.

At National, the quieter aircraft accounted for 24.9 percent of the flights at National in the three months that ended Sept. 30, up from 18 percent in the same period last year.

Silverman called that increase "heartening." "We believe the use of Stage 3 aircraft represents the community's best hope for a quieter National Airport," she said.

Nonetheless, the greater use of the new aircraft has enabled airlines to schedule more nighttime operations at National. James T. Murphy, vice president of the Air Transport Association, an airline industry group, said that airlines are merely responding to growing passenger demand..

At National there is high demand for scheduled flights in the early day and late evening, Murphy said. "This is not an outreach program. There are people with tickets out there," he said.

Airport planners expect to see 35 scheduled commercial jet flights per night by 2005, Silverman said.

Popenoe sees it another way. "The new technology is helping them get around the rules," she said.

The airports authority could change the rules, but it is unclear whether an absolute curfew would be legal, airport officials said. The legislation that transferred control of the airport from the FAA to the authority directed that National be operated as a "full-service commercial airport," Silverman said. "Some, including us, read that to mean 24 hours a day."

Airport officials reported 24 violations of the noise rules this year as of Dec. 1, about half as many as the 47 violations recorded during the first 11 months last year. A violation is a takeoff or landing between 10 p.m. and 7 a.m. by a plane that does not meet the noise standards.

Of those 24 cases, the airport imposed eight fines, ranging from $400 to $1,000. Of those, seven violations were by three major airlines; one was by a smaller aircraft.

Trans World Airlines was fined $2,500 for three flights. Eastern Air Lines was fined $1,400 for three flights. Delta was fined $1,000 for one flight.

The maximum fine was $1,000 per flight for the first half of the year, until the authority took control of the airport and raised the ceiling to $2,500 per flight.

Popenoe called the fines "piddly" and said, "National needs to gain a reputation for being stiff on violations."

But Silverman said that National "has a reputation for the most stringent antinoise measures in the United States," and that each violation is closely examined to make sure "the punishment fits the crime."