The rate of complaints about jet noise at National Airport has dropped drastically in recent months because pilots are doing a better job of following the recommended routes along the Potomac River, Federal Aviation Administration officials said.

But many pilots still are creating needless noise on takeoff by ignoring an FAA recommendation that they reduce power after reaching 1,500 feet, the officials said.

Those findings are the conclusions of studies released yesterday of airlines' conformance with the recommendations. Eastern Airlines was shown to be one of the most consistently nonconforming airlines and Northwest Airlines was one of the most observant.

Antinoise activists said yesterday they will try to organize a boycott of the offending airlines if their pilots don't improve at reducing power at 1,500 feet.

"If people . . . [knew] who the real offenders were, they'd tend to go to those airlines operating more quietly here," said Glen Rogers, an antinoise organizer from Arlington. "This would have an impact . . . . Close to a quarter of a million people are impacted by this noise."

The activists' efforts come at a time when jet noise has been lowered around National, according to the FAA, which says protest groups no longer pack auditoriums and FAA phone lines no longer are jammed.

FAA officials said one reason is that last July a retired air traffic controller was assigned to monitor departing flights on a radar screen periodically. Airlines whose flights were seen to diverge considerably from the Potomac River were notified and asked to stop the practice.

Last July, there was an average of 21 such notifications a week, FAA officials said. "Now if there's one every couple of weeks, that's a lot," said Sue Silverman, community relations director for National and Dulles International airports.

FAA Administrator Donald Engen and James Wilding, the FAA's chief of National and Dulles, have written or called airline executives in recent months to remind them about the FAA's recommendations that jets follow the river and reduce power when they reach 1,500 feet, until they pass Cabin John. These procedures are not mandatory, and the FAA has no means to enforce them except to keep reminding airline officials.

The result of the reminders is that the rate of jet noise complaints has gone down from a high of 100 a day during the 10-week test of the so-called scatter plan that ended in January 1984 -- in which departing jets were "scattered" from the traditional river route -- to five a day now, Silverman said. The great majority of current complaints come from people who live along the river, who after the scatter plan again were bombarded with the biggest share of the noise.

Three FAA tests, taken with noise monitors near Chain Bridge in Arlington in the last nine months, show that Eastern Airlines has one of the worst records. On July 12, none of its 38 flights reduced power; on October 15, two of its 32 flights did, and on Feb. 27, seven of its 37 flights did.

A reporter's attempts to reach Eastern Airlines officials for comment was unsuccessful.

Other airlines that scored poorly in the FAA tests were Delta Air Lines and Pan American World Airways.

Northwest Airlines did the best in the tests, with 12 of 14 jets in July, 17 of 17 in October, and 9 of 14 in February following the FAA guidelines. Piedmont Airlines, New York Air and Midway Airlines also performed well in the noise tests.

FAA officials warn that the statistics are sketchy and present only a tiny sampling.

C.A. Smith, Delta's vice president for flight operations, acknowledged that sometimes his pilots have "gotten lax" in observing the guidelines, but that airline officials "take pains in keeping our pilots educated" about the matter.

National is the only airport with this sort of requirement, and Smith said it "borders on impossibility" to expect pilots not to slip up occasionally. Pilots prefer not to reduce power so soon after takeoff, Smith said. "You just don't want to pull back on an airplane when you're climbing."