The Federal Aviation Administration formally notified the nation's airlines yesterday that they will have to reduce some of their flights at 22 major airports, including Washington's National Airport.

In letters to the heads of airlines operating at the 22 airports, FAA Administrator J. Lynn Helms said their schedules for flights beginning Oct. 25 "must comply" with the airport "limits" generally set by the FAA six weeks ago at the beginning of the strike by the Professional Air Traffic Controllers Organization and formalized Sept. 9.

Those limits generally reduced the number of airline flights at the 22 airports by 50 percent; since flight levels at other airports were not reduced, the FAA plan resulted in a nationwide system running at 75 percent of its normal capacity.

However, the FAA has routinely granted the airlines the authority to fly extra flights if they could be accommodated at the 22 airports. As a result, the nationwide system is back up to 82 or 83 percent of its pre-strike activity, Deputy Transportation Secretary Darrel M. Trent explained yesterday in a speech to the Aero Club. It is these add-on flights that will be cut back, he suggested, bringing the overall nationwide system back to about 78 percent of its pre-strike level.

The idea is to provide a "pad" in the system so that the FAA can reduce the workweek of all the controllers to 40 hours -- they work 48 hours every other week -- give them vacation and time off and allow some flexibility in the system to accommodate bad winter weather, Trent said. "We're quite concerned about the time being put in by the controllers," he said.

Although details were sketchy yesterday, DOT officials said some of the 22 airports would be affected very little. Others, whose add-on flights have mounted, could be affected very significantly, they said.

In his letters, Helms told the airlines he has designated Donald R. Segner, associate administrator for policy and international aviation, to review all requests for extra flights, which would be handled on a monthly basis. In a private meeting with airline officials last week, Helms was implored to leave the request-granting authority with the professional technical people in Jacksonville, Fla., who had been granting the extra-flight authority, rather than bring it to Washington where, they said, the requests could get embroiled in politics.

Helms also told them that he envisioned fixing schedules as early as 45 days before the start of a monthly schedule period.