Federal Aviation Administration officials have agreed to establish a new flight path for jets taking off south of National Airport that would route the planes down the middle of the Potomac River, away from the Alexandria shore.

The new course, to be set up at the request of Rep. Herbert E. Harris (D-Va.), would not be put into effect before public hearings are held this spring on a number of proposed new airplane flights paths into and out of National, according to FAA officials.

The public hearings or briefings on the proposed new flight paths were to have been held this month but have been delayed until at least the last week of March. They will be conducted jointly by the FAA and the Metropolitan Washington Council of Governments. A COG newsletter detailing the proposed flight paths, which would send planes over new areas around Washington in order to distribute airplane noise more evenly, will be avialable in about two weeks.

Harris met with FAA officials at National Airport last week, to urge that they not wait for the hearings to order jets out over the middle of the river when they fly by Alexandria. Harris insists that to do this would not be establishing a new flight path but simply would be requiring pilots to fly where the FAA has said all along they're supposed to fly, over the middle of the river.

A recent study by Alexandria officials found that 83 out of 100 planes taking off from National flew within a quarter mile of the Alexandria shore and six actually flew over it. The river is a mile wide at Alexandria. A separate noise study by the city last fall found planes make twice as much noise in Alexandria as they do on the Maryland shore opposite, even when all the planes are over the water.

FAA officials have admitted on several occasions recently that planes do tend to hug the Alexandria shore on take-off, when jets are noisiest, and that the FAA "could do something about it the day after tomorrow." But they have said they want to make no changes in flight paths anywhere until after the public hearings have permitted all local residents and municipalities to be heard on the subject.

"We made a commitment, a covenant with the community, to do nothing until after public hearings . . . we made it in McLean." National's chief air traffic controller, Harris G. Hubbard, told Harris. In 1975 the FAA came under fire for its decision - made without public hearings - to allow planes taking off north of National to turn away from the river over McLean.

Under 1966 noise abatement regulations, jet using National must follow the river for 10 miles north of the airport, or just beyond the Cabin John (Beltway bridge, and for 5 miles south, or just beyond Alexandria and the Woodrow Wilson (Beltway) Bridge.

Pilots are instructed to fly a "due south" (180-degree) course when taking off south of National in bad weather and to "follow the river" in good weather. But jets can fly along either the Virginia or Maryland shore without violating the 1966 noise abatement regulations.

"The 180-degree course was designed initially to achieve a middle of the river course," controller Hubbard told Harris, since the runway is on a 183-degree alignment and if planes took off in a straight line they would be even closer to the Alexandria shore.

Under the 1966 regulations, planes approaching National from the south actually are told to fly a 187-degree course until just south of Woodrow Wilson Bridge and then to "follow the river." But if they continue on the same 187-degree course, many apparently do, it takes them extremely close to Alexandria or actually over portions of it. More than 12,000 people live within six blocks of the river in the port city's old and historic district. The Maryland shore opposite contains only marshes, woods and the Blue Plains sewage treatment plant.

There are no markers or lights south of the airport to guide planes along a mid-river path, although a light was installed in the middle of Memorial Bridge to help planes stay over the center of the river north of the airport. An electronic beam or marker could easily be installed on the Woodrow Wilson bridge, Charles Foster, FAA director of environmental quality, told an Alexandria civic group last month.