The Federal Aviation Administration yesterday directed planes taking off from National Airport's south runway to fly over the middle of the Potomac River in order to reduce noise in Old Town Alexandria.

Before the directive went into effect yesterday, planes taking off from the airport frequently passed over the heavily populated old and historic part of Alexandria and, according to a survey, the airplane noise there was louder than almost anywhere else in the Washington area.

Henceforth, pilots using the south runway for takeoffs will be instructed to follow a 183 degree magnetic compass course, according to Harry T. Hubbard, FAA tower chief at National. That course will take the planes over the middle of the river for about five miles until they turn on to other courses just south of the Woodrow Wilson Bridge, Hubbard said.

Pilots landing at the airport from the south already were instructed to follow a course along the center of the river.

"We'll never get rid of all the noise, but this is an important step forward," Hubbard said. He said the new procedure for departing planes will not increase noise for residents of Prince George's County.

Donald D. Teague, president of the Alexandria Old Town Civis Association, said yesterday that it is too early to tell how much the new procedure will help. "But we think that it's something that's been needed for some time," he added. The association is pushing for additional measures to decrease noise, Teague noted.

"If you've ever been down in that area when the planes are taking off, then you know it's something else," Teague complained."If you're trying to carry on a conversation, you might as well forget it until they pass."

The FAA was prompted to do something about the takeoffs when a study conducted by Alexandria officials late last summer showed that of 100 planes taking off southward, 83 flew within a quarter mile of the Alexandria waterfront and a half-dozen flew over a portion the city. Only five flew over the Maryland side of the river.

The survey showed that noise over Alexandria was twice as loud as it is on the Maryland shore, which is almost deserted, and louder than almost anywhere else in the Washington area.

Rep. Herbert E. Harris (D-Va.) began urging the FAA to establish a mid-Potomac course last February.

"It seemed to me that if the FAA had a radial (course) to help pilots stay over the middle of the river when approaching National, then they could set up similar procedures for southbound departures," Harris said. An FFA spokesman said a specific course for northbound flights has been required since 1974.

Harris said a computerized noise monitoring system will go into operation this year. Alexandria is among the first of 12 locations where the sensing equipment, similar to microphones, will be installed, he said.

The monitoring equipment will identify planes that fail to follow the takeoff course, Harris said.

Hubbard stressed that the new takeoff procedure is "only a technical change that will provide more accurate guidance, not a procedural change." No new personnel are required, he said.

The new takeoff procedure is separate from the so-called "share the noise" proposal that was recently reviewed at public meetings held by the FAA and Council of Governments. Comments on that plan.