Alexandria residents have been given hope of some relief from jet airplane noise this spring by Federal Aviation Administration officials, who now admit National Airport jets fly unnecessarily close - and sometimes over - the city's historic Old Town section.

Two FAA spokesman, appearing at an Old Town Civic Association meeting last week with Rep. Herbert E. Harris (D-Va.), told citizens "there is no reason whatsoever" why jets taking off from National cannot fly over the middle of the river. But they asked city residents to be "patient" for a month or two until after public hearings are held on proposed new airplane flight paths into and out of National.

The series of five public hearings, expected to begin at the end of February, will consider the first major changes in airplane approach and departure routes at National since jets began using the airport in 1966. Under noise abatement regulations established then by the FAA, commercial jets must fly over the Potomac River for roughly 10 miles north of the airport and 5 miles south, although propellor planes can turn off sooner.

The proposed new flight paths, soon to be announced by the FAA in some 10,000 pamphlets distributed to Washington area civic groups, are designed to spread airplane noise more evenly over all communities and to allow many commercial jets to approach and leave National more directly.

James T. Murphy, the FAA's director of National and Dulles Airports, said following the Alexandria meeting that if no new departure flight paths south of the airport are dicided upon as a result of the public hearings, then he will almost certainly initiate FAA action to require all jets to fly a narrow middle-of-the-river route past Alexandria.

"We could do something about it the day after tomorrow," he told the civic group, "but you've endured the noise for 10 years now and I ask your patience for only several more weeks."

Charles Foster, FAA director of environment quality who appeard at the meeting with Harris and Murphy, told residents it would be simple for the FAA to install an electronic beam in the middle of Woodrow Wilson Bridge or at the airport to guide planes on an exact mid-river flight path.

In addition, he said, the FAA is installing this year permanent noise-monitoring equipment along National flight paths - including at least one on Alexandria's waterfront - to instantly record the noise made by every passing planes as well as its height and exact location.

"We will know immediately how much noise a plane has made and whether it flew near the Alexandria shore," Foster said. And while there are no penalties for airplanes that stray from a straight and narrow FAA flight path, "whether airports have noise monitors the airlines quickly come into compliance," Foster said. "It's not an airline marketing technique to be identified as the noisiest airline," Murphy added.

Under the current 1966 noise-abatement procedures, jets out of National have wide latitute while flying south of the airport and a much narrower flight path north of the airport because they are instructed only to "follow the river" which is narrow above Washington and over a mile wide near Alexandria.

In addition, planes must fly 10 miles north before turning off to their destinations but south of the airport they can turn away from the river after less than four miles, or just past the Woodrow Wilson Bridge and downtown Alexandria (the turn-off or breakaway point is theoretically five miles but is measured from the north end of National's 1.3 mile-long runway).

Thus current FAA regulations not only permit jets to fly inland over built-up areas south of the airport two and one-half times sooner - while the planes are lower and noisier - than they do north of the airport but they allow jets to fly directly along the edge of Alexandria's old and historic waterfront district, with 12,000 residents living within six short blocks of the river's edge.

FAA officials have said in the past that planes using National follow midriver routes and the FAA has commissioned studies to plot, on charts, the theoretical noise made by jets flying such mid-river courses.

However, Alexandria officials last year went out and did actual, not "theoretical," monitoring of where National's jets were flying and how much noise they were making on take-off - when jets are noisiest.

Of 100 planes surveyed by Alexandria officials, 83 were found to have flown within a quarter-mile of the city's waterfront and six actually flew over the city. In November, a spot noise survey of 19 planes taking off past the city, made when the city finally received sophisticated noise-monitoring equipment it had ordered a year earlier - found the planes made more than twice as much noise in Alexandria than they did on the Maryland shore opposite.

What particularly upset Alexandria officals was that the Maryland shore opposite the city is virtually deserted, containing only marshes, woodland and the Blue Plains Sewage Treatment Plant.

The FAA's Murphy acknowledged that south of the airport planes do seem to cling to the Alexandria shore-line. "No kidding about it . . . planes do seem to favor or disfavor the Alexandria side," he said, and suggested that planes have always done so because pilots are orally insttructed from the National Airport tower to head "due south," and because the runway is headed on a 183-degree course, either of which would take planes close to the Alexandria shore and not over the middle of the river.

In fact, Murphy said Alexandria's plight is exacerbated in summer months because jets climb more slowly in hot weather, and thus are noisier when they pass the city, and because during the summer 60 per cent of the take-offs from National are south past Alexandria due to prevailing southerly winds. "And summer is the season when aircraft noise is more noticeable generally," he said, becuase windows are open and people are outdoors more.