The Federal Aviation Administration said yesterday it will begin testing a controversial plan to spread National Airport's flight paths more evenly over the Washington area, exposing about 350,000 more residents to jet aircraft noise. The tests could begin as early as Sept. 15.

Opponents condemned the decision, saying they fear the new flight patterns will become permanent. "I think it's going to make a lot more noise for a lot more people," Rep. Frank R. Wolf (R-Va.) said.

"They said it's just a test," Fairfax Board Vice Chairman Martha V. Pennino noted. "But a test is a plan."

FAA Administrator J. Lynn Helms authorized the federally owned airport to conduct the test for up to 90 days, but said he might cancel it at any time if safety, operational or other problems arise.

Alexandria officials said yesterday they are drafting a lawsuit that the city probably will file with Arlington, seeking to stop the tests and arguing that federal rules require the FAA to make a more thorough evaluation of the environmental impact of the new flight patterns before any test begins.

Helms acknowledged in a memo released yesterday that the test has provoked bitter controversy in the area, but said the Washington Metropolitan Council of Governments, which requested the test, represents the people of the area and its wishes should be honored.

The FAA has estimated the test will expose many more communities to aircraft noise as jets turn shortly after takeoff and fly over populated areas, instead of proceeding along the Potomac River as they currently are required to do.

COG requested the plan in 1981 and reaffirmed that position this summer after some area officials raised new objections to the plan. Proponents maintain it will more fairly spread noise from National, where about 550 jetliners land or take off each weekday, and give relief to communities along the river that bear the brunt of the noise.

Fairfax and Arlington counties, Washington and Alexandria, as well as civic associations, have condemned the test, saying it would reduce safety and damage the environment in currently noise-free neighborhoods.

In his memo, Helms said, "It is not the FAA's role to make a determination that such a test would be acceptable to the various political subdivisions" of the area. But he said he might cancel the test if "other meaningful events," an apparent reference to a reversal of COG's position, arose.

Helms said that 650 comments the FAA has received on the test plan since it was unveiled last May were split equally between supporters and opponents.

Eric Bernthal, president of the Coalition of Airport Problems and a resident of Cabin John, welcomed the FAA's decision. "If this works, it will bring tremendous relief to tens of thousands of people. If it doesn't work, I hope we will learn something useful from it."

According to the FAA, the Foxhall, Spring Valley, Cleveland Park and Petworth areas of Northwest Washington will experience an increase in noise, as will Bethesda, Chevy Chase and parts of Oxon Hill in Maryland. In Virginia, McLean, Arlington, Annandale, Falls Church, Merrifield and Groveton will have more overflights.

Benefiting from the test, with less noise, would be the Cabin John and Langley areas north of the airport, and the Fort Foote, Tantallon, Camp Springs and Fort Belvoir areas south of the airport.

In an order authorizing the test, National Airport Director James Wilding said he believed it "will not produce impacts on health or on property values. While the increased overflights are predicted to be annoying to many more people than are benefited from the proposed test, that prediction is part of what the test is intended to help prove or disprove." he said.