The Federal Aviation Administration greatly underestimated the number of people who would be subjected to additional noise from low flying jets from National Airport if a controversial noise "scatter plan" is put into effect, according to a study by an Arlington-based citizens group that opposes the plan.

The study, released by the Center for Urban Education, calls the FAA's assessment of the extent of noise that would be caused by shifting jet flight paths from over the Potomac River to more densely populated areas "incomplete and misleading."

The FAA has estimated that 320,000 more residents would hear more jet noise if the scatter plan, which is intended to distribute noise more evenly over the Washington area, goes into effect. The Washington Council of Governments has endorsed a 60- to 90-day scatter plan test. The FAA, which owns National, will make the final decision on whether to go ahead with the test.

The new study, compiled by William C. Sperry, a former FAA branch chief of aircraft noise, recommended that the FAA conduct a full investigation of environmental effects the test would have on the area.

"I think their estimate is considerably on the low side," Sperry said in an interview yesterday. "My point was that they didn't make a complete or accurate assessment so we cannot tell how many more will be annoyed by the noise. . . . "

Hugh Riddle, deputy director of the metropolitan Washington airports, said yesterday that he was not aware of the study and would not comment on it until he reads it. He said he still supports his agency's findings concerning the scatter plan.

The Sperry study cautions that the test would set a dangerous precedent of permitting a federal agency to conduct a test before conducting a "properly prepared environmental assessment" for public review. The FAA has said that it believes that an environmental impact statement is not required before the plan can be tested.

Sperry contends that the FAA's analysis of the noise impact included how many seconds a person is exposed to a high-decibel sound, a method that Sperry says is not recognized by the scientific community.

Sperry argues that the acceptable method is to look at the number of persons who would be "annoyed" by aircraft noise, a measure that he contends should begin at a level of 55 decibels. Sperry contends that the FAA started determining persons affected by the noise at 65 decibels.

"The situation is annoyance," he said. "It is not a subjective term, but can be scientifically measured."

Sperry recommends that the FAA reexamine its computations and include measures of how many people will be affected by the test who live in areas that would receive from 55, 60, 65, 70 and 75 decibels of aircraft noise.

He added that the FAA should also calculate the total noise impact of the test in terms of how many people will be "highly annoyed" by the change.

Alexandria city officials said yesterday that the study, released last week, will help them in their effort to block implementation of the scatter plan.

"It came like a bolt out of the blue," said Alexandria City Councilman Donald C. Casey Jr. "It was right on target. Now we have an expert we can rely on. It's no longer a bunch of politicans saying what impact the plan will have on our areas."

City Attorney Cyril D. Calley said Sperry's study could be key in the city's attempt to obtain a court order to prevent the FAA from beginning the test.