Controllers at the Washington Air Route Traffic Control Center in Leesburg picketed the facility yesterday to protest what they call a shortage of qualified controllers.

It was the ninth anniversary of a nationwide strike that ended after President Reagan fired 11,400 controllers and began a long process of replacement that many air traffic critics say has not been completed.

The 100 or so controllers who took turns walking in circles outside the center on Route 7 claim that while the number of flights handled by the center has risen 70 percent since 1981, the number of "full performance level" controllers has fallen 30 percent.

A full performance level controller is one who has completed all training and is "checked out" to handle traffic in all of the several sectors of airspace for which controller teams are responsible.

The Washington center, one of the busiest in the nation, handles flights between airports in a 200,000-square-mile area stretching between Wilmington, N.C., and New York City, including parts of West Virginia, Pennsylvania and New Jersey.

Federal Aviation Administration officials disputed the claims by the controllers, who were marching under the banner of their union, the National Air Traffic Controllers Association.

"We actually have gained staffing," said Joyce Sexton, the center's air traffic manager. The FAA counts trainees as qualified controllers after they have qualified to handle traffic in some, but not necessarily all, sectors. Seventy-nine trainees at Leesburg fit that category.

"Depending on what level you're at in the training program, you are qualified to supplement {air traffic} staffing," said Sexton.

According to the FAA, 282 full performance controllers are on staff at the Washington center, down from the 351 controllers who worked there before the old union, the Professional Air Traffic Controllers Organization, struck in August 1981.

In 1981, controllers at Leesburg handled 1.5 million flights. This year the staff must direct an estimated 2.6 million flights, according to the union.

Paul Williams, president of the controllers' union local, which represents about 70 percent of the Leesburg controllers, said more staff is needed so controllers will not have to do more than one job at the same time. In periods of low traffic, the FAA sometimes combines jobs in the hands of one controller to reduce the number of controllers needed.

"Our contention is that controllers should never work alone," said Williams.

According to the union, there have been 74 "operational errors" -- when planes fly too close to each other -- in Washington Center airspace since January 1989, and 25 of those occurred when only one controller was at a radar position.

"We make the most effective use of our human resources that we can come up with," said Larry T. Anderson, assistant air traffic manager. If a controller asks for help, he will get it, Anderson said.

"That's impossible," said Williams. "If every controller said he needed help at once, in bad weather for instance, it would be mathematically impossible."