The union representing the nation's air traffic controllers said the Federal Aviation Administration has not corrected a dangerous radar problem that causes some planes flying over the Washington area to become invisible to controllers tracking them on radar scopes.

FAA spokesman Greg Martin acknowledged that the agency has experienced "temporary interference" with its long-range radar and is conducting flight tests over the region to see if it can pinpoint the source of the problem.

Martin said the radar glitch does not pose a threat to airline passenger safety. He said controllers have been able to safely direct planes over the region by telling them to fly at different altitudes "so you don't have any risk of collision."

The problem affects planes flying at high altitudes -- above 15,000 feet -- and occurs intermittently, according to the National Air Traffic Controllers Association, which is engaged in heated negotiations with the FAA over a new contract. The technical problem results in aircraft disappearing temporarily from the screens controllers use to direct air traffic. A tag with information about each plane remains on the screen, but it represents the plane's location inaccurately, the union said.

"It adds more stress to an already stressful occupation," said Tim Casten, a union representative for the FAA's Washington Center in Leesburg.

The union said the problem began Sept. 19 and has recurred sporadically almost every day since. When it does occur, two or three planes disappear from the screen for as long as 30 seconds at a time, according to the union. The FAA said the planes disappear from the screen for 14 seconds at most.

Casten said that if the FAA does not fix the problem, controllers will have to rely on non-radar methods to track planes, resulting in slowdowns and flight delays. "We'd only be able to push 25 percent of the airplanes" using those methods, Casten said.

Martin dismissed the union's safety concerns as a negotiating tactic.

The union and the FAA began negotiations this summer on a contract that will determine pay and work rules for the 20,000 controllers, engineers and safety employees represented by the union. The current contract was signed in 1998 and expires this year.

The FAA is set to hire hundreds of controllers to replace many who are expected to retire in the next few years. The controllers face the challenge of managing an increasingly strained air traffic system.