Air traffic controllers directing planes over Virginia called on the Federal Aviation Administration to investigate an incident Monday in which planes disappeared from radar scopes.

According to a report filed by an air traffic controller at the Leesburg facility, two Delta Air Lines planes disappeared from a radar scope as one was descending over Virginia from 35,000 feet to 29,000 feet and another nearby was flying level at 33,000 feet.

An alarm sounded in the control room signaling that the safety standard of five miles between the two planes had been breached, said Tim Casten, a representative for the National Air Traffic Controllers Association, which is in contract talks with the FAA. Further investigation revealed that the planes were a safe distance apart and that safety standards were maintained, according to the union and the FAA.

The union said the outage caused planes to disappear for at least 21/2 minutes. FAA spokesman Greg Martin said that the outage was less than two minutes but that he could not specify how long it was. But Casten said the incident highlights the tense conditions that controllers work under because of technical problems. "The computer was not receiving radar data, so the computer estimated the planes' position to be closer than five miles," Casten said. "It forced a very bad situation."

Martin said the outage is not related to an intermittent problem of outside interference with the FAA's long-range radar that has caused planes to disappear from radar over the past several weeks. In those instances, planes unexpectedly disappear for at least 30 seconds from screens used by controllers to direct traffic at the agency's Washington Center in Leesburg. The facility controls air traffic above 15,000 feet in the area bounded by New York, South Carolina and West Virginia.

Martin said that the FAA has pinpointed the radar interference problem to a specific area in Virginia and that it has not recurred since Sept. 27.

Monday's incident "is a much more common and routine incident which controllers have experienced throughout their careers," Martin said. He said the FAA is updating some of the radars covering the Washington Center's airspace, which "will help reduce those radar jumps."