The Federal Aviation Administration said it is upgrading radar equipment to help prevent aircraft from disappearing from the computer screens of air traffic controllers.

Controllers at the FAA's Washington Center in Leesburg have reported numerous instances over the past month in which a plane or several planes suddenly disappeared from radar screens for more than 30 seconds. Over the weekend, FAA officials upgraded radar in The Plains, Va., that was responsible for some of the incidents. An FAA official said the agency is also working to upgrade radar at two other locations and is ready to respond if the problem recurs, using an aircraft and vehicle equipped with spectrum analysis equipment.

"We are concerned because we want to know every anomaly that affects the national airspace system," said Steven B. Zaidman, vice president for technical operations services at FAA. "We have an active bunch of technicians and air traffic specialists who continue to look at it."

Since the radar upgrade was completed in The Plains, FAA spokeswoman Laura Brown said there continue to be problems with planes disappearing from the scopes, but she said they are "less severe."

The union representing air traffic controllers said it was pleased with the FAA's response. An FAA spokesman initially suggested that the controllers had raised the safety issue as a negotiating tactic during contract talks.

"We're very relieved the FAA is finally taking this situation seriously," said Ruth Marlin, executive vice president of the National Air Traffic Controllers Association, in a statement. "Controllers at Washington Center have been forced to scramble on too many occasions reacting to the mysterious equipment failures."

On at least one occasion, Sept. 27, multiple radars intermittently went out, which FAA officials said was highly unusual. The FAA said the problem has not occurred since then, although occasionally one radar will "jump" and a plane will disappear for about 12 seconds and then reappear.

Controllers direct traffic by referring to data on their screens that identify an aircraft and its speed and altitude. During a radar jump, the information about the plane remains on the screen, but the plane's exact location disappears for 12 to 30 seconds. Airplane information is delivered to controllers by radar that communicates with the aircraft's transponder.

Controllers in Leesburg handle air traffic for a broad region from New York to North Carolina and from the East Coast to West Virginia at 18,000 feet and higher. If controllers cannot temporarily view their planes on radar scopes, they revert to non-radar procedures, which involve spacing planes farther apart as a safety precaution. In some cases when controllers said they had to do this, they claim it resulted in small delays for some planes at some airports.