The computer system used by controllers at the Washington area's regional air traffic center quit working yesterday for almost 1 1/2 hours, the latest in a string of occasional outages over the past six months. The failure caused flight delays up to 70 minutes in the heavily traveled Northeast Corridor.

The Federal Aviation Administration, in confirming the malfunction, said that a backup computer system, which has also had problems recently, was working within 35 seconds of the outage's start and that there was no hazard.

The computer failure caused flight delays of 45 to 70 minutes at airports in the New York area, FAA spokesman Robert Buckhorn said.

There were "some delays" of shorter duration at National, Dulles International and Baltimore-Washington International airports, he said.

The backup system is supposed to prevent dangerous situations from developing when the main computer system malfunctions, freezing the symbols representing aircraft movements on controllers' screens or causing the symbols to disappear, FAA officials said.

No near-collisions or controller errors occurred during the outage, which lasted from 9:34 to 11 a.m., the height of the air traffic morning rush hour, Buckhorn said.

Controllers at the Washington Air Route Traffic Control Center in Leesburg have complained that the computer failures cause confusion and flight delays, sometimes distracting them from their primary job of preventing airplanes from colliding.

Two Boeing 727 aircraft, carring a total of 161 people, flew dangerously close to each other over Front Royal last week after a Leesburg controller directed them to fly in holding patterns because of a computer outage at the air traffic control center in Atlanta.

FAA investigators were "reconstructing events" to determine the cause of yesterday's outage, which occurred as some computer parts were being changed, Buckhorn said.

The Leesburg computer, a 15-year-old IBM 9020, has no more problems than those at the FAA's 19 other air traffic control centers across the country, officials say. But the Leesburg controllers direct flights through some of the busiest airspace in the country, over 200,000 square miles from New York to South Carolina and between the Atlantic Ocean and the Ohio River.

The more than 300 controllers at Leesburg directed 2.2 million flights last year, the FAA said.

The Leesburg center led the nation in controller errors last year, with 83.

The center has received a new computer, with four times the capacity and 10 times the speed of the old system, as part of a $12.2 billion, 12-year U.S. Transportation Department modernization program.

The new system is scheduled to begin operations Sept. 11 and be fully operational by Oct. 6, Buckhorn said.