The main air-traffic control computer for the Washington region failed yesterday morning, grounding hundreds of flights scheduled to depart from airports on the Eastern Seaboard for nearly two hours and causing delays as far away as Boston, Chicago and Florida.

Federal Aviation Administration officials said they suspect that a corrupted computer file caused a breakdown in a software system for managing flight plans, which in turn prompted the newly installed air-traffic control computer to fail. Eliot B. Brenner, assistant FAA administrator, said the glitch in operations of the computer, which gives controllers the identity and location of planes, was not a Y2K-related failure.

"It's an exceedingly rare event," he said. "It is not the new machine itself. It was not a Y2K thing."

Jack Gribben, spokesman for the President's Council on Year 2000 Conversion, said preliminary reports indicated that the airport problem was not Y2K-related but involved some other software failure.

The problem never caused controllers to lose track of aircraft on their screens, though it did require them to switch to a slower, less reliable backup system, FAA and union officials said. The main computer was restarted shortly before 10 a.m. and resumed running normally.

Trouble began at 6:45 a.m. when a difficulty arose with transmitting flight plan information to the computer at the FAA's regional center in Leesburg, which controls airspace in six states, agency officials said. That caused a stream of error messages, prompting technicians to turn off the computer system, called the Host and Oceanic Computer Replacement System.

The center switched at 8:20 a.m. to a backup system, called the Direct Access Radar Channel, that is slower and does not automatically update the information that controllers use to identify planes and guide them.

FAA spokesman Paul Takemoto said the switch did not pose an immediate hazard to air traffic because planes already airborne remained visible on the screens.

"Air-traffic controllers did not lose sight of their aircraft," he said. "They're seeing everything they would see otherwise." As a precaution, he said, the controllers ordered a freeze on takeoffs at airports along much of the Eastern Seaboard and greater spacing between planes in the air.

But a union official representing the computer specialists at the Leesburg center said the backup system did not eliminate the danger to aircraft.

"They can see the aircraft on the screens, but they can't say it's United flight such and such," said Mike Kelly, of the Professional Airways Systems Specialists. "Any time you do that, you put people at risk."

Planes across the Northeast remained grounded until shortly before 10 a.m., when the main computer was restarted. The breakdown affected airspace in part or all of New Jersey, Delaware, Maryland, Virginia, West Virginia and North Carolina.

The effect also spread to airports farther afield that had flights scheduled to pass through the airspace controlled from the Leesburg center. Departures from the three main New York area airports were severely disrupted because 40 percent of their flights enter the Washington region's airspace.

The three Washington area airports said that dozens of departures were delayed between 8:30 a.m. and 10 a.m. Though some arriving flights were allowed to land, many other airplanes bound for the Washington area were grounded at their origin, causing departures to be delayed through the early afternoon.

At Dulles International Airport, waiting passengers flipped through magazines and napped on lounge chairs. Rita Bible, 63, decided there was no point in worrying about the computer problems and munched on cheese crackers while waiting to board her plane. Bible's scheduled 11:25 a.m. flight to Charleston, S.C., was delayed until 1:10 p.m.

"I laughed when I heard because I waited [to fly] until today because of Y2K," she said.

Crosby Bonner, 47, of Alexandria, said he wondered if the Y2K bug was responsible the moment he learned his Norfolk-bound flight was delayed.

"At this time of year, after everything you heard, you expect it must be Y2K," Bonner said. "You think it must have caused some kind of problem."

At Reagan National Airport, Dennis Friedman, 21, of Potomac, languished on a US Airways plane for two hours before he was unloaded and switched to another Indianapolis-bound flight later in the afternoon. Friedman, a junior at Indiana University, said he was skeptical of FAA assurances that the glitch was not related to the millennial bug. "I think the fact of the matter is that Y2K didn't go over without a lot of problems, and we're just going to start seeing those problems now."

Among the passengers marooned on the same Indianapolis-bound flight was Rep. Edward A. Pease (R-Indiana), a member of the House aviation subcommittee who was trying to get to his district to deliver a speech.

He never made the engagement, instead spending the morning on the tarmac being served cookies and coffee.

"I'm a little frustrated," he said, after returning to his Capitol Hill office. "It reinforces what I already know: We're behind in the investment we need to be making in the infrastructure and modernization of the FAA." He said he did not know whether the age of the equipment at Leesburg was a factor in the breakdown.

The problem yesterday came three days after a computer malfunction at an FAA regional air-traffic center in Nashua, N.H., delayed flights at airports in New England and the mid-Atlantic. An agency official said that failure, which initially occurred in a disk drive on an IBM computer, was not related to the problems yesterday.

FAA officials said they could not be certain what caused the Leesburg computer to fail until after the agency completes an investigation.

Until then, spokesman Takemoto said, the FAA cannot determine whether the glitch has occurred before in the computer, which began operating at the Leesburg center in March.

The FAA finished introducing the new system at its 20 regional centers in September, he said.

Jack Ryan, vice president for air-traffic management at the Air Transport Association, said, "We don't believe this is a Y2K problem, and we've been assured by the FAA that this is not the problem."

Kelly, the union official, said he had no reason to believe the failure was related to the millennial bug but did not rule it out.

Staff writers Stephen Barr, Maria Glod, Frank Swoboda and Josh White contributed to this report.

CAPTION: Passengers at Reagan National Airport were among those who suffered flight delays of up to two hours after a malfunction in the region's main air-traffic control computer in Leesburg.

CAPTION: The glitch, which prompted delays at Reagan National and other airports, never caused controllers to lose track of aircraft on their screens, FAA officials say.