A power failure that lasted four hours pulled the plug on the holiday spirit for travelers using National Airport yesterday afternoon, reducing the airport to makeshift operations on one of the busiest travel days of the year.

Although the operations of planes and the control tower were not affected, from noon to 4 p.m. most of the airport was without many of the conveniences of modern air travel.

Computers used for ticketing went down, parking lot vending machines refused to cough up parking passes, conveyor belts lugging baggage lurched to a halt and X-ray machines went blank.

Thanks to a backup generator, the air traffic control system continued operating, but some flights were canceled because of the confusion. Many other flights were delayed as airline and airport staffs resorted to doing business the old-fashioned way -- writing out boarding passes, unloading passengers on the tarmac and patting down travelers at security checkpoints.

An unrelated electrical problem at Dulles International Airport yesterday caused lights marking the threshold of a runway to go dark for more than four hours, forcing some flights that were delayed by fog to use another runway.

The cause of the failure of the lights, situated at the south end of Runway 01 Left, could not be immediately learned.

The lights were out from 8:30 a.m. to 12:55 p.m., an airport official said.

Beth and Brad Bartholomew grumbled angrily as they left the terminal at National late yesterday afternoon after learning that their flight home to Phoenix had been scrubbed, prolonging a lengthy holiday stay with relatives.

"My parents put up with us for 10 days, and now they're going to have to put up with us for one more," said Beth, with her two small children in tow.

Her husband, a commercial pilot accustomed to flying free on his Southwest Airlines, was especially peeved at yesterday's cancellation, saying, "This is the first time I've paid full fare in four years."

The four-hour blackout was caused by airport operations staff, according to Virginia Power representatives.

The first of two power circuits that feed National Airport blew at 11:53 a.m., sending isolated areas into darkness, officials said.

Though the system compensated automatically, restoring lights in those areas by shifting the entire power burden to the remaining power line, someone in airport operations inadvertently threw a switch that blew the good circuit a few minutes past noon, officials said.

"Through operator error, a switch was closed that shouldn't have been," said Aubrey D. Tarkington, of Virginia Power.

Tarkington said that it could not yet be determined what caused the original failure, but that Virginia Power workers were able to bypass the fault and restore power on the original line.

Bob Sullivan, with National Airport operations, said the backup generators powered the lights along the runway, as well as keeping the tower in operation. "There was no impact on air operations in terms of taking off or landing safely," said Sullivan, adding that the power first flickered before "boom, it went black."

Safety concerns may have been more pronounced inside the terminal, where airport personnel used flashlights to help guide passengers down blackened corridors and darkened stairways that had no emergency lighting.

Ron Stange, acting airport manager, said, "I don't think it was anybody's fault."

Operations went on uninterrupted in National's southern end, including gates for TWA, Northwest and Midway.

Representatives at the Eastern Airline gates said passengers displayed remarkable patience, especially given the sizable crowd fed by holiday travel and the arrival of planes that were diverted from Philadelphia and New York because of bad weather.

Burke Burright sped to National three hours early so he would be sure to catch a flight to Scotland. "I thought I was crazy getting here three hours early. Now I'm glad I did," said the San Antonio resident, who scheduled a stopover in Washington to visit friends.

James J. Hare III, director of sales and services for Eastern Airlines, lent a shoulder to help load baggage onto carts. "We're finding ways to circumvent all the mechanical systems we've come to rely on," Hare said.

At the United Airlines gate, agent Shirley Price used a hands-on approach to search luggage and patted down passengers the way a police officer would look for a gun. "The power is down, but we're determined to conduct a thorough search," Price said. "I feel more safe doing a hand check."

Hare was one of several airline employees who praised travelers for their patience. "Once they recognized the seriousness of the situation, they relaxed," Hare said. "It really hasn't been that bad. Everybody is still in a Christmas, mellow mood."

Among the most mellow customers were those who gathered at Anton's bar and restaurant, one of the few pockets of the main terminal that had enough power to maintain operations.

"We have power and the tower has power. When they designed the airport, they decided those were the two most important parts," joked Mike Dugally, general manager of Anton's.

Staff writer Curt Hazlett contributed to this report.