United Airlines grounded planes nationwide for nearly two hours on Wednesday morning after a faulty computer network router disrupted its passenger reservations system.
It was the second time in five weeks that United had to ground flights because of a computer issue. The latest problem prevented United from dispatching many of its domestic flights, causing 59 flight cancellations and delaying more than 800 other flights by midafternoon.
Of the 59 cancellations, four were United flights and 55 were United Express, a regional partner, said a United spokeswoman, Jennifer Dohm.
Even though United restarted its operations at 10 a.m. Eastern time, its computer systems still suffered from lingering problems.
Ryan Ver Berkmoes, a travel writer from Long Island City, N.Y., said he and his fiancée, Alexis Averbuck, a painter, could not get their boarding passes online until 3 p.m. for a flight to San Francisco on Thursday morning to plan their wedding.
Mr. Berkmoes, who flies enough to be in United’s highest frequent-flier category, complained that “snafus happen all the time” with United, which merged with Continental Airlines in 2010 to form United Continental Holdings, based in Chicago.
“Unfortunately for the places I fly, they are my one real option, and I’m not sure that competition in the oligarchy is that much better,” he said.
The latest United problem occurred on the same day that trading was suspended for a technical problem on the New York Stock Exchange. While both incidents were reminders of how damaging computer malfunctions can be in today’s world, the United router mishap also came just a week after the Justice Department disclosed that United had been part of an investigation of possible collusion among the airlines to limit seating and prop up ticket prices.
After a series of mega-mergers, roughly 80 percent of the nation’s air traffic is concentrated among four carriers: American Airlines, United, Delta Air Lines and Southwest Airlines.
George Hoffer, a transportation economist at the University of Richmond, said the disruptions caused by United’s router problem were “a perfect example of where the mega-mergers added redundancy to the individual carrier, but really lessened redundancy for the airline system.”
“The concentration, as predicted, makes a greater percentage of the system at risk if a glitch occurs,” he added.
Blaming “network connectivity,” United ordered a ground stop to its domestic flights, as well as those flown by its regional United Express partners, at 8 a.m. Eastern time.
It restored the United Express flights at 8:25 a.m. only to halt them again at 9 a.m. The ground stop was lifted for all of United’s domestic operations at 9:49 a.m.
United’s chief executive, Jeff Smisek, said in an interview in New York that the computer issue had prevented the airline from dispatching planes. He said it took about 90 minutes for United’s computer technicians to correct the problem.
There will be residual flight delays, he said, but “we’re working hard to minimize them.”
This is the second time recently that United has had to keep some of its planes from flying because of computer issues.
Last month, about 150 United flights were grounded because pilots could not gain access to their digital flight plans. And in April, American Airlines delayed some of its flights because a bug in its iPad software meant that pilots did not have accurate airport maps.
Because of an editing error, an earlier version of this article misstated which flights were canceled. They were United and United Express flights, not United and Continental flights. The error was repeated in an earlier version of a picture caption.