United Airlines grounded planes nationwide for nearly two hours 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 61 flights to be canceled and delaying more than 1,100 other flights by midafternoon.
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.
Of the 61 cancellations, four were United flights and 57 were United Express flights, said a United spokeswoman, Jennifer Dohm. She added that there were 615 United delays, with an average of one hour, and 547 United Express delays, with an average of 27 minutes.
Even after United restarted its operations, passengers still reported 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.
On the same day, trading was suspended for a technical problem on the New York Stock Exchange.
Both United and the stock exchange confirmed that the malfunctions were unrelated internal technical issues and not a security episode.
At United, Ms. Dohm said a network router problem was the culprit. Security experts said the cause of such problems can take time to investigate, and some hypothesized that once forensics were complete, the issue could very well have been caused by a security incident.
Two weeks ago, a Polish state-owned airline grounded 1,400 travelers after hackers flooded the airline’s computer system with web traffic, but the airline has yet to disclose who was responsible. At the time, Sebastian Mikosz, Polish LOT airline’s chief executive, said the airline used the same state-of-the-art network equipment as others in the industry and warned that all airlines were vulnerable to a similar attack.
“This is an industry problem, not a LOT problem but an industry problem on a much wider scale, and for sure we have to give it more attention, if it can be given more attention,” he told Reuters.
The episode on Wednesday was the latest in a series recently for airlines. Early last month, 150 United flights were grounded because pilots could not gain access to their digital flight plans. And in April, American Airlines delayed some flights because a bug in its iPad software meant that pilots did not have accurate airport maps.
While the troubles were reminders of how damaging computer malfunctions can be, the United mishap on Wednesday 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.
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.”