The two-hour outage held up morning flights around the globe. From Los Angeles to London, Boston to San Francisco, frustrated fliers tweeted snarky remarks about the glitch. It was United's third major computer mishap this year.
"Does anyone have a Radio Shack computer or abacus to help United get their system fixed?," tweeted Lewis Franck, a motorsports writer flying from Newark, N.J. to Miami on Thursday to cover the last race of the NASCAR season.
In a subsequent phone call with The Associated Press, Franck added: "Why is there a total system failure on a beautiful day? What happened to the backup and the backup to backup?"
United said the technology problem was fixed by 10:30 a.m. EST. But early morning delays can easily ripple throughout an airline's network for the rest of the day even after the underlying cause is fixed. That's because once a plane is late for one flight it can be hard to make up for lost time.
United was not immediately able to say how many flights were delayed on Thursday. Spokesman Rahsaan Johnson said few if any flights were canceled.
The problem was that dispatchers couldn't send flight information to about half of the United's mainline flights. Johnson confirmed that the problem affected planes that came from United, which merged with Continental in 2010. Planes that came from Continental, and regional flights on United Express, were not affected. All of them together run some 5,500 flights a day.
United's biggest computer problem occurred in March, when its long-planned transition to a single computer system for passenger information caused delays and problems with frequent flier account balances. In August, 580 flights were delayed and its website was shut down for two hours because of a problem with a piece of computer hardware.
Johnson said the problems on Thursday were not related to integrating the computer systems of the two airlines. But even if Thursday's dispatching issue is separate from its earlier problems, the still leaves passengers frustrated.
Michael Silverstein, who works in finance, was supposed to be on a 6:01 a.m. flight from Los Angeles to San Francisco. The computer outage had already caused him to miss one meeting. Worried about missing another, he walked off the plane and bought a $195 last-second ticket on a Southwest Airlines flight to Oakland, Calif.
"I'm frustrated because I'm missing a meeting that I thought I had plenty of time for," Silverstein said.
CEO Jeff Smisek acknowledged on Oct. 25 that some customers avoided United over the summer because of its computer problems. He said the airline had fixed those problems by improving software and adding more spare planes to its system, among other moves.
"We expect to earn back those customers that took a detour and we expect to attract new customers as well," he said at the time.
Thursday's problems were exactly what United did not need, said airline and travel industry analyst Henry H. Harteveldt of Atmosphere Research Group. "This event shows an unacceptable lack of planning at United," he said.
"This merger has been an outright disaster on almost every count. United must make some changes in its executive leadership, starting with the CEO" and including its chief information officer if it wants to restore confidence among passengers, he said.
Shares of United Continental Holdings Inc. fell 20 cents to $19.78 on a day when shares of other big airlines rose.