Passengers saw their flight information vanish from airport screens, and thousands were stranded as United canceled 36 flights and delayed 100 worldwide.
The airline still had no explanation Saturday afternoon for the outage. But things could have been much worse.
A blizzard in the Northeast wiped out more than 10,000 flights over three days in December, a mid-January storm led airlines to cancel nearly 9,000 flights.
Friday's shutdown occurred late enough in the day that many of the canceled flights were the last planes out for the day, said Henry Harteveldt, an airline analyst with Forrester Research. On a Monday morning, the results could have been catastrophic.
"It happened as a lot of the airline was going to sleep for the night," Harteveldt said.
That doesn't mean affected travelers were happy.
"I'm just amazed at how catastrophic the failure was," said Jason Huggins, 35, who was trying to fly home to Chicago after a week working at his software company's San Francisco headquarters. "All the computer screens were blank, just showing the United logo."
Huggins paid $1,200 to book one of the last three seats left on an American Airlines flight home.
Social workers Penny Nordstrom, 57, and Emily Schaefer, 42, who were trying to get home from Cancun, Mexico, to Spirit Lake, Iowa, said their delays started with a computer problem at midday Friday in Mexico.
"We're way past 24 hours now," Nordstrom said about noon Saturday before she boarded a rebooked flight from Chicago O'Hare International Airport to Detroit for a connection to Sioux Falls. She expected to get home about midnight but hoped her travel insurance would offer some compensation.
United spokesman Charles Hobart said late Saturday afternoon that the airline didn't expect to cancel any more flights this weekend due to the computer problems, though delays might continue.
Business travelers are usually hurt less by such disruptions than people flying for vacation or personal reasons because airlines first help passengers with elite status in frequent flier programs and those who bought more-expensive, unrestricted tickets.
Jon Ryan, who had planned to fly nonstop from San Francisco to London on business Friday evening, was rebooked on partner airline Air Canada after two hours on the phone. His new itinerary: San Francisco to Toronto to Halifax to London.
"The poor, poor ticket agents were just bewildered and sitting behind the counter. Everybody was just staring and didn't know what to do," Ryan said. "The line grew and grew and then people went from sitting down to lying down."
On a typical day, United, a subsidiary of United Continental Holdings Inc., cancels 15 to 30 flights for reasons ranging from fog to maintenance problems or staffing shortages. Those are understandable. Passengers and others said a computer glitch should not have grounded the airline.
"They're infrequent, but the fact that they happen at all is puzzling. These are mission-critical," said airline analyst Robert Mann. "The idea that they would fail is troubling."
College student Jamela Wilson, on her way from Los Angeles home to Roanoke, Va., was curled inside of her hoodie sweatshirt at a gate in Chicago Saturday after an overnight wait.
"I don't know what happened, but it shouldn't have impacted me to the point I'm not home right now. It's a good 12 hours later," Wilson said.
Mary Clark, a United spokeswoman, said she couldn't say how many passengers were delayed or how many still needed to reach their destination by midday Saturday. About the outage itself, she and other airline personnel said only that it was caused by "a network connectivity issue."
Airlines rely on computers today more than ever. Reservations and customer service are largely automated, even flight paths are increasingly computer-generated. Most passengers are asked to check-in online, at airport kiosks or via mobile phone - not with an agent - and paper tickets are a thing of the past.
Airplanes also are flying fuller this summer than ever before. United's were 86.8 percent booked on average in May, which in reality meant many flew without a single empty seat. So rebooking passengers from canceled flights is much trickier and more time-consuming than in the past.
United and Continental merged in May 2010. They are slowly integrating their systems but still operate independently. So Continental was able to dispatch flights normally, though some of its airport kiosks were affected.
Naveena and Vidya Maddali's Continental flight from Seattle into O'Hare arrived on time at 5:30 a.m. Saturday. But their connecting United flight to Saginaw, Mich., was delayed repeatedly and then canceled. With the earliest flight at 1 p.m., the software engineers were fairly sure they would miss the noon high school graduation party they were traveling to attend.
"The whole purpose of the trip got ruined because of the delays," said Naveena Maddali, 27.
The couple was particularly frustrated United didn't provide more information sooner because they initially had enough time to rent a car to finish their trip.
Dave Sertich, 29, a financial analyst who lives in San Francisco, slept in a chair at an O'Hare gate Friday night but said many people had it worse. He was just puzzled by the lack of information from the airline.
"There were no agents around, no communication to passengers about what was going on, no announcements," Sertich said.
United passengers flying the rest of the weekend have been advised to print out their boarding pass at home instead of at airport kiosks in case of continuing backlogs. By 3 a.m. Eastern time, United had announced on Twitter that things were returning to normal: "Flight status and flight rebooking are fully refreshed on united.com. Thanks again for your patience." Officials did not elaborate.
The airline's customer service line at O'Hare was only four passengers long by noon, and United was letting people with tickets for travel Saturday change them for free to alleviate the crunch.
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AP Airline Writer Scott Mayerowitz reported from New York. Reach Mayerowitz at http://twitter.com/GlobeTrotScott . Reach Rodriguez at http://twitter.com/bcrodriguez .
