GLITCH GROUNDS UNITED PASSENGERS

CHICAGO - A five-hour computer outage that virtually shut down United Airlines Friday night and early Saturday is a stark reminder of how dependent airlines have become on technology.

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.

'All the computer screens were blank'

That doesn't mean affected travelers were happy.

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 before she boarded a rebooked flight from Chicago O'Hare International Airport to Detroit for a connection to Sioux Falls.

Spokesman Charles Hobart said late Saturday afternoon that the airline didn't expect to cancel any more flights this weekend because of 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.

Reliance on automated system

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 said a computer glitch should not have grounded the airline.

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.