In a nightmare for passengers, an United Airlines' computer glitch in Chicago caused a backlog of flights, stranding and frustrating travelers. The airline is expected to return to normal on Thursday.

The computer which failed in Chicago was in charge of dispatching flights from one airport to another, among other crucial details.

"That's what tells how much fuel should be onboard, where the pilot should fly, how they should fly and how much cargo can be onboard," said aviation consultant Michael Boyd. "Without that, you are shut down. You are dead."

Why it failed was anyone's guess.

"I don't know the cause," said United Airlines spokesperson Megan McCarthy. "We don't know in terms of what exactly happened."

For several hours after the computers were back up and running, lines at customer service desks in airports lengthened, and the airline still had no answer.

"They should have an official out in the first 10 minutes. That's a huge failure," Boyd said. "The airline should be out there with someone explaining it as quickly as possible."

With late departures up 10 percent this year already, and with flights on all airlines now expected to be better than 80 percent full, any delay can throw the whole system off.

"The system is wrapped so tightly right now that any disruption like this becomes a major problem for 24, up to 72 hours," said airline consultant Terry Trippler.

While United said it hopes to be back to normal by Thursday morning, the company still urges passengers to check the status of their flights before heading for the airport. That's good travel advice on any day.

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