July 2 (Bloomberg) -- United Airlines said flight schedules returned to normal at Chicago’s O’Hare Airport after a computer outage disrupted passenger check-ins, forcing delays and 100 cancellations.

The breakdown lasted from about 3 a.m. to 10:30 a.m. local time, said Robin Urbanski, a United spokeswoman. O’Hare is the world’s second-busiest airport for passenger traffic.

“We are essentially back to normal operations,” Urbanski said in an e-mail about 2:45 p.m. local time. The airline didn’t immediately know the cause of the outage, she said.

Some travelers said they felt the effects before 5 a.m. The malfunction, coming at the start of the U.S. July Fourth holiday weekend, caused “significant” delays at O’Hare, United’s largest hub, said Jean Medina, a spokeswoman for the airline, a unit of Chicago-based UAL Corp.

Passengers awaiting flights described confusion ahead of the three-day weekend. Chicago airport officials say 190,000 people will travel through O’Hare today, and United expects its planes to be as much as 94 percent full this week.

“We got here at 4:40 this morning and there was hardly anyone behind the counters, but tons of potential fliers,” said Vicki Schulz, who was still waiting at 10:10 a.m. for a 6:49 a.m. flight to Alabama to board. “The lines were terribly long. It took us over two hours to even get to security.”

Kim Cumisky of Ossining, New York, who was connecting through Chicago to get to St. Paul, Minnesota, said: “No one was giving me any instructions. I’m hungry and supposed to be in St. Paul by now. This is ridiculous.”

Limited to United

United was still advising customers on its Web site to check in online instead of at the airport, and waived a fee for passengers traveling through O’Hare today and tomorrow to change their flight plans.

No delays were reported at O’Hare beyond the United flights, said Gregg Cunningham, a spokesman for the Chicago Airport System. United has 44 percent of the daily flights from O’Hare, making it the airport’s largest carrier.

O’Hare handled 69.4 million passengers last year, placing it behind Hartsfield-Jackson Atlanta International Airport, which had 90 million travelers, according to Airports Council International, a Geneva-based group that represents airports.

About 81.5 percent of United flights arrived on time at O’Hare in April, the most recent month for which Transportation Department statistics are available. The rate was third best among major carriers that had flights to the airport that month.

For the 12 months ending in April, 75.6 percent of United flights nationally arrived on time, according to DOT. That ranked 12th among 19 major carriers, the agency said.

UAL rose 2 cents to $3.31 at 4 p.m. in Nasdaq Stock Market trading in New York. The shares have declined 70 percent this year.

To contact the reporter on this story: Mary Schlangenstein in Dallas at maryc.s@bloomberg.net

Last Updated: July 2, 2009 16:19 EDT