FAA spokeswoman Elizabeth Isham Cory said that United had ordered a ground stop of its planes on its own that ended at 7:55 a.m. The problem began about 5:15 a.m., she said.

United had told the FAA that it was a computer issue that made the airline unable to check in people for flights and resulted in an inability to get final flight information to the cockpit, Cory said.

“I only remember in my 10 years one other time” that computer glitches have caused this much trouble with flights, Cory said.

By about 9 a.m., an airline spokeswoman said that it was manually checking in travelers through mobile units and laptop computers to facilitate check-in for as many customers as possible, Szadokierski said. To get around the computer glitch, agents at the check-counters also called agents at other airport to have them check in travelers, with O’Hare agents then writing out boarding passes, she said.

The problems have since been addressed, but it took several hours and caused significant delays and some cancellations for travelers shortly before the beginning of 4th of July weekend. Larger-scale flight delays at major airports tend to have ripple effects, as schedules at many other connecting airports become affected.

This is also really expensive. In 2007 alone, flight delays were estimated to have cost the economy more than $40B.