FAA: United flights returning to normal after stoppage

United Airlines operations are beginning to return to normal.

The problem has been resolved, according to the Federal Aviation Administration's air traffic control command center, and the ground stop was lifted.

The FAA grounded flights Wednesday morning at all airports due to a computer problem; 3,500 flights were affected nationwide. United confirms this wasn't a hacking or cyberattack incident.

The ground stop was in effect for mainline aircrafts for the longest period. The stoppage was lifted earlier for United subcarriers and regional airlines.

"We experienced a network connectivity issue this morning. We are working to resolve this and apologize to our customers for any inconvenience," according to United.

Around midday, spokeswoman Jennifer Dohm said, "We fixed the router issue, which is enabling us to restore normal functions."

The Federal Aviation Administration lifted a ground-stop order after nearly two hours, allowing United planes to fly again.

United did not immediately say how many flights were affected.

United, the nation's second-biggest airline, has suffered similar technology problems before, also leading to mass delays and cancellations.

The airline briefly halted all takeoffs in the U.S. on June 2 because of a problem in its flight-dispatching system. United said then that about 150 flights were affected.

United also struggled through a series of computer outages in 2012 after switching to the passenger-information system of Continental Airlines after that carrier merged with United. Those outages caused hundreds of flights to be delayed. High-paying business travelers were outraged; United CEO Jeff Smisek apologized for failing to provide good customer service.

After a 2010 merger, United elected to combine many computer systems and frequent-flier programs all at once. Executives believed that any disruptions would thus be short-lived. By contrast, Delta and Northwest integrated their systems in stages after a 2008 merger, and American Airlines is taking Delta's same go-slow approach now as it absorbs US Airways.

Other airlines, however, have also been hit by computer problems. In April, more than 50 American flights were delayed when a software glitch prevented pilots from seeing some airport maps on their tablet computers.

After Wednesday's problems, United apologized to customers and said they could change travel plans without being charged the usual $200 reservation-change fee. In some cases, the airline said it would also waive any difference in fare for the rescheduled trip.

"We don't know everything behind this morning's issues yet, but today's incident underscores the sense that something is very wrong at United," said Gary Leff, co-founder of frequent-flier website MilePoint.

Shares of Chicago-based United Continental Holdings Inc. fell $1.25, or 2.3 percent, to $53.06 in midday trading.

(The Associated Press contributed to this report.)