The problem was severe enough that the airline asked the Federal Aviation Administration to issue "ground-stops" to prevent flights from taking off to some of its hub airports, including San Francisco, Newark and Houston.  The FAA lifted the ground-stops at 5:30 p.m. ET so that planes could take off.  The outage began just after 3 p.m. and lasted about two hours, the airline said.  "United apologizes for the disruption caused to travelers at affected airports and is re-accommodating customers as quickly as possible," the airline said.  The airline also said it would permit customers to cancel or rebook flights without having to pay a re-booking fee.  Passenger lines backed up at some airports because the problem affected their ability to check in and board.  "We have a very congested lobby," Mike McCarron, spokesman for San Francisco International Airport, said at midafternoon. "There's a backlog of passengers they've got to get through. As soon as they get planes filled, United is going to start taking off."  He said the Transportation Security Administration opened all its checkpoints to get people through. "But it's still going to be a full afternoon."  Flight-tracking firm FlightAware said the outage involved the airline's computer system, which is used for reservations, ticketing and check-ins.  The airline's website also went down during the outage, making it difficult for travelers headed to the airport to determine the status of their flights.  The outage wasn't United's first big computer glitch this year. The airline had problems with its reservations system in March after it switched to Continental's computer system as the two airlines merged operations.  Then, passengers complained of waiting to talk to ticket agents as United struggled for several days to fix the problems.
United's computer system was down for about two hours Tuesday afternoon, causing flight delays, cancellations and backlogs at some airports.