More than 700 U.S. flights were canceled and about 4,400 were running late on Tuesday as a winter storm swept across the Northeast.
Adding to travelers' woes: United Airlines experienced problems with its computer system used to check in passengers, print boarding passes and get baggage tags.
By early evening, about 650 United flights and another 1,000 flights on regional affiliate United Express had arrived late or were behind schedule, according to tracking service FlightAware.com. United said that most of the delays were due to the computer glitches.
Travelers took to Twitter to complain to United. Spokeswoman Jennifer Dohm told The Associated Press that the airline fixed the problem and was working to get passengers to their destinations. Glitches also led to widespread delays at United in early 2012, when the computer systems of United and Continental Airlines were combined after their merger.
United wasn't the only airline struggling to stay on schedule Tuesday. Southwest had more than 1,100 delays, according to FlightAware.
Southwest spokeswoman Michelle Agnew said the delays were caused by the winter storm in the Northeast and high winds that temporarily grounded flights in Denver, a key outpost for the airline. Southwest's on-time rating has fallen sharply -- dropping to last in government rankings -- since August, when it began packing more flights into peak hours of the day.
Snow and ice snarled air traffic in the Northeast. By early evening, more than 100 departures had been canceled at Newark Liberty Airport in New Jersey and dozens more at New York's LaGuardia Airport and in Philadelphia and Boston.
Several big storms have slammed travelers this winter. U.S. airlines have canceled more than 78,000 domestic flights since Dec. 1, the highest number since the government started keeping records in the winter of 1987-1988.