A major computer glitch saw United Airlines grounded for two hours earlier as the carrier struggles to integrate systems with merger partner Continental Airlines.
Reservations systems at the giant carrier collapsed, leaving queues of angry passengers at airports.
The problem was the latest in a series of glitches reported since United merged systems with Continental in Match this year.
United said at least 200 flights were delayed.
Its passenger reservation system and website stopped working for about two-and-a-half hours on Tuesday afternoon.
Planes in flight were unaffected.
At the request of the airline, the Federal Aviation Administration issued a “ground-stops” to prevent flights from taking off.
This prevented departures from hub airports including San Francisco, Newark and Houston.
This was lifted earlier, with flights now returning to normal.