Computer issues forced United Airlines to ground all morning flights nationwide for about two hours Wednesday, a problem similar to one the airline experienced last month.
A ground-stop notice went into effect about 8 a.m. at United's request and was lifted before 10:30 a.m., the Federal Aviation Administration said.
Though flights already airborne were not affected, United spokeswoman Jennifer Dohm said there were more than 800 delays.
Four United flights and 55 United Express flights were canceled.
Dohm said a router problem reduced "network connectivity" for several software applications.
The Chicago-based airline said the problem with the router spilled over into the passenger-reservations system.
Airlines typically use several linked computers instead of one giant system, said James Record, an aviation professor at Oakdale-based Dowling College.
"It's a bunch of small computer systems that have to talk to each other in order to exchange information. Once that exchange stops, the updating stops," he said.
Flights must be halted when ticketing systems malfunction because it blocks airlines from determining whether passengers are on the no-fly list, Record said.
Hundreds of United passengers Wednesday turned to social media for flight updates, to post pictures of long lines or tweet their anger.
Sabrina Almlah of East Meadow had been scheduled to leave Kennedy Airport for Los Angeles at 8:25 a.m.
"Everything was fine until we got on the plane. We sat down, did everything, then they sent out a service announcement saying that the system was down," she said.
Almlah's flight wound up leaving at 9:42 a.m.
United halted all takeoffs in the United States on June 2 due to what the airline described as computer automation issues.