Computer problems struck United Airlines again on Tuesday afternoon as the carrier's passenger reservation service went down, affecting flights across United's network.
"We have a network outage & are working to get systems back online. We'll update soon," the airline tweeted on Tuesday.
The airline, created from the merger of United and Continental Airlines in 2010, had several technical problems earlier this spring when it merged United's reservation system with Continental's program.
Some passengers have tweeted that checking in for flights is being done manually on Tuesday while others said they were stuck on the tarmac waiting for the system to come back up.