Passengers on United Airlines reported long delays after the latest in a long line of technology problems at the nation's biggest carriers.
United said Friday that it had fixed the overnight issue, yet hours later the airline still could not explain what went wrong or even how many flights were canceled or delayed.
Tracking service FlightStats Inc. said in late afternoon that United had cancelled 10 flights — not an unusual number — and delayed more than 380. However, those numbers don't include United Express flights, which are outsourced to smaller carriers.
Timing helped United. The outage spanned a late-night period when few U.S. flights were operating, and United reported it was fixed before the wave of morning departures.
United responded on Twitter to passengers who experienced delays or problems with the airline's website. The airline tweeted that it was having problems with its weight-reporting system.
Some passengers travelling late Thursday and early Friday took to social media to complain about delays as long as six hours.
United spokeswoman Maddie King said that overnight the airline "experienced a system issue causing some flights delays." She said the problem was fixed by 4 a.m. Eastern time, although there could be lingering brief delays on Friday.
Asked about the cause of the outage, King said, "We are still working to determine that."
In August, Delta Air Lines suffered a computer breakdown after a power outage in its operations center. The airline canceled more than 2,000 flights over three days. In July, Southwest Airlines also canceled more than 2,000 flights after an outage that it blamed on a failed network router. United and American Airlines both suffered computer-system problems in summer 2015.
Airline technology systems have hundreds of programs that are often of different ages and sources and are layered on top of each other. After recent outages at other carriers, outside experts have questioned whether airlines have enough redundancy in their systems and test the systems frequently enough.