The outage occurred early Friday and grounded Southwest flights around the country for nearly an hour.
A spokesman for the Federal Aviation Administration says Southwest had a problem with a system that transmits flight plans to the FAA.
By midmorning Central time, about 600 Southwest flights had been delayed, 15 percent of Southwest’s schedule and far more than any other U.S. carrier, according to FlightAware.
The airline says some of the delays are due to winter weather.
The outage and winter weather come as Southwest reports a higher than normal number of planes taken out of service for maintenance. That’s causing tension between the airline and the aircraft mechanics’ union.