“Due to a technology issue, we are holding United mainline flights at their departure airports,” the airline said.

United suffered an outage of its weight calculation system, a source familiar with the matter told Reuters.

“The issue affecting United Airlines flights has been resolved and mainline flights are departing once again,” flight tracking website FlightRadar24 said on social media at 9.19 pm.

The US Federal Aviation Administration said earlier it issued a ground stop for United flights at several US airports.

The ground stops were issued at major United hub airports including Newark, Denver, Houston, Chicago, according to the regulator’s website.

As of 9.25 pm ET, flight tracking data from FlightAware showed that 870, or 28%, of United flights had been delayed on Wednesday.

United’s outage came a few weeks after Alaska Airlines grounded all of its flights for about three hours due to an IT outage for the second time in just over a year.

In April 2024, Alaska grounded its entire fleet due to an issue with the system that calculates the weight and balance of its planes.