United Airlines Holdings Inc. started to resume flights after fixing a tech issue that temporarily grounded planes and delayed hundreds of flights across the U.S. Wednesday evening.

The Chicago-based carrier, the biggest airline in the world by capacity, said it was restarting operations to get customers to their destinations, albeit delayed, after a software problem impacted departing flights.

“The underlying technology issue has been resolved, and, while we expect residual delays, our team is working to restore our normal operations,” the airline said in a statement. The outage wasn’t related to recent instances of cybersecurity issues in the airline industry, a United spokesman said.

Airline tracking site FlightAware showed almost 1,000 United flights were delayed as of 10 p.m. New York time, accounting for 32% of the carrier’s scheduled services.

Flightradar24 earlier showed planes backing up on taxiways, either waiting to depart or getting an available gate to disembark arriving passengers in Chicago. Similar scenes playing out at the airline’s hubs in Houston and Newark.

In Washington, a United pilot told passengers on a Chicago-bound plane stuck on the tarmac that the issue was affecting all flights, before heading back to the gate. Other social media posts said the airline’s pilots were having difficulties with the system calculating the weight and balance of planes.

United Flight 1566 to Mexico City from Washington turned back to the gate at 9 p.m. local time after a nearly three-hour wait on the tarmac. The pilot addressed passengers to offer an apology on behalf of the carrier.

Flight attendants handed out snacks and water to take the edge off the delay. After returning to the gate, passengers were allowed to exit the plane to stretch their legs in Dulles International Airport’s terminal as they waited for an update on whether the flight would be cleared for departure.

Ground stops were earlier in place at airports including Denver, Houston, New Jersey’s Newark, San Francisco and Chicago, according to the U.S. Federal Aviation Administration on Wednesday evening — locations that are major hubs for United. There was no risk to planes already in the sky.

The issue came just two weeks after Alaska Air Group Inc. temporarily grounded its entire fleet due to a data center equipment failure that affected several of the carrier’s key operating systems.

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(With assistance from Catherine Larkin and Michael Shepard.)

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