A major technical glitch forced United Airlines to temporarily ground all its mainline flights on Wednesday evening, causing widespread delays and passenger frustration across key U.S. airports.

According to the airline, the system issue was resolved within hours, but residual delays were expected through the night. “We are working with customers to get them to their destinations after a technology disruption,” a United Airlines spokesperson told CNN. “The underlying technology issue has been resolved.”

Flight tracking service FlightAware reported that approximately 31% of United flights were delayed and 1% were cancelled by 9:45 p.m. (local time).

Airports in Chicago, Denver, Newark, Houston, and San Francisco — all major United hubs — were heavily impacted. The Federal Aviation Administration (FAA) issued ground stops at these locations after United reported the outage around 7:30 p.m.

Passengers took to social media to share their experiences and vent frustration as operations came to a standstill during peak travel hours.

This incident follows a growing trend of technology failures disrupting U.S. aviation. Last month, Alaska Airlines experienced a similar IT failure, while air traffic control systems in the Newark area have suffered repeated outages this year.

Aviation safety concerns have also been heightened in recent months, with a tragic mid-air collision in January involving a passenger jet and a military helicopter near Washington’s Reagan National Airport.

United Airlines says it is now working to resume normal operations and assist affected passengers, though it warns that recovery from the disruption may take time.

The latest glitch has renewed concerns over the vulnerability of critical airline infrastructure and the need for robust contingency planning in the aviation sector.