The Federal Aviation Administration (FAA) lifted a nationwide ground stop at 8:15 p.m. ET yesterday after an equipment outage disrupted air traffic control systems.

Despite the resumption of operations, the agency expects up to 5,000 flight cancellations today as airlines work to recover. Over 4,200 flights have been cancelled by midday, with delays and cancellations concentrated at major hubs like Chicago’s O’Hare, Atlanta’s Hartsfield–Jackson, and Dallas Fort Worth airports.

The outage was not linked to a cyberattack or artificial intelligence, but officials are investigating whether it stemmed from faulty equipment or installation.

The FAA Administrator, Bryan Bedford, confirmed that the current air traffic control system, which is decades old, is not sustainable and will continue to experience issues until it is replaced. Airlines are expected to restore full schedules by tomorrow night.

Travelers with affected flights may face disruptions to personal events, including weddings and birthday parties, as well as potential delays for professional sporting events.