United and American Airlines are warning pilots they could struggle to communicate due to massive IT outage

• Delta Air Lines, United Airlines, and American Airlines issued ground stops on Friday.
• United and American warned pilots that communication was intermittent.

Major airlines have warned pilots they may be unable to communicate with ground services.

American Airlines and United Airlines issued advisories to be relayed to the carriers' airborne flights early Friday morning.

They said communication was intermittent as they were experiencing issues with systems such as the Aircraft Communications Addressing and Reporting System, or ACARS, which transmits messages to planes often from air traffic control or airline operations.

Major airlines, banks, and supermarkets are experiencing widespread disruptions because of an IT outage after Microsoft reported problems with its online services, linked to an issue at the cybersecurity firm CrowdStrike.

American, United, and Delta Air Lines — the big three of US carriers — all issued ground stops on Friday, citing a communication issue. They affected all flights regardless of destination. By around 5:10 a.m. ET (9:10 a.m. UTC) Friday, American had canceled its ground stop.

Allegiant Air also issued a ground stop, as did Frontier Airlines, before later canceling it.

In a post on X, Delta said all its flights were paused as it worked through a "vendor technology issue."

"We apologize for the inconvenience as our teams work through this issue," it added. "Reports indicated that other airlines may also be impacted."

American also apologized and said it was aware of a "technology issue with a vendor that is impacting multiple carriers."

In a statement to Business Insider, United said, "A third party software outage is impacting computer systems worldwide, including at United.

"While we work to restore those systems, we are holding all aircraft at their departure airports. Flights already airborne are continuing to their destinations."