United and American Airlines warned 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.
• Several industries have been disrupted by an IT outage linked to CrowdStrike.

Major airlines warned pilots they may be unable to communicate with ground services amid an IT outage that canceled thousands of flights.

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 typically 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.

According to data from Cirium, 2,691 flights had been canceled around the world by 8 a.m. ET.

In the US, about 4.2% of flights were canceled — significantly higher than usual for that point in the morning.

137 United flights were canceled, and 307 American ones, but Delta appeared to be most affected, with 508 canceled flights — equivalent to around 10%.

Just 62% of flights were departing on time, the Cirium data also showed.

American and Delta canceled their ground stops around 5:10 a.m. ET and 7:50 a.m. ET, respectively.

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."

Allegiant Air , Frontier Airlines, and Spirit Airlines also issued ground stops but these too were later canceled.

Around 8 a.m. Delta said on X that it had resumed some flight departures but additional delays and cancellations are expected.

"We have been able to safely re-establish our operation," American said in a post.

The IT outage has caused chaos at airports. Whiteboards were used to display flight information in Belfast and Singapore, while some airline staff handwrote boarding passes.