A communications failure grounded U.S. air travel for more than four hours. Officials still haven't said what broke.

For about four hours and 20 minutes on Friday, a communications failure at the FAA's air traffic control centers kept nearly every departing flight in the United States on the ground. The FAA ordered the stop at about 1:54 PM MT (3:54 PM ET) and cancelled it at about 6:16 PM MT, according to its own advisories (USA Today and BBC give the start time; advisory 005 gives the end).

Officials say they have seen no sign of an attack. They have not said what failed.

How it unfolded

The FAA's alert said the stop was "due to extraordinary circumstances and for reasons of safety," as quoted by ABC News and NBC News. Transportation Secretary Sean Duffy wrote on X that the stop followed "a communication issue impacting all Air Route Traffic Control Centers," the high-altitude facilities that direct planes between airports, and that "FAA technicians are investigating the issue." ABC News counts 22 such centers.

The FAA's own statement, posted later, was more cautious. It referred to "multiple" centers and said the information was "preliminary and subject to change" (FAA statement). Homeland Security Secretary Markwayne Mullin said controllers could still talk to planes, but "at different levels and clarity," NBC News reported. GeekWire reported that Mullin said two areas of the country could still communicate normally.

The FAA Command Center's advisory 120, issued at 3:46 PM MT, gave the reason in two words: "communications failure." It said the stop "remains in effect" and that "as enroute facilities recover, flights will be allowed to depart." The companion operations plan, advisory 119, said traffic would be released "as conditions allow" in "a tiered approach."

What officials said

Mullin told reporters outside the White House there was "no indication as of yet" that anything "nefarious" was behind the outage (NBC News). "This isn't another 9/11. I just can't repeat that enough," he said (GeekWire). He also said officials were "not ruling out anything at this point."

He warned that a fix would not be quick. "It's going to take some time," Mullin said. "And so this isn't going to just be resolved and turned around within the hour" (CNN).

Mullin said he saw no reason to tie the stop to this week's Microsoft 365 outage. "I don't see any reason to connect those two at all. Those are separate issues" (GeekWire). Microsoft said it had "no indication that the FAA's nationwide ground stop is related to any Microsoft services" (Anadolu Agency).

Duffy and Mullin were scheduled to brief reporters at the White House on Friday evening, according to ABC News. Delay Wire had not confirmed what was said at the time of writing and will update this story.

What may have failed (unconfirmed)

Officials have not named a cause. Every account below comes from unnamed sources, and none has been confirmed by the FAA, DHS or the companies named.

The L3Harris network. The BBC, citing its U.S. partner CBS News and multiple sources, reported that the technical fault was caused by an update to the private communications network that connects the air traffic control system. CBS says the network, which it says L3Harris operates, rebooted after the update and crashed. The BBC said L3Harris had not immediately responded to its request for comment (BBC live page). The Wall Street Journal, citing people briefed on the matter, reported that a network overseen by L3Harris that carries controllers' radio frequencies "appears to be at the centre" of the outage. It was unclear whether radar feeds were also hit (via Times Now).

A software update. Bloomberg, citing one person familiar with the matter, reported that the FBI does not believe the failure was a cyberattack or a criminal act and that one line of inquiry is whether a software update caused it (via Bloomberg Línea, in Spanish).

How it started. The Air Current reported that the outages began at the Chicago center, followed in rapid succession by Boston, Washington, New York and Minneapolis, with centers losing communication, radio frequencies and radar. That is the publication's own reporting, and the FAA has not confirmed it (The Air Current).

The FAA's new AI tool. The FAA expanded its SMART routing and scheduling tool to all 22 centers this week. Asked about a link, Mullin said, "We're looking at that," and that DHS was examining it through CISA (GeekWire). Air Space Intelligence, the company behind SMART, said: "The current ground stop is not based on or a result of the SMART system, any product of ASI or our integration with FAA and ATC." No official has tied the stop to SMART.

If the reports are right, a failure in one communications network was enough to stop departures nationwide. Officials have not said whether that is what happened.

The scale

Trackers count the damage differently, and the totals were still moving when we checked. FlightAware showed 5,781 cancellations and 9,124 delays on flights within, into or out of the United States at 6:29 PM MT. Flightradar24 said at 5:30 PM MT that U.S. cancellations had passed 5,000, with Chicago O'Hare (459), Atlanta (307) and LaGuardia (305) hit hardest. ABC News said there were already nearly 800 cancellations for Saturday.

When the stop began, about 11,000 flights were still scheduled to depart, meaning more than 1.5 million passengers were stranded, according to the data firm Cirium, as reported by CNN's Pete Muntean (post on X). CBS News said more than 4,000 flights headed for U.S. airports were airborne when the stop was ordered (post on X). Those flights were allowed to land. NBC News reported that a source said Reagan National was at capacity and O'Hare was nearing it, and the BBC reported passengers saying planes that had landed could not get to gates.

The disruption spread overseas. Eurocontrol suspended U.S.-bound departures from its 42 member states at the FAA's request, covering flights leaving between 5 PM and midnight ET (USA Today; BBC).

Some flights moved during the stop

A limited number of flights were cleared while the stop was in force. The FAA's 6 PM MT operations plan said the stop "remains in effect for a large portion of the country, however, some of the coastal facilities have been able to release some international departures." The AP reported that flights within Alaska and Hawaii and some international departures were allowed to go. San Francisco said Pacific-bound flights were cleared at 3 PM local time, and Boston Logan said the FAA was permitting some transatlantic flights (NBC News; USA Today). Flightradar24 reported at 5:17 PM MT that the first aircraft in almost four hours had taken off from O'Hare (post on X).

American Airlines said at 5:53 PM MT that some flights from certain airports were beginning to get clearance, but "most departures remain paused" and it expected "unavoidable cancellations" (post on X). United said it planned to cancel the majority of its remaining flights Friday night, with exceptions for some international flights. JetBlue said it was cancelling all remaining Friday flights except Europe-bound departures from New York and Boston, and expects "additional cancellations" Saturday. Air Canada cancelled all remaining Friday flights between Canada and the U.S. (all via ABC News). Delta said customers can change flights at no extra cost (Delta statement).

A separate problem hit the Pacific. Hawaii's Department of Transportation said at 4:55 PM MT that "all west coast flights have been halted due to debris from SpaceX launch." The department said interisland flights were still running (Hawaii News Now). The department attributed that halt to the debris, not to the FAA's ground stop.

How this compares

The Washington Post and the BBC both describe this as only the second nationwide ground stop since the Sept. 11, 2001 attacks. The BBC and Bloomberg describe it as the longest since then. The first was on Jan. 11, 2023, when the FAA's NOTAM system, which sends notices to pilots, failed. The FAA ordered domestic departures paused until 9 AM ET and said at 8:50 AM ET that the ground stop had been lifted. Its preliminary review found contract personnel had unintentionally deleted files and it found no evidence of a cyberattack (FAA statements).

BBC Verify spoke to aviation professor Guy Gratton of Cranfield University, who said: "This looks like the worst since 9/11, but clearly not on that scale." He said the 2023 stop lasted 1 hour and 52 minutes, delayed about 9,500 flights and cancelled about 1,300 (BBC live page).

What travelers are owed

Under DOT rules, if an airline cancels your flight and you decline the rebooking or any voucher or credit offered, you are due a full refund of the airfare, including taxes and ancillary fees, even on a nonrefundable ticket (14 CFR 260.6). For a tarmac delay at a U.S. airport, covered airlines must provide adequate food and potable water within two hours, unless the pilot decides safety or security precludes it, and must offer a chance to deplane before three hours on a domestic flight or four on an international one, with exceptions for safety, security and air traffic control (14 CFR 259.4).

No other compensation is required for cancellations caused by a ground stop, USA Today reported, because they are outside the airline's control. Check your airline before you spend. This is general information, not legal advice.

What we don't know

The FAA has not said what failed or why. It has not said when airline schedules will be back to normal. Delay Wire will update this story as the FAA, airlines and officials share more. If you have a correction, use the comments on the live tracker.