NOTAM failed Tuesday, canceling thousands of U.S. flights for all domestic airlines, in possibly the largest travel shutdown since the terrorist attacks of September 11, 2001.

NOTAM (Notice to Air Missions) is a government transportation system which reports important adverse impacts to pilots. Flying in U.S. airspace, all pilots are supposed to consult NOTAM before takeoff. The system, which did move from telephone to online in the 2000s, is still considered antiquated.

Late January 10, 2023, the NOTAM system went down, no longer distributing new or amended travel notices to pilots, which in turn prevented them from flying. The FAA issued a ground stop during the night, grounding all civilian, non-emergency passenger flights for a few hours while they unsuccessfully rebooted the system. Overnight, the FAA set up a telephone hotline to keep information flowing, but with Wednesday morning, that system was rapidly overwhelmed by the sheer volume of daytime domestic air traffic across the country.

By 9 a.m. January 11, normal flights were starting to resume. But not before over 1,300 flights had been canceled and over 9,000 had been delayed. Major hub airports, like those in Chicago, Los Angeles, New York, and Atlanta, saw 30 to 40 percent of their flights delayed.

“We are going to see the ripple effects from that, this morning’s delays through the system during the day,” said Transportation Secretary Pete Buttigieg in an interview on CNN. “Now we have to understand how this could have happened in the first place. Why the usual redundancies that would stop it from being that disruptive did not stop it from being disruptive this time.”

There has never been a disruption to air traffic caused by technology this large before, in the world. The cause of the breakdown is yet unknown, but experts don’t yet think a cyberattack to be a likely component.

“So much of their systems are old mainframe systems that are generally reliable but they are out of date,” said Tim Campbell, an airline consultant and former VP of air operations at American Airlines about NOTAM and the rest of the FAA’s information infrastructure.

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