What is NOTAM, the FAA computer system that halted all US flights?

A critical Federal Aviation Administration computer system that experienced an outage Wednesday and briefly halted all US flights provides airlines with a digital bulletin board of crucial safety updates.

The system is known as the Notice to Air Missions or NOTAM. It sends alerts to pilots to let them know of conditions that could affect the safety of their flights. It is separate from the air traffic control system that keeps planes a safe distance from each other, but it's another critical tool for air safety.

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NOTAM messages could include information about lights being out on a certain runway, or a tower near an airport not having the required safety lights working, or an air show taking place in the air space nearby.

"It's like telling a trucker that a road is closed up ahead. It's critical information," said Mike Boyd, aviation consultant at Boyd Group International.

Although many flights take place without needing to see one of those notices, it's important that NOTAM messages reach the pilots, who are trained to check for them.

The FAA also operates the nation's air traffic control system, with air traffic controllers using radar to track all planes in their air space and radio communications with their cockpits to guide them safely. The computer systems that are the backbone of ATC system have also been known to go down. But when that happens, it typically only affects one region of country, not the entire nation's air space.

NOTAM is a national system, so its failure Wednesday meant that flights across the country were ordered not to take off for a couple of hours before they were cleared to fly again shortly before 9 am ET.

If the no new problems crop up, flights should return to normal soon, though it may take time to get all the delayed flights in the air. As of 9:30 am, tracking service FlightAware shows 4,600 delayed flights to, from and within the United States, with 827 flights canceled altogether. Aviation analytics firm Circium said there had been 23,000 domestic and international flights to or from the United States scheduled for Wednesday.

"By 6 pm this evening we won't even know it happened, I think," said Boyd.

But if the problem stretches too long, then flight crews who are standing by to fly delayed flights will run out of time in their service day. In that case, a delayed flight could turn into a canceled one if another fresh crew can't be found.

NOTAMS has been around for decades. But until December 2021, it was known by the name "Notice to Airmen," although the acronym was the same as today.

It was changed to remove the gendered term and because the notices were also then being sent to drone operators and not just pilots on board aircraft.

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