The US Federal Aviation Administration ordered airlines to "pause all domestic departures" this morning as it tries to deal with an outage in a critical computer system.

The agency said in a tweet that its Notice to Air Mission Systems (NOTAM) system was offline, affecting operations throughout US airspace. 

The FAA is working to restore its Notice to Air Missions System. We are performing final validation checks and reloading the system now. Operations across the National Airspace System are affected. We will provide frequent updates as we make progress.

- The FAA ✈️ (@FAANews) January 11, 2023

Systems have begun coming back online, but the FAA said at 0719 US Eastern Time that it was ordering domestic departures be paused until it could "validate the integrity of flight and safety information." 

Twitter users noted that planes have taken off from their gates, with some taxing onto the runway before returning to gates as departure times were pushed put.

FlightAware, a website that tracks flight status around the world, showed 1,304 delays and 48 cancellations as of writing.

It's unclear what caused the system to malfunction; the FAA has only said it's working to restore it following an outage, and that NOTAMs issued before the system failure were still viewable.

The FAA's NOTAM system is for relaying essential information to personnel concerned with flight operations, but which aren't known about enough in advance to publicize by other means.

In other words, they're pretty important. 

NOTAMs can be issued to communicate "real-time and abnormal status" in US airspace, changes in conditions or procedures at various facilities and other critical news. For some long flights, NOTAMs can run into the hundreds of pages.

The FAA said in a tweet that NOTAMs are reviewed by pilots before they take off, and as such any flights that are currently in the air or bound for US airspace are cleared to land. 

All flights currently in the sky are safe to land. Pilots check the NOTAM system before they fly. A Notice to Air Missions alerts pilots about closed runways, equipment outages, and other potential hazards along a flight route or at a location that could affect the flight.

- The FAA ✈️ (@FAANews) January 11, 2023

There are 21,464 flights carrying nearly 2.9 million passengers planned for departure in US aerospace today, Reuters said.

US airline passengers haven't seen an outage of this magnitude in some time, but shorter-term memories are likely still sore about Southwest Airline's IT failures that stranded huge volumes of passengers and grounded thousands of flights over the Christmas holiday. 

Southwest recently promoted several key leaders to more senior positions despite the holiday outage, which the company blamed on outdated flight scheduling software. 

As of 0815 ET (1315 UTC), the FAA said it was making progress, and that it was resuming departures at Newark Liberty International and Atlanta airports.

"We expect departures to resume at other airports at 0900 ET," the FAA said. ®

Updated to add at 1407 UTC

The FAA says that "normal air traffic operations" are resuming gradually across the US "following an overnight outage to the Notice to Air Missions system that provides safety info to flight crews. The ground stop has been lifted." It added: "We continue to look into the cause of the initial problem."

US press secretary Karine Jean-Pierre just said that US President Joe Biden has been "briefed by the Secretary of Transportation" on the outage. She noted: "There is no evidence of a cyberattack at this point, but the President directed [the department of transport] to conduct a full investigation into the causes."

https://www.theregister.com//2023/01/11/faa_grounds_all_us_departures/