It is still unclear what happened to the system and when it will be restored.

WASHINGTON — A critical system used by U.S. airlines failed Tuesday night, grounding all flights.

The Federal Aviation Administration, which is responsible for air travel regulations, confirmed through an advisory that its “NOTAM” system went offline shortly after 11:00 p.m. ET.

The NOTAM system is designed to provide pilots and air traffic controllers with important information as it develops, including changes in weather or conditions at a particular airport.

The NOTAM information for some flights can run to hundreds of pages, detailing runway closures, bird hazards, or low-altitude obstacles on the flight path.

It is still unclear what happened to the system and when it will be restored.

“Technicians are currently working to restore the system and there are no estimates available at this time to restore service,” the FAA’s advisory to pilots said.

The FAA said in a statement that they are working to restore the system and that the operation of the National Airspace System has been affected.

On the FAA website for the NOTAM system, a message warns users that the latest entries may not be available.

“Due to system processing delays, recently entered NOTAMs may not be displayed,” it said.

NOTAMs issued prior to the blackout were still visible Wednesday morning.

More than 500 flights were delayed Wednesday morning to or from the U.S., according to flight-tracking website FlightAware. The website did not cite the NOTAM failure as a factor in the unusually large number of delays.

The industry-wide problem comes less than a month after thousands of flights were canceled or delayed after Christmas when Southwest Airlines faced long delays due to logistics problems during the winter freeze.

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