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Grounding of domestic flights across the US on Wednesday was caused by a relatively obscure system called Notice to Air Missions (Notam), a vital air safety tool.

It is separate from the air traffic control system and warns pilots about hazards along the route.

The system flags everything from flocks of gulls in a flight path to lawnmowers operating near runways.

Pilots must .

The system went down Tuesday night, prompting the US agency responsible for air safety, the Federal Aviation Administration (FAA), to use a telephone hotline as a backup.

But that was quickly overwhelmed as flight traffic picked up on Wednesday morning.

The FAA gave little specific detail about what caused the Notam outage, but the White House said there was no evidence of a cyberattack "at this point".

How does Notam work?

Notam was created in 1947 and initially was telephone-based.

Until December 2021 the system was known as "Notice to Airmen", but authorities changed the name to remove the gender reference and reflect the fact that Notam information is used by drone operators as well as pilots onboard aircraft.

The system can be accessed via a portal on the FAA website.

Notam messages are written in a jumble of abbreviations and warn about ongoing construction, non-operational safety lights or air traffic towers, and other nearby hazards.

For instance, "AIRSPACE AEROBATIC ACFT WI AN AREA DEFINED AS 1NM RADIUS OF FSO SFC-3500FT DLY 1600-1700" indicates that an air show is taking place within a mile of Franklin County State Airport in Vermont.