The Federal Aviation Administration (FAA) allowed some flights to resume after an outage of the system that alerts pilots to any obstructions before take-off had earlier forced the civil aviation regulator to ground all aircraft in the United States.
Nearly 7,300 flights were delayed and more than 1,100 canceled because of the outage as of early Wednesday morning. US flights were slowly beginning to resume departures as a ground stop was lifted.
Here is a brief summary of what the pilot warning system does, what we know about what went wrong and background about the safety notices provided to pilots, known as NOTAM.
The FAA system that is meant to distribute notices to pilots on hazards failed at about 2 a.m. Eastern Time, officials said.
The FAA ordered airlines to put a halt on all domestic departures until 9 a.m. Eastern time while it tested whether crews had managed to restore the system and bring it back online.
The White House said that US President Joe Biden had been briefed on the outage by Secretary of Transportation Pete Buttigieg. "There is no evidence of a cyberattack at this point," White House Press Secretary Karine Jean-Pierre said in a tweet. The US Department of Transportation is conducting an investigation, she said.
What is NOTAM?
The system that failed on Wednesday is part of a nearly century-old practice originally known as Notices to Airmen originally modeled on a system for notices to mariners.
The system, which was changed to be called "Notices to Air Missions" in 2021, is meant to alert pilots to hazards, everything from snow, volcanic ash or birds near an airport.
It also provides information on closed runways and temporary air restrictions.
The NOTAMs sent by the US Federal Aviation Administration are part of a global safety system managed through the United Nations' aviation agency.
Pilots are required to review the notices, either printed on paper or on an iPad, before take-off.
The information provided can run up to 200 pages for long-haul international flights.
NOTAMs are written in a kind of encoded shorthand that had been originally designed to make communication more efficient.
When did NOTAMs begin?
While many travelers might be learning about the system due to Wednesday's mass delays, NOTAMs have been around for more than 75 years.
In 1947, the Convention on International Civil Aviation, a specialized agency of the United Nations that coordinated international air travel, agreed to begin issuing NOTAMs through telecommunications to assist with airplane safety. Originally known as the Notice to Airmen system, NOTAMs were modeled after Notice to Mariners. That system advised ship captains of hazards in navigating the high seas, according to the FAA.
When are NOTAMs issued?
A NOTAM can be issued by authorities for a variety of reasons, ranging from potential hazards such as parachute jumps, air shows and glider or micro-light flying to warning of flights from heads of state. Closed runways and taxiways, unserviceable radio navigational aids or military exercises in the area causing limited airspace could also be enough of a reason to issue a NOTAM.
It also doesn't take much for a NOTAM to flag a hazard that could interfere with the flight. The temporary erection of obstacles, such as cranes, by an airfield or airport, as well as lights that cannot be serviced on tall obstructions are also examples that could trigger a NOTAM.
How has the system changed?
The UN Civil Aviation Organization (ICAO) has been leading an effort to overhaul the system to make it easier for airlines and pilots to filter the most important warnings and present them in clearer language.
In July 2017, an Air Canada jet landed on the wrong runway at San Francisco's airport and came within seconds of colliding with four other planes.
The notice of the closure of one of the two runways at the airport had been flagged in the pre-flight NOTAM on page eight of a 27-page briefing - and missed by the pilots.
The incident, and the information overload that pilots complain the system encourages, prompted the effort to change the way the system operates.
(NOTAMs) are just a bunch of garbage that nobody pays any attention to" US National Transportation Safety Board Chairman Robert Sumwalt said at a 2018 hearing on the Air Canada incident, which helped spur a global campaign for change.
FAA officials have been involved in efforts to modernize the system in recent years.
Have there been any issues with NOTAMs?
While the system has been largely effective for years, critics of the FAA often point to a 2017 incident as to why NOTAMs are "garbage."
As Air Canada Flight 759 attempted to land in San Francisco in July 2017, the plane nearly crashed into four other airliners. The plane was able to avoid disaster after it attempted to land on a taxiway that was misidentified as a runway. The runway the pilots were looking for was closed - and that information was buried in the NOTAM they had received, according to investigators.
The National Transportation Safety Board found at the conclusion of its investigation in 2018 that NOTAMs were ignored by pilots since they were unintelligible. Robert Sumwalt, then the chairman of the NTSB, described NOTAMs as "a bunch of garbage that nobody pays any attention to," according to Reuters.
The fallout from the investigation led the International Civil Aviation Organization to reform the NOTAM system, allowing the notices to be better organized and deciphered through electronic flight planning apps.