Why did airports close in the United States?

At 9:45 a.m. Eastern time, the FAA issued an unprecedented order: all civil air traffic had to land at the nearest airport and no aircraft could take off. Over about two and a half hours, more than 4,000 aircraft were guided to the ground by controllers, while international flights en route to the United States were diverted to Canada, where towns like Gander took in thousands of passengers. Airspace remained closed until September 13 for commercial aviation, and general aviation took weeks to regain all its clearances. It was the only time in the country's history that all civil aviation was halted.

The reopening came with new measures. The TSA was created, taking over security checks previously handled by the airlines; cockpit doors were reinforced; armed air marshal programs were expanded and passenger watchlists were tightened. Traffic figures took years to recover and several historic airlines ended up bankrupt or merged.

Outside that episode, what usually closes are specific airports for short periods. Hurricanes force the closure of airports in Florida, Texas or Louisiana for a day or two; snowstorms paralyze Chicago, New York or Boston; in January 2023 a failure of the FAA's NOTAM system halted all takeoffs in the country for a few hours. Federal government budget shutdowns don't close airports either, but they leave controllers and security agents without pay, with absences that in 2019 and 2025 forced traffic to be limited. Each of these closures is announced via NOTAM and ends as soon as the cause disappears.