system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What was the process like to land over 4,000 planes on 9/11 after that order was given?

That many planes needing to land almost immediately...was there a plan in place for that? (I assume there was/is). What about international flights inbound to the states?
gpt
US domestic flights got grounded so there were none taking off.  Everyone else landed asap.  It was not that significant an operation because the airports were closed to departures.

All US airports were immediately closed to all International flights.  The flights were turned back, or if low on fuel routed to Canada.  There were around 500 in the air at the time and about half could not turn around.  227 aircraft (33,000 passengers) landed in Canada in Operation Yellow Ribbon.