How long was air space closed in America on September 11 2001?

On September 11, 2001, U.S. airspace was closed for nearly three days. The Federal Aviation Administration (FAA) ordered a complete shutdown of commercial air traffic shortly after the terrorist attacks began, and the airspace remained closed until September 13, 2001. This unprecedented closure marked the first time in U.S. history that the entire national airspace was shut down.