After American Airlines Flight 11 and United Airlines Flight 175 crashed into the twin towers of the World Trade Center and American Airlines into the Pentagon, Sliney gave the order to land every plane in the air over the US at the time (implementing the SCATANA plan), effectively shutting down US airspace.
Following the crashes of American Airlines Flight 11 and United Airlines Flight 175 into the World Trade Center towers and American Airlines Flight 11 into the Pentagon, Sliney ordered all planes in US airspace to land (implementing the SCATANA plan), effectively shutting down US airspace.