The term was also used in a USA Today article quoting an official immediately after 9/11 atrocities supposedly referring to the blanket ban on aviation although the term is not as all embracing as that.
When ATC services are unavailable flight requiring ATC cannot take place. The term is used on occasions, if it is ever used, as short hand to refer to a situation where all aircraft going to any destination inside the airspace must re-route, and any aircraft already in the airspace must either land or fly outside the affected airspace. Perhaps military and emergency service aircraft may be authorized to be airborne in the area.
Flight in uncontrolled airspace is unaffected when ATC services are unavailable.
- Levin, Alan; Adams, Marilyn; Morrison, Blake (2002-08-13). "Part I: Terror attacks brought drastic decision: Clear the skies". USA Today. Retrieved 2010-09-11.