In 1992, The United Kingdom and the United States intervened in Kurdish-Iraqi dispute in northern Iraq by establishing a no-fly zone in which Iraqi aircraft were prevented from flying. The intent of the no-fly zone was to prevent possible bombing and chemical attacks against the Kurdish people by the Iraqi regime. While the enforcing powers had cited United Nations Security Council Resolution 688 as authorising the operations, the resolution contains no explicit authorization. The Secretary-General of the UN at the time the resolution was passed, Boutros Boutros-Ghali called the no-fly zones "illegal" in a later interview with John Pilger.
In the United States, the phrase "no-fly zone" has no legal meaning. What most people would consider a "no-fly zone" is termed by the Federal Aviation Administration a "Prohibited Area." Prohibited Areas are permanent until canceled and are published in the Federal Register and at http://sua.faa.gov, and are depicted by blue hashed boundaries on aeronautical charts.
Active Prohibited Areas:
The FAA may also issue Temporary Flight Restrictions, or TFRs, which are similar to Prohibited Areas but which are typically effective for a few days or weeks, versus the essentially permanent nature of a Prohibited Area. For example, a TFR is typically issued to prohibit flights near the President's destination when he travels outside Washington. TFRs are also issued to ensure a safe environment for firefighting operations in the case of wildfires and for other reasons as necessary. A TFR was quickly issued around the crash site of Cory Lidle's airplane in New York City. Later, a broader TFR was issued to require pilots traveling over the East River to obtain air traffic control clearance.
TFRs are issued by Notice to Airmen (NOTAM), and are available at http://tfr.faa.gov or by calling a Flight Service Station.
There are active TFRs that have been effective since 2003 over Disneyland and Walt Disney World, and an indirect TFR that prohibits flight below 3000' above ground level, and within a three nautical mile radius of a stadium having a seating capacity of 30,000 or more "in which a major league baseball, national football league, NCAA division one football, or major motor speedway event is occurring" from one hour before to one hour after the event, with exceptions for essentially anyone connected with the event sponsor.
In addition to areas completely off limits to civil aviation, a variety of other airspace restrictions exist in the United States. Some notable ones include the Flight Restriction Zone (FRZ) encompassing all airspace up to within approximately of Ronald Reagan National Airport around Washington, D.C. Flights within this airspace, while not entirely prohibited, are highly restricted. All pilots flying within the FRZ are required to undergo a background check and fingerprinting. An additional area encompassing most of the Baltimore-Washington D.C. metropolitan area requires the filing of a flight plan and communication with air traffic control.