CONTROLLED AIRSPACE

'Controlled airspace' is an aviation term used to describe airspace in which traffic levels are such that it has been determined that air traffic control (ATC) must provide some form of separation between aircraft. It is the opposite of uncontrolled airspace.
Controlled airspace usually exists in the immediate vicinity of busier airports, where aircraft used in commercial air transport flights are climbing out from or making an approach to the airport, or at higher levels where air transport flights would tend to cruise. Some countries (significantly the U. S.) also provide controlled airspace almost generally, however in most countries it is common to provide uncontrolled airspace in areas where significant air transport or military activity is not expected.
ICAO classifies airspace in seven classes from classes A to G. Controlled airspace is classes A to E, in order of decreasing ATC regulation of flights. Flight under Instrument Flight Rules (IFR) is allowed in all airspace, controlled or uncontrolled; flight under Visual Flight Rules (VFR) is permitted in all airspace except A. Pilots of aircraft need to obtain an Air Traffic Control clearance before they enter airspace of classes A, B, C or D. If flying under IFR they also need to obtain a clearance to fly in class E airspace.

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★ Special use airspace