Controlled airspace is an aviation term used to describe airspace in which ATC has the authority to control air traffic, the level of which varies with the different classes of airspace. Controlled airspace is established mainly for three different reasons:
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 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 controlled airspace (some countries also permit IFR in uncontrolled airspace); flight under visual flight rules (VFR) is permitted in all airspace except A. Most countries use only a subset of the seven ICAO classes to classify their airspace.