Questions about how airspace is divided into classes with differing levels of control over the aircraft therein, how these classes differ from each other, and how they are used.
The entirety of the world's airspace is divided into non-overlapping three-dimensional blocks, each of which is placed into exactly one of seven airspace classes. The International Civil Aviation Organization (ICAO) defines the seven classes as follows:
Controlled airspace (5 classes)
- Class A: Only IFR flights are allowed. All flights require ATC clearance and are kept safe distances from each other by ATC. See class-a-airspace.
- Class B: IFR, VFR, and SVFR flights are all allowed. All flights require ATC clearance and are kept safe distances from each other by ATC. See class-b-airspace.
- Class C: IFR, VFR, and SVFR flights are all allowed. All flights require ATC clearance. ATC is responsible for keeping IFR and SVFR flights safe distances from each other and from VFR flights, but not for keeping VFR flights safe distances from each other. Instead, ATC issues traffic advisories to VFR flights; it is the VFR pilots' responsibility to "see and avoid" other VFR traffic. See class-c-airspace.
- Class D: IFR, VFR, and SVFR flights are all allowed. All flights require ATC clearance. ATC is responsible for keeping IFR and SVFR flights safe distances from each other, but not for keeping VFR flights safe distances from anything. Instead, ATC issues traffic advisories to all flights; it is the VFR pilots' responsibility to "see and avoid" other traffic, including other VFR traffic, and it is every pilot's responsibility to "see and avoid" VFR traffic. See class-d-airspace.
- Class E: IFR, VFR, and SVFR flights are all allowed. IFR and SVFR flights require ATC clearance, but VFR flights do not. ATC is responsible for keeping IFR and SVFR flights safe distances from each other, but not for keeping VFR flights safe distances from anything. Instead, ATC issues traffic advisories to all flights, as far as is possible (although these may be incomplete, due to the possibility of unknown VFR traffic); it is the VFR pilots' responsibility to "see and avoid" other traffic, including other VFR traffic, and it is every pilot's responsibility to "see and avoid" VFR traffic, even if no specific traffic advisory has been given. See class-e-airspace.
Uncontrolled airspace (2 classes)
Class F (most countries do not use this airspace class): IFR and VFR flights are allowed. ATC clearance is not required. ATC is responsible for keeping IFR flights safe distances from each other, as far as is possible, and can give pilots advisories on known nearby traffic if and when requested, but it is the pilots' responsibility to "see and avoid" other traffic, even in the absence of any guidance from ATC. See class-f-airspace.
Class G: IFR and VFR flights are allowed. ATC can give pilots advisories on known nearby traffic if and when requested, but their ability to provide guidance may be very limited in that regard, and it is the pilots' responsibility to "see and avoid" other traffic. Completely uncontrolled; ATC can only give advice to flights in class G airspace, not orders. See class-g-airspace.