Airspace Classification

Airspace Classes at a Glance

VMC minima vary by altitude, day/night, and state. The above are ICAO Doc 4444 general minima. Always check national AIP for specific requirements in your operating area.

Controlled Airspace (Classes A–E)

Class A

The highest level of control. IFR only — VFR flight is not permitted in Class A airspace. All flights are separated from all other flights. ATC clearance is required before entry and must be obtained before reaching the boundary.

• Typically used for upper airways and oceanic tracks.
• In the USA: FL180 to FL600 (the “high-altitude structure”) is Class A nationwide.
• In the UK: airways and controlled areas above FL195 are Class A.
• Transponder with Mode C (altitude reporting) is required.

Class B

Both IFR and VFR are permitted. All flights receive separation from all other flights — the same level as Class A but accessible to VFR. Specific ATC clearance is required before entry. A transponder with Mode C is required.

• Used primarily in the USA around major hub airports, typically from the surface to FL100.
• Not used in the UK — UK uses Class C and D around busy airports instead.
• Entry clearance must be explicitly received — “cleared into the Bravo” — prior to crossing the boundary.

Class C

Both IFR and VFR are permitted. IFR flights are separated from all other IFR and VFR traffic. VFR flights are separated from IFR traffic and receive traffic information on other VFR flights. Mandatory two-way radio contact is required.

• Typical use: around busy regional airports and terminal areas.
• ATC clearance required for entry by IFR aircraft. VFR must establish radio contact and receive acknowledgement before entering.
• Used extensively in the UK for terminal control areas (TCAs) and control zones (CTRs) around major airports.

Class D

IFR separated from IFR only. No separation is provided between VFR and VFR, or between VFR and IFR aircraft — traffic information is provided instead. This is the most common class around regional airports with a control tower.

• Radio contact is mandatory. ATC clearance required for IFR. VFR requires two-way radio and ATC permission to enter.
• Speed restriction: 250 kt IAS below FL100 (or where national rules impose tighter limits).
• Common in the UK and Europe around busy general aviation and regional airports.

Class E

Controlled for IFR; uncontrolled for VFR. IFR flights are separated from other IFR flights. VFR flights receive no separation service — pilots are responsible for see-and-avoid. VFR does not generally require radio contact except to transit certain designated Class E areas.

• Used for transitional airspace and en-route low-altitude airways.
• Common in Germany and other European states as the default low-altitude controlled airspace structure.
• IFR must be cleared into Class E; VFR may enter without clearance but VFR VMC minima apply.

Uncontrolled Airspace (Classes F and G)

Class F (Advisory)

An ICAO provision not used by all member states. An advisory service is available for IFR traffic, but participation is voluntary — pilots may request and receive advisory separation, but it is not guaranteed or mandatory.

• No mandatory radio requirement, though use of the advisory frequency is strongly recommended.
• Where used: en-route advisory airspace in some European states, including historic use in parts of the UK.
• VFR pilots fly under see-and-avoid with no ATC service.

Class G (Uncontrolled)

The default airspace where no ATC authority exists. A Flight Information Service (FIS) may be available on a best-efforts basis to provide weather, NOTAMs, and traffic information, but no separation is provided to any flight.

• No clearance required. No mandatory radio requirement (though recommended for VFR; required by IFR in many states).
• All flights operate under see-and-avoid or self-separation principles.
• IFR may still operate in Class G but receives no separation — pilots must ensure terrain and traffic clearance themselves.
• In the UK, Class G exists from the surface upward wherever Class A/C/D/E airspace is not designated.
• In the USA, Class G exists from the surface to the base of Class E (typically 700 or 1200 ft AGL).

Transition Altitude and Transition Level

RVSM — Reduced Vertical Separation Minima

In RVSM airspace (FL290–FL410), vertical separation between aircraft is reduced from the traditional 2,000 ft to 1,000 ft. This doubles the number of available flight levels in the busiest upper airspace and increases capacity without compromising safety.

Aircraft operating in RVSM airspace must be RVSM-certified with approved altimetry equipment and specific ATC authorisation. Non-RVSM state aircraft (e.g., certain military types) must coordinate with ATC before entering RVSM airspace. RVSM is in effect in most ICAO member state upper airspace, including all of European airspace and North Atlantic Tracks (NAT HLA).

Special Use Airspace