The letter on the chart tells you, before anything else, who is in charge of the air you are about to enter and what you can expect from them.
This is general educational information, not operational, legal, or regulatory advice. Rules differ by authority and change over time. Always verify against current official sources and follow your operator's approved procedures.
The ICAO framework
Airspace is classified so that pilots and controllers share a common language about who may fly where and what service comes with it. ICAO Annex 11 (Air Traffic Services) defines seven classes, A to G, in its Appendix 4. Each class answers four questions: which flights are permitted (instrument flight rules, visual flight rules, or both), which flights are separated from which, whether an air traffic control clearance is required, and what service is provided.
The first dividing line is controlled versus uncontrolled. Classes A to E are controlled airspace, where at least instrument flights receive a control service and a clearance. Classes F and G are uncontrolled, where the service is advisory or informational and no clearance is given.
What each class provides
Summarising ICAO Annex 11, Appendix 4, reproduced for Europe in the UK CAA's text of SERA.6001:
- Class A. Instrument flight only. All flights are separated from each other. A clearance is required. This is the most controlled airspace.
- Class B. Instrument and visual flight. All flights are separated from each other. A clearance is required.
- Class C. Instrument and visual flight. Instrument flights are separated from each other and from visual flights; visual flights are separated from instrument flights and receive traffic information about other visual flights. A clearance is required.
- Class D. Instrument and visual flight. Instrument flights are separated from each other and given traffic information about visual flights; visual flights receive traffic information. A clearance is required.
- Class E. Instrument and visual flight. Instrument flights are separated from each other and need a clearance; visual flights need no clearance and are not separated, receiving traffic information as far as practical. Class E is controlled airspace, but only for the instrument traffic.
- Class F. Instrument and visual flight. Instrument flights receive an air traffic advisory service if they request it, and a flight information service is available. No clearance is given. This is uncontrolled.
- Class G. Instrument and visual flight, both receiving a flight information service on request. No clearance, no separation. This is the open, uncontrolled airspace.
The detailed visual-meteorological-conditions minima that go with flying visually in each class are a topic of their own, covered in our guide to VFR weather minima.
The same letters, different implementations
The classes are an ICAO baseline; how a State applies them is a national decision, and this is where assumptions go wrong.
In the United States, the FAA AIM and the FAA's AIP ENR 1.4 describe a system using Classes A, B, C, D, E, and G, and the FAA explicitly states that Class F is not used in the United States. There, Class A is the airspace from 18,000 ft up to FL600, Class B surrounds the busiest airports, Class C and D surround other towered airports by size, Class E is the general controlled airspace, and Class G is what is left below it.
In Europe, EASA's Standardised European Rules of the Air (SERA) transpose the ICAO classification into EU law through SERA.6001. National authorities then choose which classes to apply: the United Kingdom, for instance, uses Classes A, C, D, E, and G, and does not use Class B or, in practice, Class F. The service definitions are the ICAO ones; the map of where each class sits is national.
Common pitfalls
- Controlled does not mean separated from everyone. In Class D and E, visual flights are not separated from you; you still look out.
- Class E catches visual pilots out, because you can be in controlled airspace, mixing with instrument traffic, without needing a clearance or a radio call.
- The class letter is not the geography. Class A over the United States and Class A over the United Kingdom mean the same service but cover very different airspace.
- Equipment and entry requirements vary by class and by State (transponder, radio, clearance), so check the national rules for the airspace you are actually entering.
In Pilot EFB
Pilot EFB is a study and planning aid that keeps your weather, NOTAMs, flight time, and logbook in one offline-first place; it is not an airspace or charting authority and is not a certified Electronic Flight Bag. Use the official AIP, current charts, and your national rules to confirm the class, dimensions, and entry requirements of the airspace on your route, and treat this article as background to read them with. Saved data stays readable offline; pulling fresh data needs a connection.