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. (Read that last sentence as ICAO and Europe; the United States is the exception, and the next section explains why.)
- 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. (Again, ICAO and Europe.)
- 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 defined by 14 CFR 71.33(a) as the airspace from 18,000 feet MSL to and including FL600: note both halves of that, because the datum is MSL, not AGL, and FL600 is inside Class A rather than the first level above it. 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.
The US deviation worth knowing cold: Class C and D need contact, not a clearance
The ICAO table above says a clearance is required in Class C and Class D. In the United States that is not the rule for a VFR flight. The FAA AIM, 3-2-1 and the FAA's AIP ENR 1.4 require only that two-way radio communication be established with the controlling facility before you enter: the ATC unit calling you back by your callsign is the entry condition, and no clearance is issued or needed. It is the single most-confused difference between the two systems, and it cuts both ways.
The practical test is a familiar one to US pilots. If you call "Bravo Tower, Cessna 12345, ten miles south, inbound", and the tower replies "Cessna 12345, standby", that is not two-way communication established, because they have not used your callsign in a way that acknowledges you: you stay out. If they reply "Cessna 12345, remain clear of the Class D and standby", they have acknowledged you, but they have also told you to stay out. Only a reply that acknowledges your callsign without an instruction to remain clear lets you in. A European pilot flying in the US who waits for the word "cleared" will hold outside indefinitely; an American pilot flying in Europe who enters Class D on a callsign readback alone has entered controlled airspace without a clearance. Same letter, genuinely different rule.
In Europe, EASA's Standardised European Rules of the Air (SERA) transpose the ICAO classification into EU law through SERA.6001, and here the ICAO table means what it says: a clearance is required for Class C and Class D, VFR included. 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.
- Assuming Class C and D always need a clearance. Under ICAO and in Europe they do. In the United States, VFR entry to Class C and Class D needs two-way radio communication established, not a clearance, and "standby" does not count as established.
- 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.
- US Class A includes FL600. 14 CFR 71.33(a) runs it from 18,000 feet MSL to and including FL600, so FL600 is inside it, not above it.
- 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.