When I'm reading about aviation, it's very common to see references to Class B or Class C airspace and airports.
What are the different classes and what are the differences between them?
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Although Lnafziger's answer is correct, I'd like to elaborate on the purpose of the airspace classes.
* Technically, airspace above 60,000 feet is Class E, but nobody goes up there except scientific and spy/military flights.
The short version, for non-aviation people is as follows:
The answers given are correct for the USA. If you're in one of the other 199 (or so) countries, it's more likely that something closer to the original ICAO (International Civil Aviation Organisation) definition of airspace applies.
They specify (a) what sort of traffic is allowed, and (b) who is responsible for avoiding other traffic, namely ATC (when separation is provided) or the pilot. Below, IFR refers to Instrument Flight Rules, while VFR refers to Visual Flight Rules.
Individual countries can (unfortunately...) deviate quite substantially from these airspace classes, and/or impose further structure on top of them.
Frequently, certain air space classes will have specific speed limits, equipment requirements, or minimum visibility requirements for VFR traffic.
This question is quite broad, as airspace classes are used in varying ways by different jurisdictions. The only near-constant is what the airspace classes mean in terms of the services Air Traffic Control must provide and the conditions under which planes may use the space.
First, you must understand four basic modes of aircraft operation:
As defined by ICAO, the classes are:
In the United States, the classes are used as follows: