Ever wondered what the difference is between the various "classes" of airspace that you keep communicating with air traffic control about actually mean? Airspace in the United States is divided into five classes:

Class A - Airspace from 18,000 feet MSL up to and including FL600, including the airspace overlying the waters within 12 nautical miles of the coast of the 48 contiguous States and Alaska. Unless otherwise authorized, all persons must operate their aircraft under IFR.

Class B - Airspace from the surface to 10,000 feet MSL surrounding the nation's busiest airports.

Class C - Generally that airspace from the surface to 4,000 feet above the airport elevation. Each person must establish two-way radio communications with the ATC facility providing air traffic services prior to entering the airspace and thereafter maintain those communications while within the airspace. VFR aircraft are only separated from IFR aircraft within the airspace.

Class D - Airspace from the surface to 2,500 feet above the airport elevation. Unless otherwise authorized, each person must establish two-way radio communications with the ATC facility providing air traffic services prior to entering the airspace and thereafter maintain those communications while in the airspace. No separation services are provided to VFR aircraft.

Class E - Airspace is not Class A, Class B, Class C, or Class D, and it is controlled airspace, it is Class E airspace. Unless designated at a lower altitude, Class E airspace begins at 14,500 MSL but does not include the airspace 18,000 MSL or above.

Class G - (uncontrolled airspace) That airspace not designated as Class A, B, C, D, or E.

In Flight Simulator, the US system is used throughout most of the world. In real aviation, different countries each have their own system. The classes for Canadian airspace are:

Class A - At and Above 18,000' asl. Airliners and other IFR traffic

Class B - Between 12,500' and 18,000' asl.

Class C - Clearance required to enter

Class D - Dialogue required -- Don't enter without talking to the controller first.

Class E - You don't need to talk to anyone.

Class F - Special use airspace

Class G - Uncontrolled airspace

Classes A through E are controlled. Class F can be controlled or uncontrolled. Class G is always uncontrolled.

The difference between C and D is that you need a clearance to enter Class C airspace but not to enter Class D. Two-way communication is all you need to enter class D.