Airspace in the US is classified into the following categories:
A, B, C, D, E, and G. The airspace is ordered from Class A-Tightest
control to Class G-No control.
Class A airspace is anywhere from 18000 feet to flight lever 600
(approx. 60000 feet); all aircraft in class A airspace must fly IFR
(instrument flight rules) and stay in contact with ATC. The
required separation is also greater in class A airspace.
Class B airspace surrounds larger airports, and consists of 3 or
more "shelves". Class B airspace looks somewhat like an upside down
wedding cake. In Class B airspace, all aircraft require a specific
clearance to enter or fly through the airspace.
Class C airspace Surrounds smaller airports, and looks like an
upside down cake with only two layers. Aircraft entering class C
airspace need only establish two way radio contact with ATC before
entering.
Class D airspace has the same restrictions as class C, but has
only one layer.
Class E airspace encompass everything from 1200 feet to 18000
feet that is not Class B, C, D, or, G. Sometimes the base elevation
is lower or higher that 1200 feet, but 1200 is the normal base.
Class E also encompasses everything above Flight Level 600. VFR
(visual flight rules) aircraft are not required to contact ATC in
class E airspace. IFR aircraft maintain ACT contact in class E.
Class G airspace is uncontrolled, and usually extends to
anything below 1200 feet. Even IFR aircraft do not contact ATC in
class G airspace.
