The increase in numbers of airplanes flying over the united states prompted
the creation of demarcations on airspace, to provide for better traffic
management and ensure the safety of all aircraft involved. The following
classifications are determined and regulated by the FAA and enforced via the
several Air Traffic Control Centers throughout the US.
Class A Airspace
Extends from 18000 feet to 60000 feet. All aircraft in this zone must
adhere to Instrument Flying Rules (IFR) and must file a plan accordingly to the
appropriate ATC and receive clearance for it. They must also reset their
instrument so that all the aircraft will have the same reading on their altimeters to
ensure proper separation. This airspace affect all 48 contiguous states
and 12 nautical miles.
Class B Airspace
Extends from the surface to 18000 feet. This is most often modified to
meet the needs of the airports it surrounds. In order to enter Class B
airspace, all aircraft must receive clearance from ATC and strictly stick with
the heading, altitude and speed assigned to them for safety, since it's usually
the busiest and most crowded.
Class C Airspace
Only found in airports that have an operating tower and can be serviced by
radar approach control and support IFR operations. It is individually
tailored to meet the needs of airports. Pilots must establish and
maintain two way radio communications at all times with ATC controllers until
they are handed off to the tower for landing.
Class D Airspace
Goes from the surface to 2500 feet above certain airports. It is necessary
to establish two way communications with ATC, although no separation
services are provided to aircraft flying under Visual Flying Rules (VFR).
It is also present in uncontrolled airports (without a tower). In this
case, a broadcast frequency will be used for pilots to inform other aircraft of
their intentions (such as, landing, departing, etc.) Responsability for
aircraft separation falls in the hands of those flying the planes.
Class E Airspace
Controlled airspace that does not falls into the previous categories is
called Class E. It is usually used when transitioning between other
controlled arieas under IFR. VFR planes can use this airspace so long as
they can mantain VFR conditions for separations, and olny up to 17500 feet.
Class G Airspace
This is uncontrolled airspace. IFR doesn't operate here. VFR
planes can operate in this airspace if weather conditions allow for it.
Sources: FAA web site. http://www.asy.faa.gov/safety_products/airspaceclass.htm
Airspace Classifications. http://www.allstar.fiu.edu/aero/airspace.htm