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Before World War II, airspace in the United States was not a major concern, since the number of aircraft flying was relatively small. With the number of aircraft flying over the United States today, proper airspace usage is critical for flight safety and efficient service to pilots and the flying public. To assist in this goal, the airspace is divided into five classifications.
CLASS A Airspace
Class A Airspace is the airspace from FL 180 or 18,000 feet to FL 600 or 60,000. All pilots flying in Class A airspace shall file an Instrument Flight Rules (IFR) flight plan and receive an appropriate air traffic control (ATC) clearance. When climbing through 18,000 feet, the pilot will change the altimeter setting from the local altimeter (30.01 for example) to 29.92. This ensures all aircraft flying in class A airspace have the same altimeter setting and will have proper altitude separation.
CLASS B Airspace
Class B Airspace is generally the airspace from the surface to 10,000 feet. This airspace is normally around the busiest airports in terms of aircraft traffic such as Chicago or Los Angeles. Class B airspace is individually designed to meet the needs of the particular airport and consists of a surface area and two more layers. Most Class B airspace resemble an upside down wedding cake. Pilots must contact air traffic control to receive an air traffic control clearance to enter Class B airspace. Once a pilot receives an air traffic control clearance, they receive separation services from other aircraft within the airspace.
CLASS C Airspace
Class C Airspace is the airspace from the surface to 4,000 feet above the airport elevation. Class C airspace will only be found at airports that have an operational control tower, are serviced by a radar approach control, and that have a certain number of IFR operations. Although Class C airspace is individually tailored to meet the needs of the airport, the airspace usually consists of a surface area with a 5 nautical mile (NM) radius, an outer circle with a 10 NM radius that extends from 1,200 feet to 4,000 feet above the airport elevation and an outer area. Pilots must establish and maintain two-way radio communications with the ATC facility providing air traffic control services prior to entering airspace. Pilots of visual flight rules (VFR) aircraft are separated from pilots of instrument flight rules (IFR) aircraft only. Anchorage International airport, located in Anchorage, Alaska, has a Class C airspace.
CLASS D Airspace
The fourth airspace is Class D Airspace which is generally that airspace from the surface to 2,500 feet above the airport elevation. Class D airspace only surrounds airports that have an operational control tower. Class D airspace is also tailored to meet the needs of the airport. Pilots are required to establish and maintain two-way radio communications with the ATC facility providing air traffic control services prior to entering the airspace. No separation services will be provided to pilots of VFR (Visual Flight Rules) aircraft. Pilots operating under VFR must still use "see-and-avoid" for aircraft separation. Airports without operating control towers are uncontrolled airfields. Here pilots are responsible for their own separation and takeoff and landings. Uncontrolled airports use a "UNICOM" frequency that pilots will transmit their intentions to other aircraft using the airport. EXAMPLE: "CESSNA 1870 VICTOR (the aircraft's callsign) DEPARTING UNION CITY (the uncontrolled airport) RUNWAY 17 (the pilot's intentions).
CLASS E Airspace
The fifth airspace to discuss is Class E Airspace which is generally that airspace that is not Class A, B, C, or D. Class E airspace extends upward from either the surface or a designated altitude to the overlying or adjacent controlled airspace. If an aircraft is flying on a Federal airway below 18,000 feet, it is in Class E airspace. Class E airspace is also the airspace used by aircraft transiting to and from the terminal or en route environment normally beginning at 14,500 feet to 18,000 feet. Class E airspace ensures IFR aircraft remain in controlled airspace when approaching aircraft without Class D airspace or when flying on "Victor airways" -- federal airways that are below 18,000 feet. NOTE: VFR aircraft can fly up to 17,500 feet IF they can maintain VFR weather clearance criteria (and the aircraft is equipped to fly at 17,500 feet).
CLASS G Airspace
Class G Airspace is uncontrolled airspace. IFR aircraft will not operate in Class G airspace*. VFR aircraft can operate in Class G airspace.
Chart of Anchorage, Alaska airspace
*Ronald Wheeler, a contributing editor to IFR Magazine sent the following
Aircraft on an IFR flight plan do operate in Class G airspace to airports where there is an instrument approach. For hire pilots, such as those operating under FAR Part 135, are normally restricted from doing so. However, company operations specifications produced by the FAA establish guidelines for the company to operate in into uncontrolled airspace for the purpose of shooting an approach or departing.
Pilots operating under Part 91 are also allowed to shoot an approach into and depart from such an airport under IFR conditions on an IFR flight plan.
The interesting point is that ATC can't insure separation in class G airspace, at least as far as I know, since it's uncontrolled, but yet there are published instrument approaches into airports with the facilities and lie within uncontrolled airspace. One point Mr. Wheeler makes is: you never know who is flying about down there in one mile visibility, clear of clouds and in the traffic pattern, at night, at the class G airport. (FAR Part 91.155(b)).
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Updated: June 28, 2007