Airspace class (United States)

:"This article describes the implementation of International Civil Aviation Organization airspace classes in the United States of America. For a general discussion of airspace classes, see Airspace class."

The United States implementation of International Civil Aviation Organization (ICAO) airspace classes defines Classes A through E and Class G. Class F is not used in the United States. The other U.S. implementations are described below. The United States also defines categories of airspace that may overlap with classes of airspace. Classes of airspace are mutually exclusive. Thus, airspace can be Class E and Restricted at the same time, but it cannot be both Class E and Class B at the same location and at the same time.

Note: All airspace classes except Class G require air traffic control (ATC) clearance for Instrument flight rules (IFR) operations.

Airspace classes

Class A

Class A airspace extends from convert|18000|ft|m|-2 mean sea level (MSL) to FL600 (approximately convert|60000|ft|m|-3 MSL) throughout the United States.

Unless otherwise authorized by ATC, all flight operations in Class A airspace must be under ATC control, and must be operating IFR, under a clearance received prior to entry.

Since Class A airspace is normally restricted to instrument flight only, there are no minimum visibility requirements.

Class B

Class B airspace is defined around key airport traffic areas, usually airspace surrounding the busiest airports in the U.S.A. according to the number of flight operations and passengers passing through. The exact shape of the airspace varies from one Class B area to another, but in most cases it has the shape of an inverted wedding cake, with a series of circular “shelves” of airspace of several thousand feet in thickness centered on a specific airport. Each shelf is larger than the one beneath it. Class B airspace normally begins at the surface in the immediate area of the airport, and successive shelves of greater and greater radius begin at higher and higher altitudes at greater distances from the airport. Many Class B airspaces diverge from this model to accommodate traffic patterns or local topological or other features. The upper limit of Class B airspace is normally convert|10000|ft|m|-3 MSL.

All aircraft entering Class B airspace must obtain ATC clearance prior to entry and must be prepared for denial of clearance. Aircraft must be equipped with a two-way radio for communications with ATC and an operating Mode C transponder, furthermore aircraft overflying the upper limit of any Class B airspace must have an operating Mode C transponder. Visual Flight Rules (VFR) flights may proceed under their own navigation after obtaining clearance but must obey any explicit instructions given by ATC. Some Class B airspaces include special "transition routes" for VFR flight that require communication with ATC but may not require an explicit clearance. Other Class B airspaces include "VFR corridors" through which VFR flights may pass without clearance (and without technically entering the Class B airspace).

VFR flights operating in Class B airspace must have three miles (5 km) of visibility and must remain clear of clouds (no minimum distance).

Pilots operating in Class B airspace must have a private pilot's certificate, or a student certificate with a special endorsement. Some Class B airspaces prohibit student pilots.

Some Class B airspaces prohibit Special VFR flights. Certain Class B airports have a Mode C veil, which is airspace within thirty nautical miles of the airport in which all aircraft must have an operating Mode C transponder (up to convert|10000|ft|m|-3 MSL).

Class C

Class C airspace is defined around airports of moderate importance that have an operational control tower and is in effect only during the hours of tower operation at the primary airport. Class C space is structured in much the same way as Class B airspace. The vertical boundary is usually convert|4000|ft|m|-2 above the airport surface. The core surface area has a radius of five nautical miles (9 km), and goes from the surface to the ceiling of the Class C airspace. The upper “shelf” area has a radius of ten nautical miles, and extends from as low as convert|1200|ft|m|-1 up to the ceiling of the airspace.

All aircraft entering Class C airspace must establish radio communication with ATC prior to entry, generally about convert|20|mi|km out. The aircraft must be equipped with a two-way radio and an operating Mode C (altitude reporting) radar transponder, furthermore aircraft overflying above the upper limit of Class C airspace must have an operating Mode C transponder. VFR flights in Class C airspace must have three miles (5 km) of visibility, and fly an altitude at least convert|500|ft|m|-1 below, convert|1000|ft|m|-2 above, and convert|2000|ft|m|-2 laterally from clouds.

Pilots operating in Class C airspace must have at least a student pilot certificate. Aircraft speeds must be below 200 knots (230 mph) at or below convert|2500|ft|m|-1 above the ground, and within convert|4|nmi|km|0 of the Class C airport.

Class D

Class D airspace is circular in form and normally extends from the surface to convert|2500|ft|m|-1 above the ground and to a variable radius (generally 5 statute miles) around airports with an operational control tower and not otherwise in Class C or Class B airspace. Class D airspace reverts to Class E during hours when the tower is closed, or under other special conditions.

Two-way communication with ATC must be established before entering Class D airspace, generally at 5 to convert|10|mi|km out, but no transponder is required. VFR cloud clearance and visibility requirements are the same as Class C.

Class E

In most areas of the United States, Class E airspace extends from convert|1200|ft|m|-1 AGL up to but not including convert|18000|ft|m|-2 MSL, the lower limit of Class A airspace. There are areas where Class E airspace begins at either the surface or 700 AGL that are used to transition to/from the terminal or en route environment (around non-towered airports). These areas are designated by VFR sectional charts.

In some areas of the western United States, class E airspace begins at convert|14500|ft|m|-1 MSL. These areas are usually either very remote or very mountainous. (e.g. the airspace over many areas of Montana).

Most airspace in the United States is Class E. The airspace above FL600 is also Class E.

No ATC clearance or radio communication is required for VFR flight in Class E airspace. VFR visibility requirements are as they are for Class C and Class D airspaces, except that the visibility requirement is extended to five miles (8 km) above convert|10000|ft|m|-3 MSL, and the cloud clearance requirement above convert|10000|ft|m|-3 MSL is extended to convert|1000|ft|m|-2 below, convert|1000|ft|m|-2 above, and one mile (1.6 km) laterally.

Class G

Class G airspace includes all airspace not otherwise classified below flight level 600. There are no entry or clearance requirements for Class G airspace, even for IFR operations. Class G airspace is typically the airspace very near the ground, beneath Class E airspace.

Radio communication is not required in Class G airspace, even for IFR operations.

VFR visibility requirements in Class G airspace are one mile (1.6 km) by day, and three miles (5 km) by night, for altitudes below 10,000' MSL. Beginning at 10,000' MSL, five miles (8 km) of visibility are required, day and night. Cloud clearance requirements are to maintain an altitude that is convert|500|ft|m|-1 below, convert|1000|ft|m|-2 above, and convert|2000|ft|m|-2 laterally below convert|10000|ft|m|-3 MSL; at or above convert|10000|ft|m|-3 MSL, they are convert|1000|ft|m|-2 below, convert|1000|ft|m|-2 above, and one mile (1.6 km) laterally. By day at convert|1200|ft|m|-1 AGL and below, aircraft must remain clear of clouds, but there is no minimum distance.

Special use airspace

Some airspace categories have no correlation with ICAO airspace classes but are nevertheless important in United States airspace. The airspace class (A, B, etc.) in which special use airspace is found still controls the requirements and procedures for flying into/through it.

Alert and warning areas

Alert and Warning areas contain special hazards that pilots must take into consideration when entering the areas. They do not require a special ATC clearance, but ATC can advise on the status of the area in some cases (whether or not hazardous activities are in progress, for example).

Restricted airspace

Entry into restricted areas is prohibited under certain conditions without a special clearance obtained from the controlling agency obtained directly or via ATC. Examples of restricted areas include test firing ranges and other military areas with special hazards or containing sensitive zones.

Prohibited airspace

Entry into prohibited areas is forbidden under all circumstances. Prohibited areas exist over a handful of extremely sensitive locations, such as the White House.

Military operation area (MOA)

Military operation areas (MOA) are areas in which military activities are regularly conducted. No special clearance is required to enter MOAs, but pilots should verify with ATC or Flight service station that no hazardous activity is underway before entering an MOA.

Terminal radar service area (TRSA)

Terminal radar service area, or TRSA, is general controlled airspace wherein ATC provides radar vectoring, sequencing, and separation on a full-time basis for all IFR and participating VFR aircraft. Service provided at a TRSA is called Stage III Service. TRSA's are depicted on VFR aeronautical charts. Pilot participation is urged but is not mandatory.

They are designated in high volume traffic areas where radar services are available but not otherwise designated as B or C class airspace, such as the Palm Springs Valley in Southern California where high mountainous terrain channels air traffic into the same busy space.

When VFR, you need not contact TRACON prior to entry or while in any TRSA, however it is recommended you do so. There are no specific equipment requirements to operate VFR in a TRSA. If you ask for and receive radar services from TRACON, you must comply with heading and altitudes assigned or cancel the service.

Temporary flight restriction (TFR)

Temporarily restricted airspace is designated by NOTAM and used to provide a safe airspace environment for emergency aircraft operations in situations such as forest fires, disasters, or during Presidential movement.

VFR chart notation

Specific conventions are used to indicate airspace boundaries on VFR sectional and terminal area charts (TACs) for the United States.

Class A

Class A airspace is not shown on VFR charts, but it is assumed to extend from convert|18000|ft|m|-2 MSL to convert|60000|ft|m|-3 MSL everywhere.

Class B

Class B airspace is delimited by a heavy cyan border. Each distinct segment of Class B airspace contains figures indicating the upper and lower altitude limits of that segment in units of one hundred feet, shown as a fraction, e.g., 100 over 40 indicates a ceiling of convert|10000|ft|m|-3 MSL and a floor of convert|4000|ft|m|-2 MSL (SFC indicates that the segment begins at the surface). In some areas each segment may also be assigned a letter for identification during communication with ATC.

In many cases the boundaries of Class B airspace segments are coincident with specific radials from a specific VOR station or with specific distances from such a station; these are normally marked on the chart. In other cases, the boundaries may follow natural topological features or may be defined in other ways, which may or may not be explicitly indicated on the chart.

Class C

Class C airspace is delimited by a heavy magenta border. Each distinct segment of Class C airspace contains figures indicating the upper and lower altitude limits of that segment in units of one hundred feet, shown as a fraction, e.g., 100 over 40 indicates a ceiling of convert|10000|ft|m|-3 MSL and a floor of convert|4000|ft|m|-2 MSL. (SFC indicates that the segment begins at the surface, and T indicates that the ceiling ends where overlying Class B airspace begins.)

In many cases the boundaries of Class C airspace segments are coincident with specific radials from a specific VOR station or with specific distances from such a station; these are normally marked on the chart. In other cases, the boundaries may follow natural topological features or may be defined in other ways, which may or may not be explicitly indicated on the chart.

Class D

Class D airspace is delimited by a thin, dashed cyan line, generally in the form of a circle centered on an airport. A number enclosed in a box surrounded by a similar dashed line (ceiling value) and usually within the Class D area gives the upper limit of the airspace in hundreds of feet (the lower limit of Class D is always the surface). A MINUS ceiling value indicates surface up to but not including that value.

Class E

Class E airspace is delimited in different ways depending on its lower altitude limit. Airspace in this class that begins at the default altitude of convert|14500|ft|m|-1 MSL is not delimited. Class E airspace that begins at convert|1200|ft|m|-1 AGL is delimited by a broad, shaded cyan border. Class E airspace that begins at convert|700|ft|m|-1 AGL is delimited by a broad, shaded magenta border. Airspace in Class E that begins at the surface is delimited by a thin, dashed magenta line (this type of Class E is most often seen as an extension to Class D airspace that facilitates control of IFR routes to and from an airport).

In many cases, the expanse of airspace that is Class E beginning at convert|1200|ft|m|-1 AGL is so large that only the areas that differ are marked on the chart. Thus, one may see only external borders within the chart, with the convert|1200|ft|m|-1|sing=on region extending off the chart.

When Class E airspace begins at altitudes other than convert|1200|ft|m|-1 AGL, convert|700|ft|m|-1 AGL, or 14,500 feet MSL, a delimiting border resembling links in a chain in dark cyan separates the areas, and specific altitudes are marked within them.

References

* Federal Aviation Administration. [http://www.faa.gov/library/manuals/aviation/instrument_flying_handbook/ "Instrument Flying Handbook."] FAA, FAA-H-8083-15A, 2007.
* Federal Aviation Administration. [http://www.faa.gov/airports_airtraffic/air_traffic/publications/ATpubs/AIM/index.htm "Aeronautical Information Manual."] FAA, February 16, 2006.
* Aircraft Owners and Pilots Association Safety Center. "Airspace Definitions." AOPA, SA02-9/05, September, 2005.
* National Archives and Records Administration. [http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=a19e3a27523fc90b2ef96206ab9709c7&tpl=/ecfrbrowse/Title14/14cfr91_main_02.tpl "Electronic Code of Federal Regulations] , December 13, 2007, 14 CFR 91.126 through 14 CFR 91.135