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ICAO DEFINITIONS On March 12 1990 , ICAO adopted the current airspace classification scheme. The classes are fundamentally defined in terms of Flight Rules and interactions between Aircraft and Air Traffic Control (ATC). Some key concepts are:
The classifications adopted by ICAO are:
Classes A-E are referred to as Controlled Airspace . Classes F and G are Uncontrolled Airspace . As of 2004, ICAO is considering a proposal to reduce the number of airspace classifications to three, which roughly correspond to the current classes C, E and G. USE OF AIRSPACE CLASSES Each national aviation authority determines how it uses the ICAO classifications in its airspace design. In some countries, the rules are modified slightly to fit the airspace rules and air traffic services that existed before the ICAO standardisation. United States See Also: Airspace class (United States) The U.S. adopted a slightly modified version of the ICAO system on September 16 1993 , when regions of airspace designated according to older classifications were converted wholesale. The exceptions are some Terminal Radar Service Areas (TRSA), which have special rules and still exist in a few places. With some exceptions, Class A airspace is applied to all airspace between and Flight Level 600 (approximately 60,000 ft). Above FL600, the airspace reverts to Class E (Reference Order 7400.9P, Subpart E). The transition altitude (see '' Flight Level '') is also consistently . All operations in US Class A airspace must be conducted under IFR. SVFR flight in Class A airspace is prohibited. Class B airspace is used around major Airport s, in a funnel shape that is designed to contain arriving and departing commercial air traffic operating under IFR, up to above MSL (12,000 feet above Denver, Colorado ). Class C airspace is used around airports and military air bases with a moderate traffic level. Class D is used for smaller airports that have a control tower. The U.S. uses a modified version of the ICAO class C and D airspace, where only radio contact with ATC rather than an ATC clearance is required for VFR operations. Other controlled airspace is designated as Class E - this includes a large part of the lower airspace. Class E airspace exists in many forms. It can serve as a surface-based extension to Class D airspace to accommodate IFR approach/departure procedure areas. Class E airspace can be designated to have a floor of 700' AGL (above ground level) or 1,200' AGL. Class E airspace exists above Class G surface areas from 14,500' MSL (mean sea level) to 18,000 MSL. Federal Airways from 1,200 AGL to 18,000 MSL within of the centerline of the airway is designated Class E airspace. Airspace at any altitude over 60,000' (the ceiling of Class A airspace) is designated Class E airspace. The U.S. does not use ICAO Class F. Class G airspace (Uncontrolled) is mostly used for a small layer of airspace near the ground, but there are larger areas of Class G airspace in remote regions. Canada Canada generally follows the United States in application of airspace with some differences. For example, Canadian class "C" airspace is procedurally equivalent to United States class "B" airspace. Additionally, the term "Class F" is used for Special Use Airspace, this includes Advisory airspace and Restricted airspace. Germany In Germany , Classes A and B are generally not used at all. Class C is used for all Airspace above Flight Level (FL) 100 (or FL 130 near the Alps ). Airspace is divided into ''lower airspace'' below FL 285 and ''upper airspace'' above FL 285.
Lithuania In Lithuania , Classes A and B are generally not used at all. Classes C and D are used in the following areas of controlled airspace of the Republic of Lithuania:
Source: Airfield Guide Lithuania, 29 SEP 2005, ENR 1.1-1 United Kingdom Class A
Class C All UK airspace between FL 195 and FL 660. N.b: The Upper Flight Information Region (UIR) boundary begins at FL 245 Class D
Class E Parts of the Belfast and Scottish TMAs and a small part of the Durham Tees Valley CTR. Class F "Advisory Routes" (ADRs): regularly used routes similar to airways but where traffic levels are not high enough to warrant establishment of an airway. Class G All remaining airspace, comprising by far the largest part of the airspace below FL 195. The UK is unusual in that IFR flight in Class G airspace is relatively common and ATC units may provide an "as far as is practical" form of separation between some such flights. In addition the UK has a couple of special classes of airspace that do not fall within the ICAO classes: Aerodrome Traffic Zones (ATZ) are zones of between 1.5 nm and 2.5 nm from the surface to AAL set up around an airport, where aircraft must obey the instructions of the tower controller (if present), must make radio contact with the Information Officer or Air/Ground radio unit on the airport before entering the zone (in the case of an uncontrolled airfield), or must obey ground signals if non-radio. Military Air Traffic Zones (MATZ) are zones from the surface to AAL set up around military air bases in class G airspace. Military aircraft treat these as if they are controlled airspace; civilian traffic is advised but not obliged to do the same. Australia Australia has adopted a civil airspace system based on the United States National Airspace System (NAS):
In addition, Australia has a non-standard class of airspace for use at the Capital City General Aviation airports, called a General Aviation Airport Procedures Zone (GAAP Zone). A control tower provides procedural clearances for all aircraft inside the zone. Additionally, any aircraft operating within 5 nm of the zone must obtain a clearance. VFR aircraft arrive and depart using standard arrival and departure routes, while instrument arrival and departure procedures are published for IFR operations. During VMC, IFR aircraft are not provided with full IFR services. During IMC, or marginal VMC, VFR operations are restricted in order to facilitate full IFR service for IFR aircraft. AIRSPACE CLASSES AND VFR Authorities use the ICAO definitions to derive additional rules for VFR cloud clearance, visibility, and equipment requirements. For example, consider Class E airspace. An aircraft operating under VFR may not be in communication with ATC, so it is imperative that its pilot be able to see and avoid other aircraft (and vice versa). That includes IFR flights emerging from a cloud, so the VFR flight must keep a designated distance from the edges of clouds above, below, and laterally, and must maintain at least a designated visibility, to give the two aircraft time to observe and avoid each other. The low-level speed limit of 250 Knot s does not apply above , so the visibility requirements are higher. On the other hand, in Class B airspace, separation is provided by ATC to all flights. Now the VFR flight only needs to see where it is going, so visibility requirements are reduced and there is no designated minimum distance from clouds. Similar considerations determine whether a VFR flight must use a two-way Radio and/or a Transponder . SPECIAL-USE AIRSPACE Each national authority designates areas of special use airspace (SUA), primarily for reasons of national security. This is not a separate classification from the ATC-based classes; each piece of SUA is contained in one or more zones of letter-classed airspace. SUAs range in restrictiveness, from areas where flight is always prohibited except to authorized aircraft, to areas that are not charted but are used by military for potentially hazardous operations (in this case, the onus is on the military personnel to avoid conflict). Refer to the external links for more specific details. EXTERNAL LINKS
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