c.
Controlled airspace is established according to the needs and available facilities in different areas. It is an airspace of defined dimensions within which Air Traffic Control service is provided. Controlled airspace may appear in various forms.

(1)
A Control Area (CTA) is an airspace of defined dimensions extending upwards from a specified height above the earth. The Lower Limit is not less than 200 meters, or in some cases, 700 feet above the earth. The Lower Limit does not necessarily have to be established uniformly in a given CTA. It may laterally encompass an entire Flight Information Region or only a portion of one, or it may extend along designated airways. An upper limit is established when Air Traffic Control will not be provided above such limit, or the CTA, is situated below an Upper CTA (UTA). This situation is similar to the Flight Information Region/Upper Information Region relationship previously described. They both have common functions and the UTA may cover several CTAs, Flight Information Regions or portions thereof.

(2)
A Control Zone (CTLZ) laterally encompasses those portions of the airspace not within Control Areas, containing the paths of IFR flight arriving and departing airports. A CTLZ's lower limit is the ground, and if located within the lateral limits of a Control Area, extends vertically to at least the lower limit of the Control Area.

(3)
A Terminal Control Area (TCA) is a portion of a Control Area normally situated at the confluence of airways in the vicinity of one or more major airports. A TCA supplements a Control Zone and is established to protect IFR flights over a wider area in the vicinity of an airport.

(4)
An Advisory Route (ADR) is merely a recommended route in uncontrolled, advisory airspace. No control will be

offered, but "advice" or "suggestions" will be made to facilitate flights. This service does not afford the degree of safety and cannot assume the same responsibility as Air Traffic Control service in respect to the avoidance of collisions. It is for the pilot to decide and to inform the unit providing this service whether or not he will comply with the advice or suggestions. When flying ADRs, entry clearance into Control Areas must still be obtained.
d.
Corridors are designated by national authority and are normally the only routes in which aircraft may operate, unless otherwise specified, corridors should be considered positive control route. Uncontrolled corridors within Controlled Airspace may be provided if necessary to enable uncontrolled VFR traffic access through Controlled Airspace or to aerodromes in a Control Zone.

e.
New airspace classifications were developed by ICAO to provide standard designations to airspace and Air Traffic Services worldwide. These airspace classifications are published in ICAO Annex 11 and were authorized for implementation on 14 Nov 91, but not all ICAO contracting states implemented the classifications on that date. Remaining ICAO Contracting States will implement the airspace classifications on yet undertermined future dates. ICAO Contracting States may select only those airspace classifications appropriate to their needs.

(1)
Implementation of the airspace classifications by an ICAO Contracting State will be indicated in the country entry of the appropriate Area Planning book under VISUAL FLIght RULES and INSTRUMENT FLIGHT RULES, e.g., INSTRUMENT FLIght RULES. The United Kingdom has implemented the ICAO Annex 11 airspace classifications.

(2)
Air Traffic Service (ATS) airspaces. Airspaces of defined dimensions, alphabetically designated, within which specific types of flights may operate and for which Air Traffic Services and rules of operation are specified. States shall select those airspace classes appropriate to their needs.

a Air Traffic Service airspaces shall be classified and designated in accordance with the following:
Class A. IFR flights only are permitted, all flights are subject to Air Traffic Control service and are separated from each other.
　
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