Definition

A controlled airspace extending upwards from the surface of the earth to a specified upper limit.

Source: ICAO Annex 11 - Air Trafffic Services

Description

While not explicitly stated in the definition, controlled zones (CTRs) are generally established around aerodromes. Their main purpose is to accomodate departing and arriving IFR traffic by encompassing the airspace which includes the flight paths of those flights that is not part of a control area (i.e. a TMA or a CTA). This provides continuity of the air traffic control (ATC) service. The airspace of a CTR is normally class C or class D (class A would not allow for VFR flights, class E is forbidden for such use and classes E and F are not controlled airspace). This is the only airspace structure that can accomodate special VFR flights.

A CTR may be established around a single aerodrome or a group of two or more aerodromes situated close together. The lateral limits extend to at least 5 NM from the centre(s) of the aerodrome(s) concerned in the directions from which approaches may be made.

The lower limit of a CTR is, by definition, the surface of the earth (by contrast, the lower limit of a TMA or a CTA is a defined level above the surface). A CTR is often located below a control area. In such cases, the upper limit of the CTR extends at least up to the lower limit of that area but a higher upper limit can also be established.

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Examples of CTR vertical limits coinciding with the lower limit of a control area (left) and above that limit (right).

When the upper limit of a CTR is above 3000 ft MSL, it often coincides with a VFR cruising level (e.g. FL35/FL45/...).

A CTR is normally identified by the name of the ATC unit having jurisdiction over the airspace which is usually a control tower.

Detailed descriptions of CTRs can be found in the relevant AIPs.