In aviation, a control area (CTA) is "a controlled airspace extending upwards from a specified limit above the earth", not less than 200m (700 feet), although the limit does not need to be established uniformly within a control area.[1] ICAO recommends for the lower level to coincide with a VFR cruising level if the lower level limit of a control area is above 900 m (3000 feet) MSL (mean sea level), and the VFR cruising level to not have to be lowered to less than 200 m (700 feet) due to atmospheric conditions.[1] The upper limit should be established when either air traffic control service will not be proveded above that level, or there is another (upper) control area above this control area.[1]
Control areas are established in any areas when the density of air traffic is high:[2]
A control area usually is situated on top of a control zone (CTR) and provides protection to aircraft climbing out from the airport by joining the low-level control zone to the nearest airways. In the UK, they are generally class A, D or E.
Control areas are particularly useful where there are busy airports located close together. In this case, a single CTA will sit over all of the individual airports' CTRs. In larger-scale cases, this is known as a terminal manoeuvring area (TMA, or TCA).