Visual observation from a control tower constitutes the primary method of controlling air traffic on the ground and in close proximity of an airport. The control tower is a tall, windowed structure that offers the air traffic controllers a panoramic view covering the airport and its surroundings. Aerodrome controllers - or "tower controllers" - are responsible for the separation and efficient movement of aircraft and vehicles operating on the taxiways and runways of the airport, as well as for aircraft in the air in the vicinity of the airport.
Radar displays are also available to aerodrome controllers at some airports. They may use a radar system called "ground radar" to survey and guide the aircraft and other vehicles that move on the aerodrome as well as a so-called approach radar in order to keep an eye on approaching and departing traffic. These screens display the aircraft positions and data tags that include aircraft identification, speed and altitude.
The areas of responsibility for tower controllers fall into three general operational disciplines; Ground Control, Tower Control, and Clearance Delivery. While each tower procedure is different and several teams may control multiple runways in important towers, there is a general concept of delegation of responsibilities within the tower environment.
- Departure clearance (or clearance delivery) :
Clearance delivery is the position that takes up the coordination with on the one hand the Eurocontrol Central Flow Management Unit (CFMU) which is responsible for the overall management of the air traffic flows in Europe and on the other hand the en-route air traffic control centre to deliver clearance for departing aircraft. Under normal conditions, this is performed quasi automatically. In case of bad weather conditions or when there is a very high demand for an airport or a specific air traffic control sector, there may be traffic rerouting, and in some extreme cases even ‘slots' (a time window and therefore a delayed take-off) are used to make sure that the system never gets overloaded. The primary responsibility of the clearance delivery position is to ensure that the aircraft receives the proper route and slot for take-off. This information is transmitted to the ground controller in order to ensure the aircraft reaches the runway within the time limits assigned by the CFMU.
- Ground control :
Ground Control is responsible for the airport "movement" areas. These include all taxiways, holding areas, and some manoeuvring areas or intersections where aircraft arrive after having left the runway or the departure gates. Responsibilities and working areas allotted to each air traffic controller are clearly defined in local documents and agreements specific to each airport. Aircraft and vehicles changing their position within these movement areas, are required to have clearance from the ground controller. This is normally done via radio contact, but there may be special circumstances where other methods are used, such as communication through visual signals.
Efficient ground control is vital to smooth airport operations because, on top of his most important mission which is to ensure the safety of ground movements, the ground controller is responsible for optimising the order in which the aircraft are sequenced to depart at the runway threshold and this in order to accelerate the take-off rhythm. Some very busy airports dispose of electronic systems designed to detect and identify aircraft and vehicles on the ground. The ground controller uses them as an additional tool to control ground traffic, particularly at night or in poor visibility. These advanced systems can display a detailed overview of the ground operations, where every vehicle detected by the radars is identified by a data blocks. Furthermore, computerized alarm signals warn the ground controller of every potential danger. In addition, the AMS (Airport Movement System), developed by Belgocontrol, supplies the air traffic controllers with all relevant information regarding the controlled aircraft in a concise and efficient manner.
- Air control :
The Air Controller is in charge of the movements on the runways as well as for the air traffic in the vicinity of the airport. He clears aircraft for take-off or landing, thereby ensuring that the assigned runway is clear for the foreseen manoeuvre. Regarding the traffic in the air, the air controller is responsible for managing a controlled airspace called "CTR" that covers the airport and in which he ensures the safety of the approaching or departing aircraft by giving adequate instructions. If the air controller detects potentially unsafe conditions, he can tell the pilot of an aircraft in landing phase to go around or even order an aircraft to abort departure.
At the tower, a highly disciplined communication and collaboration process between air and ground controllers is an absolute necessity. But the co-ordination with other air traffic controllers does not stop there: it is also advisable to co-ordinate with the radar controllers the distances separating approaching aircraft in view of creating the necessary distance for aircraft taking off or for crossing the runway.
The generic term "aerodrome control" - or "tower control" - refers to air traffic control that is performed from the tower and does not distinguish between the air, ground and clearance delivery positions.
Belgocontrol manages the control towers at the airports of Antwerp, Brussels, Charleroi, Ostend, Liege and Kortrijk (Flight Information Services).