The sky is huge, the aircrafts are quite small, so why do whe need ATM? Mainly because many aircrafts want to depart, or to arrive at the same points at mostly the same time. That points are the main airports, and the airport is quite small, compared with the sky, so all the trajectories merge in a very reduced space. So all in all, we need something to establish an order and guarantee a safe separation between aircrafts. There are also other functions to be performed by the ATM, but for the moment, we can go with this fundamental idea. 

To illustrate that, look at the animation, it represents 24H traffic arriving-departuring from Madrid airport, condensed in just 30 seconds. Does it seems easy to manage?

From the airport stand to the next airport stand, several control units need to take part, and these, in turn, are divided to perform specific functions or to be responsible for a portion of the flight. This interactive animation shows step by step how it works.

Each flight phase is controlled by one or more Air Traffic Controllers, an those divide into: Aerodrome, Approach and En-route controllers. Remeber: each ATCO has an unique VHF frequency to communicate with, althought sometimes and ATCO may be responsible of more than an area/volume and then use several VHF frequencies at the same time.

When an aircraft approaches the boundary between two control sectors, two processes come into action. First, controller-to-controller coordination takes place; then, once the transfer is agreed upon, the controller-pilot communication transfer occurs. However, the actual transfer of control does not take effect until the aircraft crosses the boundary.

A golden rule to remember: transfer of communication always precedes transfer of control. By how much? It depends on several circumstances, but usually ranges between 5 and 15 minutes.