To stay safe, both in the air and on the ground, aircraft need to keep their distance from one another and from other vehicles and obstacles. Air traffic control is responsible for this so-called “separation”.
On the ground
At the airports air traffic control takes care of the separation of traffic in the so-called manoeuvring area. This part of the airport site is intended for take-off, landing and taxiing of aircraft, with exception of the platforms. The boundary between the manoeuvring area and the platforms is indicated with red lines.
In the air
In the air, the separation between aircraft is expressed in terms of both horizontal and vertical distance. So-called separation minima – minimum distances – have been established to maintain safety, whilst at the same time making optimum use of airspace. These minima create buffers, which reduce the risk of a mid-air collision. Air traffic control is responsible for maintaining them. To do this, it provides pilots with instructions on their course, altitude (flight level) and speed.
Exceptions to separation rules
In the control zone (CTR) around an airfield – the area in which air traffic is managed from the local control tower – visual separation may be used. This is when the air traffic controller can see both flights or the two pilots can see one another’s aircraft and so determine visually that they are sufficiently far apart. It is a technique commonly used for the separation of successive take-offs from the same runway and when dealing with aircraft that are flying “by sight” – more properly, under visual flying rules (VFR) – in the immediate vicinity of an airfield.
Another exception to the standard separation rules applies in the case of parallel landings. Amsterdam Schiphol Airport, for example, has three parallel runways, known as the Polderbaan (18R), the Zwanenburgbaan (18C) and the Aalsmeerbaan (18L). Two aircraft on simultaneous approaches to 18R and 18C respectively will breach the normal separation minima because of the distance between these runways, but are permitted to do so as long as they are on the designated approach path and are using the runway’s instrument landing system (ILS).
In aviation, we draw a distinction between traffic flying “by sight” and “on instruments”. Most of the former consists of light aircraft and is known as VFR (visual flight rules) traffic, whilst the latter is referred to as IFR (instrument flight rules) traffic.
To keep these two different types of traffic as far apart as possible, geographical separation can be enforced. This means that specific sections of airspace are designated for particular uses. For example, a VFR sector and ILS areas on the approaches to runways with an instrument landing system.
Reduced separation is permitted when the aircraft concerned comprise so-called “aerodrome traffic” – that is, they are operating in the immediate vicinity of an airfield – and:
The runway controller is able to maintain sufficient separation because he or she has uninterrupted visual contact with all the aircraft concerned, or
The pilots themselves have visual contact with the other aircraft and report that they are thus able to maintain sufficient separation, or
In the case of two aircraft following one another, the pilot of the one in the rear reports that he or she has visual contact with the other one and so is able to maintain sufficient separation.