ATC Theory - The Basics

Following popular demand, we are going to (re)write some basic training stuff. As you know, all of this work is voluntary and may take some time to be completed. If you want to make a contribution, or you have any questions, corrections and/or suggestions, please drop a line to our communications team and we'll have a look at it as soon as we can. Hope you can find this useful :)


"Got clearance, Clarence"

Basically any instruction give to an aircraft on the ground and/or in the air is called a clearance. They can be issued both enroute and before being airborne.

Any pilot who wants to fly in controlled airspace (except class E) has to get permission from the controller first. In order for the controller to know what the intentions of a certain aircraft are, the pilots must send a flightplan. VFR flights are not required to send a flightplan, but when requesting clearance will have to give all the information over the radio; practice which is not a common practice in our online environment.

Clearances vary in content, but in any case, it is very important that the controller ensures there will be no risk when clearing the aircraft for something.


"VLG16A, identified."

In order to establish a succesful radio contact, each aircraft shall be uniquely identified during its flight. On the radio, this is done by means of a callsign.

There are different forms of callsigns. Airline flights typically use a callsign made up of their airline ICAO code (VLG for Vueling, EIN for Aer Lingus -Shamrock-, and so on...) followed by some numbers, which are specific for the flight path they're following. On the other hand, smaller aircraft (normally for non-airline flights) normally use the aircraft registration as their callsign. In this type of callsigns, the first 2 letters stand for the country, and the following 3 identify each aircraft. For example, the first two letters "EC" mean that it is a spanish aircraft, "TC" a turkish one, etc...


Well, you now know how an aircraft is distinguished from the rest on the radio, but how is this done on the radar screen?

To do this, ATCs use 2 different types of radar: Primary and Secondary.

  • Primary radar: This type of radar is the one everybody knows. Basically a pulse of sound is sent and after bouncing on an object, it is received back. By performing some calculations it is then possible to detect where something is, and even what its groundspeed is. But, what is it that has been detected? At what altitude? Where is it going? Here is where the other type of radar comes into game.
  • Secondary radar: Aircraft are equipped with a piece of equipment called "transponder". When the secondary radar sends a signal and this piece of equipment receives it, it responds with a unique 4-digit code (squawk, which is normally assigned together with the flightplan clearance but can also be given while enroute) and the altitude which it's at. Depending on its technology, it can also give more data, like for example heading or actual groundspeed.

The combination of this 2 different radars is then "correlated" to show a unique blip on the radar screen with all the necessary information about it.

It is also important to know that because of the way transponders work, they only allow the use of the numbers 0 to 7. This means that there is 4096 combinations possible; some of which are reserved for special use.


Standard Instrument Departure

A SID is a pre-defined route towards a specific waypoint from an airport. They are named using a special system.

ERIVA4E is an example of a SID. ERIVA is the waypoint at which the departure ends and 4 is its version number. If it gets updated, next one will be called ERIVA5E, for example. Even though, changes are usually only minor adjustments, and if a pilot only has ERIVA3E available, it normally isn't much of a problem allowing him to fly that one. Note that in some cases the SID's name may be shortened; for example, HISA1K departure in Antalya, which wouldn't end in a waypoint called "HISA", but "HISAR" instead.

It is also common that the last letter of the SID corresponds to a certain runway, but this is not always the case. In Arlanda for example, all  SIDs ending with "G" depart from runway 19R, but in the case of Copenhagen, all "A" departures are valid for both runways 04L and 04R.

SIDs are very useful, as they reduce the workload of the controller. They are carefully studied to minimize potential risks and conflict between aircraft, including both inbound and outbound flights.


Standard Terminal Arrival

We already know that when departing an aerodrome, SIDs are followed to minimize conflicts and therefore improve efficiency.

STARs are the same thing as SIDs, but instead of for departures, they are for arrivals. They join certain waypoints to the IAF (Initial Approach FIX), and in some cases extend all the way to the FAF (Final Approach FIX). In this cases, the controller is only responsible for the aircraft's descent, and shall only vector it if it deems necessary or if it ends at the IAF.

As with everything, there are some exceptions. Some STARs don't end up at any Approach Reference Points, but at a navigational point some distance away from the runway, from which the controller will then vector it in.

If when a pilot reaches the clearance limit the controller still hasn't cleared him for the approach, the pilot has to start holding at the last point of the STAR. Try to avoid this as much as possible.

ATS Routes

ATS Routes are predefined routes which connect waypoints to each other. They are named by a letter followed by 2 or 3 numbers (e.g: N872).

If it is a route used in the upper airspace, a "U" is added at the beginning (e.g: UN872).

Some of them restrict their direction of travel, so that aircraft can only fly in one direction along them.


  • Precision approaches: As the name states, this type of approaches are "precise". They provide a guidance to the runway in both the horizontal and vertical direction. The most common type of precission approach is the ILS. Unfortunately, not all airports have this systems, so "non-precision approaches" are used.
  • Non-precision approaches: This type of approaches are normally used in those cases where there is no ILS, it is non-functional or simply, where the pilots want to fly a more "exotic" approach. There are different type of non-precision approaches, such as VOR, VOR/DME and NDB approaches. The one most commonly used is the VOR approach, where the pilots fly on a radial towards the VOR, which provides them horizontal guidance with the runway.
  • Visual approach: The most simple approach type. It is done without any navigational aids; only with visual references.


During an approach, it is required that a pilot has visual contact with the runway at a certain height, which varies depending on the approach type, pilots, aircraft, etc... if when reaching it the pilots still have no visual contact with the runway or its lights, they must call a missed approach. In precision approaches this height is called Decision Height/Altitude (DH/DA), whereas in non-precision approaches it is named Minimum Descent Height/Altitude (MDH/MDA).


Runway Visual Range

RVR is only measured when the visibility drops below 1500m. In these cases, a pilot is allowed to comence the approach, regardless of the conditions, but is not allowed to go beyond the outer marker or equivalent if the reported RVR/visibility is below the minima.

If after passing the outer marker the RVR/visibility falls below the applicable minimum, then it is possible to continue the approach towards the  DA/H or MDA/H.


Missed Approach Point

In non-precision approaches, a MAP is designated. If when reaching this point the pilots still haven't got visual contact with the runway, they must go around.

ILS Categories



In real life, there are another 3 limiting factors when performing an ILS approach:

  1. Pilot qualification
  2. Aircraft qualification
  3. Ground equipment qualification

Flightplan & Route

As you willl see later onwards, in order to enter most classes of controlled airspace, it is necessary to file a flightplan in order for the controller to know where an aircraft is going and thus clear it, with some exceptions.

A flightplan contains all useful information about the flight an aircraft will make, including speed, altitude, route, etc...

As in many cases aircraft find it necessary to climb/descend and change their cruise speed during their flight, there is a certain way of writing this in a flightplan.

For speeds:



For altitudes:

Flight LevelFxxxF320
Standard Metric Level (tens of metres)SxxxxS1100
Metric Altitude (tens of metres)MxxxxM0120
VFR (unspecified)VFRVFR


More to come...

A VATSIM Europe Division service.
Content updated: 13. November 2019.