Aircraft that fly under the supervision of Air Traffic Control fly in what is called controlled airspace. These are predefined areas of airspace in which no aircraft can fly unless properly equipped and the pilot suitably qualified. Within these areas, aircraft can get within very close proximity and to control that proximity air traffic controllers use a set of procedures and regulations to keep aircraft separated safely at all times and communicate with them through radio.

Types of Airspace

Depending on the airspace in which the aircraft is flying and set of rules under which the pilot is carrying out the flight, depends on whether separation will be applied to the aircraft by Air Traffic Control. There are 7 classes of airspace:

  • Class A – This is controlled airspace, no aircraft flying under visual flight rules (VFR) are allowed to fly here (although ‘special VFR’ may be issued with permission to transit these zones visually). This covers all airways up to Flight Level (FL) 195 (19,500 feet), Terminal Control Areas around London, Birmingham and Manchester, London Control Zone and some CTAs.
  • Class B – No class B airspace used in the UK.
  • Class C – All airspace in the UK between FL 195 and FL 660 (between 19,500 and 66,000 feet).
  • Class D – CTRs and CTAs around larger airfields, covering a few other airways in the UK which are in less busy areas.
  • Class E – TMAs, clearance from aircraft flying under visual flight rules (VFR) do not need to contact air traffic control to pass through, however they are strongly advised to contact them. Belfast and Scottish TMAs are currently Class E airspace.
  • Class F – These are known as “advisory routes” (ADRs) which are regularly routes like airways but air traffic is not busy enough to warrant creating an official airway.
  • Class G – Everything else! This covers most airspace in the UK below FL 70 (7000 feet). This is known as a FIR (flight information region). Aircraft can request information about other aircraft and their location from air traffic control but participation in a constant service is not required. This is where a majority of general aviation traffic can be found in the UK.

Classes in red text are areas of controlled airspace. Classes in green text are areas of uncontrolled airspace.

Horizontal Separation

Aircraft flying in controlled airspace A-E are under the instructions of ATC. This means that they have to follow the instructions given by their Air Traffic Control Officer (ATCO). The ATCO has to use the radar to keep these aircraft within a certain limitation either horizontally or vertically.

When flying horizontally, aircraft flying en route, usually within higher altitude airspaces are kept separated by a minimum of 5 nautical miles. The ATCO controls heading, altitude and speed of aircraft to maintain this separation.

Horizontal separation in air traffic tracking

When aircraft are operating in busier airspace, near airports and at slower speeds, this separation comes down to 3 nautical miles.

Approach to Heathrow screenshot 3 nautical miles

Horizontal separation must be maintained when vertical separation is not. There must always be one dimension of separation present between two aircraft at any one time.

Vertical Separation

Aircraft that are within 3 nautical miles or 5 nautical miles of each other laterally (as above) must be separated vertically from one another. This ensures the aircraft are always a safe distance from each other. Within controlled airspace the ATCO uses radar to keep separation vertically by providing instructions on climb, descent and cruise speed as well as providing vectors for aircraft to follow.

  • Aircraft flying between FL290 (29,000 feet) and FL410 (41,000 feet) must be separated by at least 2000 feet. However, most airliners are suitably equipped with advanced avionics that means they may fly to Reduced Vertical Separation Minima (RVSM) which allows separation of 1,000ft between these flight levels.
  • Aircraft flying below FL290 must be separated by 1000 feet.

You can see these rules in action all the time on PlaneFinder:

2,000 feet separation rules in air traffic tracking

Radar separation in aircraft

Separation in air traffic 2,000 feet


To allow aircraft to manoeuvre with each other strictly under control, the ATCO issues directional or heading instructions to pilots within controlled airspace. For example, aircraft approaching Heathrow for final approach to landing arrive in the control area via a navigational beacon and a controller tells them which direction to head in using a compass heading. The pilot then follows this and any other instructions provided. You can see this in the map below as a flight is vectored in to intercept final approach at Heathrow. The likely reason this controller has taken them in an “S” pattern is to slot the aircraft between two others, therefore altering its direction of travel to allow the aircraft to ‘slot’ in and maintain separation between other aircraft already on final approach.Aircraft vectored to final approach at Heathrow

Watch it all on Plane Finder’s flight tracker

All of the above air traffic manoeuvres can be seen in action on PlaneFinder’s flight radar. If you zoom out so that you can see the area of Europe, you can spot where the controlled airspace and airways are:

Flight Tracking in Europe

Thank you to Richard Buckby from Derbyshire for making suggestions to update the above content. We have duly done so!

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