Separation (air traffic control)

In air traffic control, separation is the name for the concept of keeping an aircraft outside a minimum distance from another aircraft to reduce the risk of those aircraft colliding, as well as prevent accidents due to secondary factors, such as wake turbulence. Separation can also apply to terrain, obstacles, and controlled airspace, wherein an aircraft must stay at a minimum distance from a block of airspace; as an example, all aircraft must be approved by the controller who "owns" the airspace before the aircraft is approved to enter that sector.

Air traffic controllers apply rules, known as separation minima to do this. Pairs of aircraft to which these rules have been successfully applied are said to be separated: the risk of these aircraft colliding is therefore remote. If separation is lost between two aircraft, they are said to be in a conflict.

When an aircraft passes behind or follows another aircraft, wake turbulence minima are applied due to the effect of the wingtip vortices of the preceding aircraft on the following aircraft. These minima vary depending on the relative size of the two aircraft. This is particularly acute on final approach with a smaller aircraft following larger aircraft.

It is a common misconception that air traffic controllers keep all aircraft separated. Whether aircraft actually need separating depends upon the class of airspace in which the aircraft are flying, and the flight rules under which the pilot is operating the aircraft. As stated by the U.S. FAA, The pilot has the ultimate responsibility for ensuring appropriate separations and positioning of the aircraft in the terminal area to avoid the wake turbulence created by a preceding aircraft.

There are three sets of flight rules under which an aircraft can be flown:

Public transport flights are almost exclusively operated under IFR, as this set of rules allows flight in regions of low visibility (e.g. cloud). On the other hand, a large amount of private flying in light aircraft is done under VFR since this requires a lower level of flying skill on the part of the pilot, and meteorological conditions in which a pilot can see and avoid other aircraft. As its name suggests, SVFR is a special infrequently-used set of rules. For the purposes of separation, controllers consider SVFR to be the same as IFR.

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