In air traffic control, separation is 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 responsible for 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 acute on final approach with a smaller aircraft following larger aircraft. Wake turbulence categories are typically used.
Need 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:
Visual Flight Rules (VFR) Special Visual Flight Rules (SVFR) Instrument Flight Rules (IFR) 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. SVFR is a special infrequently-used set of rules. For the purposes of separation, controllers consider SVFR to be the same as IFR. Airspace exists in seven classes, A to G, in decreasing order of air traffic control regulation. Classes A to E are controlled airspace and class G is uncontrolled airspace. In class A and B airspace, all aircraft must be separated from each other, while in class G airspace there is no requirement for any aircraft to be separated. In the intermediate classes some aircraft are separated from each other depending on the flight rules under which the aircraft are operating. For example, in class D airspace, IFR aircraft are separated from other IFR aircraft, but not from VFR aircraft, nor are VFR aircraft separated from each other.
Vertical separation Between the surface and an altitude of 29,000 feet (8,800 m), separation rules state that no aircraft should come closer vertically than 300 metres, unless some form of horizontal separation is provided (For countries that measure altitude in feet, a 1,000 feet minimum is observed). Above 29,000 feet (8,800 m), the minimum vertical distance is increased to 600 m (or 2,000 feet), except in airspace where Reduced Vertical Separation Minima (RVSM) can be applied.
RVSM
In certain airspace, between 29,000 and 41,000 feet (12,500 m), pairs of aircraft equipped with more modern altimeter and autopilot systems can be vertically separated by minimum of 1,000 feet (300 m) rather than the standard 2,000 feet (600 m). RVSM airspace encompasses Europe, North America, parts of Asia and Africa and both the Pacific and Atlantic oceans. In areas where RVSM capabilities exist, 1,000 feet of vertical separation may be utilized up to FL410 (41,000 ft), and 2,000 between FL410 and FL600 (60,000 ft). 5,000 ft vertical separation must be applied to military aircraft above FL600, RVSM or not. Military Assumes Responsibility for Separating Aircraft (MARSA) separation can be applied by military aircraft, which overrides all of these rules. Under MARSA conditions, air traffic controllers protect only a block of airspace around multiple military aircraft. They are treated as one, and given only one data tag on the controller's scope.
Horizontal separation If any two aircraft are separated by less than the vertical separation minimum, then some form of horizontal separation must exist.
Procedural separation Procedural separation, or temporal separation, is separation based upon the position of the aircraft, based strictly on timing. It therefore does not necessarily require the use of radar to provide air traffic control using procedural separation minima. In procedural control, any period during which two aircraft are not vertically separated is said to be "level change". In some cases, procedural separation minima are provided for use with radar assistance.
Lateral separation Lateral separation minima are usually based upon the position of the aircraft as derived visually, from dead reckoning or internal navigation sources, or from radio navigation beacons. In the case of beacons, to be separated, the aircraft must be a certain distance from the beacon, measured by time or by Distance Measuring Equipment (DME), and their tracks to or from the beacon must diverge by a minimum angle. Other lateral separation may be defined by the geography of pre-determined routes, for example the North Atlantic Track system.
Longitudinal separation If two aircraft are not laterally separated, and are following tracks within 45 degrees of each other, then they are said to be following the same route and some form of longitudinal separation must exist. Longitudinal separation can be based upon time or distance as measure by DME. No two aircraft following the same route must come within 15 minutes flying time of each other. In areas with good navaid cover this reduces to 10 minutes; if the preceding aircraft is faster than the following one then this can be reduced further depending on the difference in speed. Aircraft whose tracks bisect at more than 45 degrees are said to be crossing. In this case, longitudinal separation cannot be applied as lateral separation will soon exist again.
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