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 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.
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. At one end of the scale in classes A and B airspace, all aircraft must be separated from each other. At the other end of the scale in class G airspace there is no requirement for any aircraft to be separated from each other. 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.
Between the surface and an altitude of 29,000 feet (8,800 m), no aircraft should come closer vertically than 300 metres, unless some form of horizontal separation is provided (Note: for countries that measure altitude in feet, a 1,000 feet minimum is observed rather than the 984.252 feet found in 300 meters). Above 29,000 feet (8,800 m), no aircraft shall come closer than 600 m (or 2,000 feet), except in airspace where Reduced Vertical Separation Minima (RVSM) can be applied.
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.
"MARSA" separation can be applied by military aircraft, which overrides all of these rules. Under MARSA conditions (Military Assumes Responsibility for Separating Aircraft), 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.
If any two aircraft are separated by less than the vertical separation minimum, then some form of horizontal separation must exist.
Procedural separation is separation based upon the position of the aircraft, based upon reports made by the pilots over the radio. 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, however it is important not to get this mixed up with radar separation as in the former case the radar need not necessarily be certified for use for radar separation purposes, the separation is still procedural.
In the case of beacons, to be separated, the aircraft must be a certain distance from the beacon (measured by time or by 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.
If two aircraft are not laterally separated, and are following tracks within 45 degrees of each other (or the reciprocal), 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. The golden rule is the 10-minute rule: 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 of 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 it will not be very long before lateral separation will exist again.
Radar separation is applied by a controller observing that the radar returns from the two aircraft are a certain minimum horizontal distance away from each other, as observed on a suitably calibrated radar system. The actual distance used varies: 5NM (9.26km) is common in en route airspace, 3 NM is common in terminal airspace at lower levels. On occasion, 10 NM may be used, especially at long range or in regions of less reliable radar coverage.
As a practical example of the rules, US FAA rules are published in the FAA Order 7110.65; the rules are complicated, and they allow different separation distances (from 3NM to 10NM) depending on the aircraft, distance from radar antenna, type of radar used, flight level, etc.
In certain special cases, controllers may reduce separation below the usually required minima.
Aerodrome or "Tower" controllers work in tall towers with large windows allowing them, in good weather, to see the aircraft flying in the vicinity of the aerodrome, unless the aircraft is not in sight from the tower (e.g. a helicopter departing from a ramp area). Also, aircraft in the vicinity of an aerodrome tend to be flying at lower speeds. Therefore, if the aerodrome controller can see both aircraft, or both aircraft report that they can see each other, or a following aircraft reports that it can see the preceding one, controllers may reduce the standard separation to whatever is adequate to prevent a collision.
Under its SESAR project, Eurocontrol is optimizing the slack of the separation, reducing it from 3 to 2.5 nmi (5.6 to 4.6 km): a simulation increased throughput by up to 14% at Zurich's Runway 14, which cannot have any high-speed exit, inducing a longer runway occupancy.
A conflict is an event in which two or more aircraft experience a loss of minimum separation. This does not in itself suggest that the aircraft are at any risk of collision. However, the separation minima are set for risk mitigation and therefore it is central to a controller's job to prevent this situation from occurring. Conflicts are detected by data assistants, who report them to the actual controllers; the data assistants suggest how to solve the conflict, but the controller is not obligated to follow the suggested instructions. A conflict occurs when the distance between aircraft in flight violates a defining criterion, usually considered as 5 nautical miles (9 km) of horizontal and/or 1,000 ft (300 m) of vertical separation. These distances define an aircraft's protected zone, a volume of airspace surrounding the aircraft which should not be infringed upon by any other aircraft.
An opposite conflict occurs if two aircraft are flying towards each other from opposing directions. Looking at the information on the flight progress strips, a controller can detect an opposite conflict by checking: