Separation (air traffic control)
Encyclopedia
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 wake turbulence
.
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.
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 classes F and G are 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.
of 29000 feet (8,839.2 m), no aircraft should come closer vertically than 300 metres or 1,000 feet (in those countries that express altitude in feet), unless some form of horizontal separation is provided.
Above 29000 feet (8,839.2 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.
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.
or internal navigation sources, or from radio navigation
aids ('beacons').
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.
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.
system. The actual distance used varies: 5 nmi (9 km) 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 cover.
By US FAA Rules, when an aircraft is:
RVSM airspace encompasses Europe, North America, parts of Asia and Africa and both the Pacific and Atlantic oceans.
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 1000 feet 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 another aircraft.
s, a controller can detect an opposite conflict by checking:
Air traffic control
Air traffic control is a service provided by ground-based controllers who direct aircraft on the ground and in the air. The primary purpose of ATC systems worldwide is to separate aircraft to prevent collisions, to organize and expedite the flow of traffic, and to provide information and other...
, separation is the name for the concept of keeping an aircraft
Aircraft
An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...
outside a minimum distance from another aircraft to reduce the risk of those aircraft colliding, as well as prevent accidents due to wake turbulence
Wake turbulence
Wake turbulence is turbulence that forms behind an aircraft as it passes through the air. This turbulence includes various components, the most important of which are wing vorticies and jetwash. Jetwash refers simply to the rapidly moving gases expelled from a jet engine; it is extremely turbulent,...
.
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
Wake turbulence
Wake turbulence is turbulence that forms behind an aircraft as it passes through the air. This turbulence includes various components, the most important of which are wing vorticies and jetwash. Jetwash refers simply to the rapidly moving gases expelled from a jet engine; it is extremely turbulent,...
minima are applied due to the effect of the wingtip vortices
Wingtip vortices
Wingtip vortices are tubes of circulating air that are left behind a wing as it generates lift. One wingtip vortex trails from the tip of each wing. The cores of vortices spin at very high speed and are regions of very low pressure...
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.
Which aircraft need separating?
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 RulesVisual flight rulesVisual flight rules are a set of regulations which allow a pilot to operate an aircraft in weather conditions generally clear enough to allow the pilot to see where the aircraft is going. Specifically, the weather must be better than basic VFR weather minimums, as specified in the rules of the...
(VFR) - Special Visual Flight RulesSpecial visual flight rulesSpecial visual flight rules are a set of aviation regulations under which a pilot may operate an aircraft. It's a special case of operating under visual flight rules .- Use in different regions :...
(SVFR) - Instrument Flight RulesInstrument flight rulesInstrument flight rules are one of two sets of regulations governing all aspects of civil aviation aircraft operations; the other are visual 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
Light aircraft
A light aircraft is an aircraft that has a maximum gross take-off weight of or less.Many aircraft used commercially for freight, sightseeing, photography and scheduled flights are light aircraft.Examples of light aircraft include:...
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
Airspace
Airspace means the portion of the atmosphere controlled by a country above its territory, including its territorial waters or, more generally, any specific three-dimensional portion of the atmosphere....
exists in seven classes
Airspace class
The world's navigable airspace is divided into three-dimensional segments, each of which is assigned to a specific class. Most nations adhere to the classification specified by the International Civil Aviation Organization and described below...
, A to G, in decreasing order of air traffic control regulation. Classes A to E are controlled airspace
Controlled airspace
Controlled airspace is an aviation term used to describe airspace in which ATChas the authority to control air traffic, the level of which varies with the different classes of airspace. Controlled airspace is established mainly for three different reasons:...
and classes F and G are uncontrolled airspace
Uncontrolled airspace
Uncontrolled airspace is airspace where an Air Traffic Control service is not deemed necessary or cannot be provided for practical reasons. According to the airspace classes set by ICAO both class F and class G airspace are uncontrolled...
. 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.
Vertical separation
Between the surface and an altitudeAltitude
Altitude or height is defined based on the context in which it is used . As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The reference datum also often varies according to the context...
of 29000 feet (8,839.2 m), no aircraft should come closer vertically than 300 metres or 1,000 feet (in those countries that express altitude in feet), unless some form of horizontal separation is provided.
Above 29000 feet (8,839.2 m) no aircraft shall come closer than 600 m (or 2,000 feet), except in airspace where Reduced Vertical Separation Minima
Reduced Vertical Separation Minima
Reduced Vertical Separation Minima or Minimum is an aviation term used to describe the reduction of the standard vertical separation required between aircraft flying above FL285 and up to FL410 from 2,000 feet to 1,000 feet...
(RVSM) can be applied.
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 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 radarRadar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
to provide air traffic control using procedural separation minima
Procedural control
Procedural control is a method of providing air traffic control services without the use of radar. It is used in regions of the world, specifically sparsely-populated land areas and oceans, where radar coverage is either prohibitively expensive or is simply not feasible...
. 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.
Lateral separation
Lateral separation minima are usually based upon the position of the aircraft as derived visually, from dead reckoningDead reckoning
In navigation, dead reckoning is the process of calculating one's current position by using a previously determined position, or fix, and advancing that position based upon known or estimated speeds over elapsed time, and course...
or internal navigation sources, or from radio navigation
Radio navigation
Radio navigation or radionavigation is the application of radio frequencies to determine a position on the Earth. Like radiolocation, it is a type of radiodetermination.The basic principles are measurements from/to electric beacons, especially...
aids ('beacons').
In the case of beacons, to be separated, the aircraft must be a certain distance from the beacon (measured by time or by DME
Distance Measuring Equipment
Distance measuring equipment is a transponder-based radio navigation technology that measures distance by timing the propagation delay of VHF or UHF radio signals....
) 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 (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
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 radarRadar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
system. The actual distance used varies: 5 nmi (9 km) 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 cover.
By US FAA Rules, when an aircraft is:
- 1. Less than 40 miles from the [radar] antenna, horizontal separation is 3 miles from obstructions or other aircraft.
- 2. 40 miles or more from the [radar] antenna, horizontal separation is 5 miles from obstructions or other aircraft.
- 3. Terminal Area For single sensor ASR-9Airport Surveillance RadarAn airport surveillance radar is a radar system used at airports to detect and display the position of aircraft in the terminal area.-Digital Airport Surveillance Radar :...
with Mode S, when less than 60 miles from the antenna, horizontal separation is 3 miles from other aircraft.
Reduced separation
In certain special cases, controllers may reduce separation below the usually required minima.In the vicinity of an aerodrome
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.RVSM
In certain airspace, between 29,000 and 41000 feet (12,496.8 m), pairs of aircraft equipped with more modern altimeter and autopilot systems can be vertically separated by minimum of 1000 feet (304.8 m) rather than the standard 2000 feet (609.6 m).RVSM airspace encompasses Europe, North America, parts of Asia and Africa and both the Pacific and Atlantic oceans.
Conflicts
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 1000 feet 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 another aircraft.
Local conflict
A local conflict occurs if two or more aircraft pass a certain given point (in nearly all cases a certain town). A local conflict occurs, if at least one of the following conditions are met:- The distance in time is 4 minutes or less, and
- The distance in space is 30 flight units or less.
Opposite conflict
An opposite conflict occurs if two aircraft are flying towards each other from opposing directions. Looking at the information on the flight progress stripFlight progress strip
- General :A flight progress strip is a small strip of paper used to track a flight in air traffic control . While it has been supplemented by more technologically advanced methods of flight tracking since its introduction, it is still used in modern ATC as a quick way to annotate a flight, to...
s, a controller can detect an opposite conflict by checking:
- If one aircraft is flying from city A to city B and another from city B to city A,
- If comparisons of the temporal distance of the first plane over city A with that of the second plane over city B and that of the second plane over city A with the first plane over city B lead to a separation of 4 minutes or less at any time during their flights, or
- If comparisons of the topical altitude of the first plane over city A with that of the second plane over city B and then the altitude of the second plane over city A with that of the first plane over city B give a separation of 30 flight units or less at any time during their flights.