Separation (aeronautics)

In air traffic control, separation is the name for the concept of keeping an aircraft in 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.

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 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. 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.

Vertical separation

Between the surface and an altitude of 29,000 feet (8,800 m), no aircraft should come closer vertically than 1,000 feet or 300 metres (in those countries that express altitude in metres), unless some form of horizontal separation is provided. Above 29,000 feet (8,800 m) no aircraft shall come closer than 2,000 feet (or 600 m), except in airspace where Reduced Vertical Separation Minima (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 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.

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 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

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 15 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 radar 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 separtion 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-9 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 (i.e. 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

Main article: Reduced Vertical Separation Minima

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.

See also

• Traffic Collision Avoidance System