Produce Definitions on Spacing, Separation and Segregation

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Produce Definitions on Spacing, Separation and Segregation

47TH ANNUAL CONFERENCE, Arusha, Tanzania, 10-14 March 2008

WP No. 96

Produce Definitions on Spacing, Separation and Segregation

Presented by TOC


The IFATCA Technical and Operations Committee (TOC) has been tasked to investigate the terms separation, spacing and segregation. These terms are used in many ICAO documents yet there is a concern that clear and consistent definitions of these terms may not exist. This paper investigates each term starting at a common English definition then further clarifies each term based on ICAO specific documentation and usage in the aviation environment. The paper also considers how separation, spacing and segregation relate to one another.

TOC has concluded via this paper that there are deficiencies in the ICAO provided definitions for separation and spacing and the lack of a definition for segregation. As such this paper contains draft recommendations to address these concerns.


1.1.  This paper stems from the concern, raised by the Technical and Operations Committee (TOC), that there are various concepts of operation and documentation that use the terms separation, spacing and segregation without providing a clear understanding of what those terms mean and their relationship to one another.

1.2.  The intent of this paper is to investigate current definitions and applications of separation, spacing and segregation, highlight any inconsistency or ambiguity and, if necessary, provide clarification of the terms in the current environment. This paper will also attempt to identify any relationship these terms may have.

1.3.  Each term will initially be discussed separately with the Oxford English Dictionary referred to in the first instance for a definition.



Separation • noun, the action of separating or the state of being separated.

Separation is a derivative of:

Separate • verb, move or come apart. Divide into constituent or distinct elements.

These definitions outline the basic act carried out by separators, be they controller within controlled airspace, or pilot outside of controlled airspace when in positions of conflict with other traffic, obstacles or terrain. However, the definitions are not detailed enough to clearly define separation for aviation use.

2.1.1  Separation is defined in ICAO Doc 9713 The International Civil Aviation Vocabulary:

“Separation – Spacing between aircraft, levels or tracks”

This definition does not progress us beyond the dictionary based interpretation. It is also potentially confusing if we consider the use of the word spacing which is increasingly obtaining its own interpretation when applied in aviation environments.

2.1.2  Assistance in defining ‘separation’ may be achieved by defining the term it is often associated with in many documents, that is minima. Minima is the plural of minimum.

Minimum • noun, the least or smallest amount, extent, or intensity possible or recorded.

adjective, smallest or lowest in amount, extent, or intensity.

If we consider the two terms together – separation minima – we come some way towards a clear understanding of an aviation specific use of separation. That is, to move or divide objects apart by the least or smallest amount possible or recorded. What the least or smallest amount is must be such that it keeps the risk of collision at an acceptable level and is recorded via documentation.

2.1.3  ICAO Doc 9689 Manual on Airspace Planning Methodology for the Determination of Separation Minima provides an aviation specific reference for separation minima under Chapter 1.2 Separation Considerations:

“The required separation between aircraft is generally expressed in terms of minimum distances in each dimension which should not be simultaneously infringed.”

Additionally ICAO Doc 9854 Global Air Traffic Management Operational Concept Chapter 2.7.5 defines separation minima as:

“The minimum displacement between an aircraft and a hazard which maintain the risk of collision at an acceptable level of safety.” …

In the definition above, a hazard is an object or an element that an aircraft can be separated from.

2.1.4  ICAO summarizes the process of minima specification in ICAO Doc 9689 Manual on Airspace Planning Methodology for the Determination of Separation Minima Chapter 1, Factors affecting the development of an airspace planning methodology 1.17 and 1.18:

“Three of the main interdependent parameters that affect the achievement of such a predetermined level of airspace system safety (target level of safety — TLS) for a given traffic density are:

a) aircraft navigation performance;

b) ground and airborne communications performance; and

c) surveillance performance.

These performance capabilities are used to determine airspace design (separation minima/route spacing/sectorization), instrument procedures and air traffic control intervention capability.”

2.1.5  ICAO details methods for States to define minima in ICAO Doc 9689 Manual on Airspace Planning Methodology for the Determination of Separation Minima. Further to this ICAO researches new separation minima concepts via the Separation and Airspace Safety Panel (SASP). Beyond this ICAO specifies minima for use in Chapters 5, 6, 7 and 8 of ICAO Doc 4444 Air Traffic Management and ICAO Annex 11 Air Traffic Services. Current promulgated minima are designed around aircraft vs. aircraft separation or, to a limited extent, aircraft vs. terrain.

IFATCA currently has no policy for the safety components in a separation minima however there is a working paper (WP 83 Santiago 1999) which details the concerns that have been identified with ICAO defined separation minima design process. IFATCA does have policy with regards to the publishing and promulgation of separations specifically lateral and longitudinal separation, IFATCA Manual Page 3273:

“Publications promulgating separation standards are required to include guidance material on practical methods of application and the associated phraseology.”


2.1.6  An important factor in minima design is the Controller Intervention Buffer (CIB) – the buffer required within a separation minimum which allows a controller to identify and react and, for an aircraft to respond so as to avoid a collision when separation minima are breached. IFATCA has policy on CIB in the IFATCA Manual Page 3276:

“The controller Intervention Buffer is defined as:

The time required for the Air Traffic Controller to intervene ensuring that a collision would be averted in the event that a separation standard being applied breaks down. This will include an allowance:

  • to recognize the ‘blunder’,
  • formulate a solution,
  • convey instructions to the pilot,
  • or the pilot to react and cause the aircraft to achieve the required change of trajectory.

A Controller Intervention Buffer should be included in the development and specification of any separation minima where controller intervention is used as a risk mitigator.

All of the parameters used to determine the Controller Intervention Buffer should be part of the documentation of any such separation minima.

ATC systems should be developed with the capability to monitor relevant Controller Intervention Capability (CIC) parameters and warn controllers when they fall outside the values used in determining separation standards in use so that alternative standards can be applied.”


It is important that CIB are quantified in any separation minima in which the controller is identified as risk mitigation for reduction in what would otherwise be the minimum distance required. ICAO Doc 9689 Manual on Airspace Planning Methodology for the Determination of Separation Minima does reference CIBs throughout the document and makes specific reference in some of the formula provided to determine minima. However, ICAO Doc 9689 Manual on Airspace Planning Methodology for the Determination of Separation Minima does not provide the level of detail that is provided in the IFATCA Policy statement on the factors that require inputting when decided what the CIB value is.

2.1.7  The preceding paragraphs give an indication of a more strategic application for separation. That is, separation minima are designed and defined at higher levels for application in a tactical sense by Air Traffic Services (ATS). ICAO gives an example of this strategic delineation within ICAO Doc 9613 Performance-Based Navigation Manual, Chapter 3.1:

“Stakeholders of Performance Based Navigation use the concept at different stages:

  • At a strategic level, airspace planners and procedure designers translate ‘the PBN concept’ into the reality of route spacing, aircraft separation minima and procedure design.”

2.1.8  Another important aspect of separation is responsibility for provision. By responsibility for provision it is meant, ‘who has responsibility for the application, monitoring and maintaining of separation of an aircraft from a specific hazard or hazards?’ ICAO is quite specific in detailing that the default provider of aircraft to aircraft separation within controlled airspace is Air Traffic Control. Delegations are identified within specific procedures, such as flights maintaining own separation in Visual Meteorological Conditions (VMC). This process of application may be considered the tactical aspect of separation.

2.1.9  Considering all the above it is not possible to find a single ICAO definition that fully encompasses the meaning of separation. ICAO does provide a section in Doc 9689 Manual on Airspace Planning Methodology for the Determination of Separation Minima – Separation Considerations Chapter 1.1 which encompasses many of the factors outlined above:

“Separation is the generic term used to describe the action on the part of air traffic services (ATS) to keep aircraft operating in the same general area at such distances from each other that the risk of collision is maintained below an acceptable safe level. Such separation can be applied horizontally and vertically.”

This section identifies that separation is defined by a limit that beyond which the risk of collision is considered too high. It also highlights separation as an ‘action’. The word ‘action’ may be considered either strategically for the design of separations or tactically for the application of promulgated separations.

TOC considered the above statement as a starting point for a definition but felt it deficient in the following areas:

  • Separation is only considered as an ATS function
  • Separation is considered only with regards to aircraft vs. aircraft situations x Other hazards such as terrain are not identified
  • Separation is considered only as a distance. Time is not identified
  • It does not specified where separation is applied

To resolve these deficiencies the statement requires some modification to become a viable definition:

  • Although it is critical that airspace users know who is separating aircraft from hazards, separation provision is considered beyond the scope of defining what separation is. Therefore the definition does not consider provision.
  • The hazards that aircraft require separating against require specification. However it is unadvisable to list specific hazard names within the definition as these may change over time. Therefore highlighting that the action of separating aircraft requires it to be against defined hazards was included.
  • As separation can be applied in 4 dimensions (longitudinally, laterally, vertically and time) a term that encompasses these dimensions is required. The term must also give an indication of the effect of separating – to move apart. Displacement is a term that encompasses both needs.
  • As to where separation can be applied again specific details are not desired as these may change over time. However that an airspace user must know where they are separating or being separated is required.

Considering the above the following is put forward as a useable definition for separation:

The action within defined airspace of keeping aircraft at such displacements from defined hazards that the risk of collision is limited to an acceptable safe level.

2.2 Spacing is a derivative of the word space

Space • noun 1 unoccupied ground or area. 2 a free or unoccupied area or expanse. 3 the dimensions of height, depth, and width within which all things exist and move. • verb 1 position (two or more items) at a distance from one another.

The definition provides us with basic descriptors of what spacing is or how it is done but lacks enough detail to be useful in the aviation environment. ICAO does define spacing in ICAO Doc 9713 The International Civil Aviation Vocabulary:

“Espacement: Le terme espacement comporte une notion de temps en plus de celle de distance.”

Translated this reads:

“Spacing: The term spacing comprises a concept of time in addition to that of distance”

Although this definition introduces the concept of time as a method to define spacing it does not further assist us in placing spacing clearly in an aviation perspective. However aspects that define spacing can be found in other ICAO references.

2.2.1  That spacing can have a dynamic nature is highlighted in ICAO Doc 4444 Air Traffic Management Chapter 8.7.2 Separation Application:

“Note.— Factors which the controller using an ATS surveillance system must take into account in determining the spacing to be applied in particular circumstances in order to ensure that the separation minimum is not infringed include aircraft relative headings and speeds, ATS surveillance system technical limitations, controller workload and any difficulties caused by communication congestion.”

This statement is important as it indicates that in a surveillance environment, spacing must be larger than separation minimum to ensure that the minimum is not infringed. This can also be said for non-surveillance environments where time and distance separations still set a limit for aircraft spacing.

2.2.2  The most obvious example of the dynamic consideration above is that of spacing required for sequencing aircraft on the approach. Under the current environment succeeding flights must be spaced at such distances as to ensure that once vertical separation is relaxed they will not breach longitudinal separation with the aircraft ahead. Either because:

  • For similar types as the preceding slows the succeeding will catch up;
  • Of dissimilar aircraft types with a slower aircraft in front;
  • Other factors such as the desire by the tower to depart aircraft.

ICAO Doc 4444 Air Traffic Management Chapter Interval Between Successive Approaches provides greater detail on such factors:

“In determining the time interval or longitudinal distance to be applied between successive approaching aircraft, the relative speeds between succeeding aircraft, the distance from the specified point to the runway, the need to apply wake turbulence separation, runway occupancy times, the prevailing meteorological conditions as well as any condition which may affect runway occupancy times shall be considered. When radar is used to establish an approach sequence, the minimum distance to be established between succeeding aircraft shall be specified in local instructions. Local instructions shall additionally specify the circumstances under which any increased longitudinal distance between approaches may be required as well as the minima to be used under such circumstances.”

Another example of spacing’s dynamic nature is the increased spacing applied by controllers during aircraft uncertainty situations. This spacing may vary depending on the conditions that are encountered and is used as a buffer to mitigate the increased risk of a separation breach. This application is commonly referred to as increased separation. ICAO Doc 4444 Air Traffic Management Chapter

Larger separations than the specified minima should be applied whenever exceptional circumstances such as unlawful interference or navigational difficulties call for extra precautions. This should be done with due regard to all relevant factors so as to avoid impeding the flow of air traffic by the application of excessive separations.

In effect these ‘larger separations’ are perhaps more accurately described as the increased spacing of flights as the separation minima have not changed and the new spacing chosen has no methodology to back up its determination beyond the controller’s assessment.

2.2.3  Spacing aircraft on an approach or application of increased separation are examples of the tactical use of spacing and are currently, in the majority of cases, carried out by the controller. However, spacing can also be used strategically. In the strategic sense a basic spacing example is as part of an Air Traffic Flow Management (ATFM) system via slot allocation. ICAO Doc 9426 Air Traffic Services Planning Manual, Part Two Chapter

“Measures to regulate demand may take several forms. In simpler systems, ACCs exercising flow control to surrounding area control centres disseminate notices requesting that affected aircraft be spaced at prescribed intervals, e.g. one every ten minutes.”

This example also shows spacing being applied in a rigid or non-dynamic way. In this case by way of time at 10 minute intervals.

2.2.4  Strategic spacing applications also include route spacing. That is the process of defining the spacing between route structures particularly parallel or near parallel routes. ICAO Doc 9613 Performance-Based Navigation Manual, Volume 1 Chapter 3.2:

“…route spacing can generally be described as being a function of three factors: navigation performance, aircraft’s exposure to risk and the mitigation measures which are available to reduce risk…”

Additionally ICAO Annex 11 Air Traffic Services Chapter 2.12.1 Establishment and identification of ATS routes:

“When ATS routes are established, a protected airspace along each ATS route and a safe spacing between adjacent ATS routes shall be provided.”

As such route spacings should include additional distance above the separation minima that are applicable to the environment (en-route or terminal) to cater for the inaccuracy that can exist in the variables of Communication, Navigation and Surveillance (CNS) and any mitigating factors.

Thus we see that spacing is generally a dynamic application of either distance or time between aircraft that is greater than the separation minima applicable to the environment. This summary of spacing is reflected in ICAO Doc 9854 Global Air Traffic Management Operational Concept which defines spacing as:

“Any application of a distance or time between an aircraft and a hazard at or above separation minima in order to maintain a safe and orderly flow of traffic.”

Within TOC discussion it was decided that this definition could provide a basis for an IFATCA definition. However some changes are required:

  • To remain consistent with the definition established for separation, and to which spacing relates, the term displacement should be used;
  • The ICAO definition defines spacing at or above the separation minima. TOC discussed this issue and felt that having spacing at separation minima confused the terms. The definition needs to clarify that spacing is an additional displacement to separation minima;
  • The use of separation minima was seen as too generic. There is a need to clarify that spacing is applied to each separation minimum as required.

Considering the above the following is put forward as a useable definition for spacing:

The application of an additional displacement to a specific separation minimum between an aircraft and a hazard.

2.3 Segregation is derived from the word segregate

Segregate • verb, set apart from the rest or from each other

This definition is perhaps the closest of the three terms to one that could be used as is in an aviation context and is distinct from separation and spacing by setting a certain item or group of items apart from the rest. That is to say segregation creates an exclusive subset of a set.

2.3.1 Although ICAO does use the term segregation throughout its documents there is no stand alone definition. Definition can only be obtained by reference to applications.

The most obvious example of segregation’s use is in the division or reserving of airspace. There are several examples of this:

  • The segregation of aircraft in controlled airspace and those outside of it by way of an ATC clearance;
  • The establishment of restricted, military or special use airspace. In this example defined airspace is allocated for the exclusive use of one group of users while excluding the rest. Approval for access is required, either by the separation provider or an airspace governing authority.

2.3.2 Other common uses for segregation are the division of traffic flows for reasons of safety, controller workload or increasingly, efficiency. Under current airspace design and ground based navigation aid focus, this has been used as a reasonably blunt tool. Examples are:

  • Structured route systems. Designed to harmonise traffic flows and reduce controller workload by reducing crossing or merging tracks. Standard Instrument Departures, STandard Arrival Routes (SIDs and STARs) and one way route structures are prime examples of this type of segregation. ICAO Doc 9426 Air Traffic Services Planning Manual Doc, Part II Section 2 Chapter 4 Appendix A, 1.1 identifies this:

“Standard departure and arrival routes should:

a) segregate traffic operating along different routes, and such traffic from traffic in holding patterns;”

  • Segregation of levels i.e. ‘odds and evens,’ aligned to the predominant bi-directional traffic flow of a region are commonly used. These are rigidly applied, reduce controller workload within controlled airspace and allow safe operation in uncontrolled environments;
  • Segregated parallel runway operations. As defined by ICAO Doc 4444 Air Traffic Management, Chapter 1 Definitions:

“Segregated parallel operations. Simultaneous operations on parallel or near-parallel instrument runways in which one runway is used exclusively for approaches and the other runway is used exclusively for departures.”

2.3.3  Segregation can also be used to facilitate greater efficiency and economic saving beyond a straight aircraft operating cost. Transponder mandatory airspace is an example. ICAO Doc 9426 Air Traffic Services Planning Manual, Part I – Planning factors Section 2, Chapter 9.3.8 – Advanced ATS systems:

“As the use of SSR expands and takes account of developments in other fields aimed at the segregation of air traffic according to types of operation, it may be possible to arrive at situations where all the traffic, operating within a defined portion of the airspace, i.e. at higher altitudes, will be SSR equipped. If such conditions are created and transponder reliability has reached a point where failures are rare, it may be feasible to rely on SSR without primary radar. Such an arrangement would present a very appreciable economy in the investments required.”

Regulation for enforcement of equipage and facilitating non-compliant airspace users are both large issues with this type of segregation application.

2.3.4  All of the examples above give an indication of segregation as a strategic tool. By strategic it is meant that, if applied rigidly, segregation can reduce the frequency of conflict and subsequently the need for controller intervention and increase efficiency. . ICAO Doc 9426 Air Traffic Services Planning Manual implies this strategic use when identifying segregation as a tool used in the application of Airspace Management (ASM), Part Two section 1 Chapter 1.2.3:

“The objective of ASM is to maximize, within a given airspace structure, the utilization of available airspace by dynamic time-sharing and, at times, segregation of airspace among various categories of users based on short-term needs” …

2.3.5  This does not mean that there is not a tactical aspect to segregation. Though procedures may be designed at a strategic level, to facilitate the most efficient use of airspace the ATM system will still rely on controllers to apply such procedures in the tactical environment.

2.3.6  Another important aspect of segregation is that it can be applied without reference to separation or spacing. For example in the design of some special use airspace both the aircraft within or outside of the segregated area may operate up to the boundary. What limits movement across the boundary is an operational characteristic such as mode of operation (military etc) equipage (Transponder mandatory airspace) or clearance (controlled airspace). The reason for segregation may vary but in all applications the effect is the same.

2.3.7  Whether separation/spacing play a part or not the effect of segregation is still the same an exclusive subset of airspace users is created. This may not always be reflected in a physical sense but may be by way of an airspace user’s validity for a procedure or route.

2.3.8  When considering the above TOC required a definition that encompassed the basic meaning of segregation but highlighted how this was achieved in the ATM environment. The following is put forward as a useable definition for spacing:

The application of procedures and design with the purpose of setting apart an exclusive subset of users of the ATM system, based on a defined operational characteristic.

2.4 Finally it is important to highlight the relationships that the three terms have with one another. As spacing is a buffer for variables within the ATM system its displacement may vary but must always include the separation minimum. The separation minimum defines the lower limit for spacing. When the lower limit is reached the separator no longer provides spacing but only separation and will have decided there are no variables within the system that can affect that particular separation.

Segregation may be applied without reference to separation or spacing as it works by looking at airspace users only by way of operational characteristics such as in the design of special use airspace or Land And Hold Short (LAHSO) avoidance procedures. Separation minima may be designed for application from segregation applications such danger or restricted areas. Segregation can be used as a tool to affect the levels of separation and spacing application by the creation of route procedures or structures that inherently include separation minima and spacing buffers.


3.1  It is possible to view separation, spacing and segregation as having strategic and tactical applications. Strategic generally refers to the design and promulgation process while tactical refers to application by ATC of such promulgated procedures.

3.2  Separation, spacing and segregation all affect one another. Separation minima define the limit for spacing while separation (and therefore spacing) may be applied with due regard to any segregation of airspace. Segregation can be applied without regards to separation and spacing or may be used directly to affect separation and spacing application levels via route or airspace design/eligibility criteria.

3.3  The current default provider of separation in controlled airspace is ATS but this can be delegated in certain instances for defined periods. Such delegations are promulgated and specify the target of delegation.

3.4  Creation of separation minima is critical. Both separation minima and spacing applications use CNS factors as the basis for design and application respectively.

3.5  Current ICAO definitions for separation do not provide clarification on separation’s meaning. There are several ICAO references that provide statements which can be used as the basis of a definition.

3.6  Spacing can be applied dynamically or as a rigid value. Either way it must be applied so that the times or distances used are greater than the applicable separation minima.

3.7  Although segregation can be applied in different ways it essentially achieves the same result on each application. That is to create an exclusive subset of a group of airspace users.

3.8  The lack of an ICAO definition for segregation is a concern as it is widely used in many documents.


It is recommended that;

4.1  Separation is defined as:

The action within defined airspace of keeping aircraft at such displacements from defined hazards that the risk of collision is limited to an acceptable safe level.

and is included in the IFATCA Manual on page 3 2 7 7.

4.2  Spacing is defined as:

The application of an additional displacement to a specific separation minimum between an aircraft and a hazard.

and is included in the IFATCA Manual on page 3 2 7 7.

4.3  Segregation is defined as:

The application of procedures and design with the purpose of setting apart an exclusive subset of users of the ATM system, based on a defined operational characteristic.

and is included in the IFATCA Manual on page 3 2 7 7.

Last Update: September 29, 2020  

April 11, 2020   279   Jean-Francois Lepage    2008    

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