Study Visual Separation on Approach

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Study Visual Separation on Approach

51ST ANNUAL CONFERENCE, Kathmandu, Nepal, 12-16 March 2012

WP No. 91

Study Visual Separation on Approach

Presented by TOC


After a presentation on Airborne Separation Assistance System (ASAS) during the annual IFATCA Conference in Amman in 2011, it was felt necessary to investigate the concept of the ASAS application Visual Separation on Approach (ATSA-VSA). TOC has studied the subject and addressed the concept of ATSA-VSA in regard to ICAO regulations and guidelines for visual separation. This paper recommends IFATCA Policy on the naming of ATSA-VSA.


1.1 During the annual IFATCA Conference in Amman in 2011 a presentation was given on Airborne Separation Assistance System (ASAS). It was felt needed to study the concept of visual separation in regard to the use of ASAS as an outcome of this presentation.

1.2.  The Technical and Operations Committee (TOC) was tasked to investigate the use of Visual Separation on Approach, and to study the consequences when using this concept in new technologies like ASAS-applications in terminal airspace.

1.3.  This study is intended to address the proper use and procedures of visual separation applied by air traffic controllers and/or pilots in regard to ASAS-applications.


2.1 Visual separation on approach is often used by ATC and pilots to apply reduced separation, and thereby to increase landing capacity and/or (fuel) efficiency.

2.1.1  ICAO allows the use of visual separation, under specified conditions, as a form of reduced separation. The actual distance between aircraft applied with visual separation is in general considerably lesser than the standard separation methods.

2.1.2  With visual separation the responsibility for the separation can be transferred from ATC towards flight crew. In ATM this concept is also known as “to maintain own separation”. Nevertheless, with new technologies this concept could not only be limited to visual separation, but could be extended with surveillance techniques like ADS-B (e.g. ITP) and new technologies like ASAS.

2.1.3  ASAS is an aircraft system that aims to enable flight crew to maintain separation from one or more other aircraft, and the system provides flight information concerning the surrounding traffic. An application of ASAS, which is under development, is Air Traffic Situation Awareness (ATSA). This application can be divided in ATSA-AIRB (basic airborne), ATSA-VSA (enhanced visual separation on approach) and ATSA-SURF (basic surface). ATSA is believed to enhance the operation of visual separation by providing improved and reliable visual acquisition of aircraft of interest and thereby extending the use of “own separation” clearances. In addition to the traffic information provided by ATC, a Cockpit Display of Traffic Information (CDTI) will support the flight crew in the search of reference aircraft whenever these other aircraft are equipped with ADS-B OUT.

Figure 1 Functional Diagram Cockpit Display of Traffic Information

Figure 2 Cockpit Display of Traffic Information  Pilots should always maintain visual contact with the reference aircraft and ATSA-VSA cannot replace this “visual contact”. When “visual contact” with the reference aircraft is lost, this should immediately be reported to ATC and from that moment on the operation of “maintaining own separation” must be terminated. In general the use of ATSA-applications by flight crews is expected to be according standard procedures and regulations. However, the system could also tempt flight crews in unlawful and/or incorrect use of the system. Changing meteorological conditions (e.g. in and out clouds), inadequate identification procedures and non-existing standard phraseology are some of the key factors that could influence the correct use of ATSA-applications.  Flight crews will require sufficient training how to interpret the traffic displayed during all common situations (e.g. during en-route, approach, departure and taxi). Vehicles on an aerodrome, operational military aircraft and even ships (in a close by harbor) could show up on a CDTI with unknown labels and thereby confusing the flight crew if they lack adequate training. For identification purpose flight crews require also training in ICAO designators for Aircraft Operator Agencies (e.g. BAW007, JTR115, SAA212), and knowledge about the ICAO telephony designators, like in the case of the examples; Speedbird, Herbie and Springbok. Although pilots receive training in Wake Turbulence Categorization (WTC) separation, it is unlikely that all pilots are familiar with the required minimum WTC separation in nautical miles in regard to all aircraft types as published by ICAO. Furthermore the use of the CDTi of ATSA-VSA will increase head-down operations on the flightdeck.

2.2 ICAO

2.2.1 ICAO Doc 4444, Chapter 5 Separation methods and minima:

5.9 Clearances to fly maintaining own separation while in visual meteorological conditions.

Note 1.— As indicated in this Section, the provision of vertical or horizontal separation by an air traffic control unit is not applicable in respect of any specified portion of a flight cleared subject to maintaining own separation and remaining in visual meteorological conditions. It is for the flight so cleared to ensure, for the duration of the clearance, that it is not operated in such proximity to other flights as to create a collision hazard.

Note 2.— It is axiomatic that a VFR flight must remain in visual meteorological conditions at all times. Accordingly, the issuance of a clearance to a VFR flight to fly subject to maintaining own separation and remaining in visual meteorological conditions has no other object than to signify that, for the duration of the clearance, separation from other aircraft by air traffic control is not provided.

When so requested by an aircraft and provided it is agreed by the pilot of the other aircraft and so authorized by the appropriate ATS authority, an ATC unit may clear a controlled flight, including departing and arriving flights, operating in airspace Classes D and E in visual meteorological conditions during the hours of daylight to fly subject to maintaining own separation to one other aircraft and remaining in visual meteorological conditions. When a controlled flight is so cleared, the following shall apply;

a) the clearance shall be for a specified portion of the flight at or below 3.050 m (10.000 ft), during climb or descent and subject to further restrictions as and when prescribed on the basis of regional air navigation agreements;

b) if there is a possibility that flight under visual meteorological conditions may become impracticable, an IFR flight shall be provided with alternative instructions to be complied with in the event that flight in visual meteorological conditions (VMC) cannot be maintained for the term of the clearance;

c) the pilot of an IFR flight, on observing that conditions are deteriorating and considering that operation in VMC will become impossible, shall inform ATC before entering instrument meteorological conditions (IMC) and shall proceed in accordance with the alternative instructions given.

2.2.2 ICAO Doc 4444, Chapter 6 Separation in the vicinity of aerodromes:

6.1 In addition to the circumstances mentioned in Chapter 5, 5.11.1, the separation minima detailed in Chapter 5, 5.4.1 and 5.4.2, may be reduced in the vicinity of aerodromes if:

a) adequate separation can be provided by the aerodrome controller when each aircraft is continuously visible to this controller; or

b) each aircraft is continuously visible to flight crews of the other aircraft concerned and the pilots thereof report that they can maintain their own separation; or

c) in the case of one aircraft following another, the flight crew of the succeeding aircraft reports that the other aircraft is in sight and separation can be maintained. Radar monitoring shall not be terminated until:

a) visual separation is applied, provided procedures ensure that both radar controllers are advised whenever visual separation is applied; […] ICAO normally considers “in the vicinity of aerodromes”, in regard to visual separation, to be the “traffic pattern”. The requirement that aerodrome controllers must have the relevant aircraft visible obviously restricts the applicable range for visual separation.

2.2.3  ICAO Doc 4444 Chapter 12 Phraseologies: Approach instructions:


2.2.4  ICAO Doc 9426 Air traffic Services Planning Manual:

1.1.7 In any case, the determination of the prescribed separation minima is a complex process, which needs to take account of numerous factors, many of which are outside the scope and competence of ATC. Frequently it will be left to the individual controller to determine, based on sound judgement, what separation is adequate for a specific situation. However, once separation minima are established by the competent authority, it is incumbent upon ATC to ensure that the established minima are not infringed upon.

2.3 IFATCA Policy

2.3.1  IFATCA has no Policy on “visual separation”.

2.3.2  IFATCA Policy on ASAS (WP 93 – Istanbul 2007):

Where ASAS-applications are implemented, a clear and unambiguous statement for separation responsibility is required.

ASAS-applications shall meet all appropriate safety levels.

The publication of MOPS (Minimum Operational Performance Specifications) for CDTIs by ICAO, or another internationally recognized organisation is urgently required.

New ICAO phraseology shall be developed before ASAS applications are deployed. Identification issues need to be resolved prior to implementation of ASAS applications.

Procedures shall be in place to ascertain that any action taken by a crew in the use of ASAS applications will not generate additional conflict.


2.3.3  TOC is of the opinion that the IFATCA Policy on ASAS is still valid and solid. IFATCA Policy addresses the responsibility for separation and contains adequate and solid requirements for the safe use of ASAS-applications, and thus consequently for ATSA.

2.3.4  An important aspect in the safe operation of ASAS-applications is the reliability and availability of surveillance data. Although this reliability and availability is addressed in IFATCA Policy with the requirement for ASAS-applications to meet all appropriate safety levels, TOC expects still a considerable amount of work to be done before this requirement will be achieved, and therefore IFATCA should continue to monitor the future implementation of ASAS-applications.

Figure 3 Required data for the correct operation of ASAS-applications

2.3.5 Another safety aspect with the reliability and availability of surveillance data is the awareness of flight crews and ATC with this possible absence of correct surveillance data. For example, not all aircraft will carry this technology and are therefore undetectable.

2.4 IFALPA Policy

2.4.1 IFALPA Policy on “visual separation” and “maintaining own separation”:

The Federation maintains that Air Traffic Control systems should evolve so as to progressively reduce the number of occasions where clearances to fly maintaining own separation while in Visual Meteorological Conditions render an operational advantage.

The Federation discourages pilots from accepting clearances to maintain their own visual separation because the risk of collision is increased.

2.5 A quote from an FAA Air Traffic Bulletin article on visual separation dated Summer 2000:

“visual separation is where you place aircraft on a certain path where you can monitor their progress and assure that they are in no danger of colliding, and maintaining this vigilance until lateral, vertical or pilot separation is in place.”

2.5.1  The “pilot separation” in this previous quote refers to pilots instructed and/or requesting to maintain their own separation, which consequently could be visual separation or even on the basis of new surveillance methods. The concept of “maintaining own separation” in visual meteorological conditions with new techniques like ADS-B and ASAS is not yet addressed by ICAO.

2.5.2  In the opinion of TOC, the quote in the FAA Air Traffic Bulletin is an excellent definition of the concept of visual separation.

2.6 According to UPS, the company already uses the concept of ATSA-VSA at Louisville in the USA. UPS has equipped almost its whole fleet with ADS-B CDTi to enable the operation of ATSA-VSA. The Radio Technical Commission for Aeronautics (RTCA) has published a document in 2008, RTCA DO-314 Safety, Performance and interoperability Requirements Document for Enhanced Visual Separation on Approach, on which the FAA has accepted the operation of ATSA-VSA in the USA.

2.6.1 Although UPS estimates benefits, by the use of the CDTi at Louisville, to be an increase of 20% of the throughput, local ATC claim that nothing has changed in their operation of visual separation with UPS and also does not observe any difference in pilot behaviour. Furthermore ATCOs at Louisville have received no training or briefings regarding the operation of ATSA-VSA at Louisville, which should have been realized according to RTCA DO-314 (approved by the FAA) before the implementation of this concept.

2.7 The implementation of ASAS-applications in aircraft increases the situational awareness of pilots in regard to the surrounding traffic. This could assist in the visual acquisition of conflicting traffic and support the use of visual separation. According to published guidance material from Eurocontrol, the CASCADE Programme expects some more benefits through the use of ASAS- applications:

a) Reduced voice communications associated with traffic information;

b) Improvement of safety of operations, as it is anticipated that ASAS will decrease the likelihood of wake turbulence encounters;

c) Efficiency benefits to be derived when the preceding and succeeding aircraft are approaching the same runway because of a reduction in the number of missed approaches.

2.7.1  TOC is of the opinion that the expected benefits by the CASCADE Programme of Eurocontrol are a bit too ambitious:

  • Voice communication associated with traffic information will probably not be reduced much, as long as ATC is required, by ICAO SARPs, to issue (essential) traffic information.
  • Encounters of wake turbulence are more likely to increase due to the fact that ATC is bonded to greater (WTC) separation than the reduced visual separation which is encouraged by the use of ASAS-applications.
  • In the opinion of TOC in regard to separation, many of the missed approaches occur not due to insufficient separation on final, but due to the runway occupancy time of preceding aircraft. Due to the expected reduced separation, missed approaches are even more likely to increase. Although statistics (see next paragraph) also show that the majority of go-arounds are caused by other than separation reasons.

2.7.2  Causes of go-arounds at Schiphol Amsterdam:

Figure 4 Causes Go-Arounds at Schiphol

Considering the reports of go-arounds at Schiphol in 2010, only 6% of the go-arounds is caused by separation on final. The percentage of runway occupancy of leading aircraft is 11%, however a large part of this runway occupancy is caused by the leading aircraft (not vacating the runway in time), and ATSA-VSA would not have prevented the majority of these specific go-arounds.

2.8  The use of ASAS-applications for the purpose of identification and applying visual separation must be regulated and in accordance with ICAO guidelines. By developing standard procedures for identification via ASAS-applications, the risk of misidentification through the use of ASAS- applications should thereby be kept to an absolute minimum. And although the ICAO requirements for applying visual separation are very clear, there is a clear and present danger that flight crews will ignore some of these requirements for their (operational) benefit.

2.9  In regard to the implementation of ASAS-applications it must be very clear and unambiguous who is responsible for separation, the controller or the pilot. Phraseology and procedures for identification and transfer of separation responsibility need to be developed before implementation of new techniques like ASAS and ATSA, and the level of safety must be demonstrated.

2.10  The use of ASAS-applications in terminal airspace could increase efficiency and landing capacity. Nevertheless, the workload of the flight crew is especially high in terminal airspace. This high workload should be taken into consideration when the delegation of separation responsibility is considered, and not only with the use of ASAS-applications, but also with the common use of delegating responsibility of (visual) separation by ATC.

2.11  A study of the FAA in the USA (“Situation Awareness in Air Traffic Control”) on the free-flight concept proved that systems like ASAS-applications only operate, without compromising safety, if appropriate support within the ATC system is established.

2.12.1  Another aspect, which needs to be considered, is the harmonization of visual approaches. Aircraft operators tend to have different “standards” in regard to all kind of operational procedures. For example, different standards in crosswind limitations or in low visibility. This is also the case with visual approaches. Some aircraft operators do not allow visual approaches according to their “company rules”.

2.12.2  When ATC clears pilots for a visual approach; the exact procedure is generally not prescribed in detail. Pilots are normally free to determine at which point they turn to base leg, speeds are often also pilot’s discretion and altitudes are not always assigned during a visual approach. Pilots tend to have different habits about how to fly a visual approach. For example, ATCOs normally expect pilots, flying a visual approach, to fly a reasonable short visual circuit. However, sometimes pilots extend the downwind leg more than ATC would expect. These unknown factors could also complicate the operation for ATC.

2.13.1  TOC considers the present state and development of ASAS-applications yet too immature to be implemented as separation methods in current ATM. However presently, the ASAS-concepts can be considered and used as a sensible way to improve traffic situational awareness of flight crew. With the development of ASAS-applications, ICAO should develop SARPs and guidelines for the safe implementation of these applications, especially when implemented as separation methods. Whereas IFATCA Policy provides solid and valid requirements in regard to the future implementation of ASAS-applications

2.13.2  According to TOC the naming “ATSA for Enhanced Visual Separation on Approach (ATSA-VSA)” is misleading in regard to the wording “enhanced visual separation”. The objective of ATSA-VSA is, in the opinion of TOC, merely to support the flight crew (with the help of the CDTi) in visually acquiring the position of surrounding aircraft when performing a visual approach. According to TOC, the concept of ATSA-VSA should mainly be considered as a tool, just like a binocular is for ATC.

2.13.3 The wording “enhanced visual separation” in ATSA-VSA might suggest that present visual approach procedures are changed and/or even improved. Nevertheless, ICAO has not changed any SARPs in regard to “maintaining own separation while in visual meteorological conditions” (ICAO Doc 4444), also responsibilities and procedures for both ATC and flight crew do not change with ATSA-VSA.

2.14 With the fast development of modern technology it is to be expected that the operation of ATC be subject to changes. Nevertheless, the delegation of separation is of such essence that changes in responsibility will require substantial preparation and establishment of solid regulations.


3.1  According to ICAO, “Visual separation” in the vicinity of aerodromes and in Class D and E airspace up to 10.000 ft is considered as an acceptable method for separating air traffic by ATCOs. According ICAO publications, “maintaining own separation in visual meteorological conditions” by pilots is not considered as “visual separation” as in an air traffic control separation method, but as a delegation of responsibility for separation to the pilot.

3.2  According to ICAO publications, ATCOs must have each relevant aircraft continuously visible when applying visual separation. This clearly restricts the applicable range for using visual separation by ATC in the vicinity of aerodromes. However pilots, out of the visual range of ATC, can also apply visual separation themselves. And this must be under visual meteorological conditions, specified airspace classification and authorized by the appropriate ATS authority.

3.3  Visual separation applied by ATC can be considered safe when the aircraft concerned are continuously visible to the tower controller, and can be placed on a certain path where they will be continuously monitored and therefore providing the assurance that they are in no danger of colliding.

3.4  Flight crew “maintaining own visual separation” instructed by ATC can be considered safe when the traffic of interest is continuously visible to the flight crew, and that their own aircraft is placed on a certain path to provide the assurance that they are in no danger of colliding.

3.5  ICAO should address separation methods and associated phraseology to be implemented with new technologies. Till then the concept of “visual separation” will remain the same, and “enhanced” visual separation does not exist.

3.6  The description “ATSA for Enhanced Visual Separation on Approach (ATSA-VSA)” is misleading in regard to the wording “enhanced visual separation”. To prevent misunderstanding, the description for ATSA-VSA, “Enhanced Visual Separation on Approach”, shall be changed to: “Enhanced traffic situational awareness during visual separation”.

3.7  ICAO procedures on Visual Separation are adequate and sufficient. Before the implementation of ASAS-applications as separation methods, ICAO should develop SARPs and guidelines to accommodate this.

3.8  IFATCA Policy on ASAS-applications is valid, and relevant to ATSA-VSA. IFATCA should continue to monitor the development and implementation of ASAS-applications.


It is recommended that;

4.1. IFATCA Policy is:

The description for ATSA-VSA, “Enhanced Visual Separation on Approach”, shall be changed to: “Enhanced traffic situational awareness during visual separation”.

And is included in the IFATCA Technical and Professional Manual.

Last Update: September 30, 2020  

April 19, 2020   167   Jean-Francois Lepage    2012    

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