Co-operative Separation

Co-operative Separation

43RD ANNUAL CONFERENCE, Hong Kong, China (SAR), 22-26 March 2004

WP No. 158

Co-operative Separation


This topic has been known in the past as Transfer of Control Functions to Pilots, and has been on SC4’s working programme since the 37th Annual Conference held at Toulouse, France, in 1998.

Existing IFATCA Policies are:

Transfer of separation function – human factors aspects and legal aspects (page 4125 para 2.7 and page 4412 para 1.1.10):

“From a Human Factors aspect IFATCA has strong concerns over the transfer of control responsibility to the cockpit for the following reasons:

If separation functions are transferred to the cockpit the situation awareness and skills base of the ATCO will be degraded to the point when intervention will not be possible. Aircrew workload will increase by fulfilling additional tasks, which are currently carried out by ATC. This might lead to overload situations in cockpit workload when other, higher priority, tasks have to be taken care of by the crew. Responsibility for the control function cannot simply be handed back to the controller.”


Legal Liability (page 4415 para 1.2.21 till 1.2.23):

“States must have in place regulations detailing procedures to be followed before Separation Assurance can be transferred to the cockpit.

The legal responsibility for Separation Assurance must be defined in the regulations governing the use of the procedure.

The Initial and final points at which Separation Assurance is transferred from ATC to the pilot must be accurately defined in all cases.”


Working Papers 160/02 and 157/03 described the concept of Transfer of Control Functions to Pilots, and advised that this concept was to be called Co-operative Separation and that the Air Traffic Management Operational Concept Panel (ATMCP) of ICAO was mandated to undertake the necessary studies for identifying the best technically practical and operationally feasible ICAO provisions in developing a gate-to-gate air traffic management operational concept that would facilitate the evolutionary implementation of a seamless, global ATM system. The planning horizon is up to and beyond year 2025. IFATCA is represented on the ATMCP, through the EVPT.

Cooperative Separation was on the agenda of the 11th Air Navigation Conference (ANC) held in Montreal from 22 September to 3 October 2003. Committee C at Buenos Aires recommended that this Topic, Co-operative Separation, be kept on the work programme of SC4.


Economic growth is demanding ATM to enhance air traffic capacity and flight efficiency while providing the same level of safety. However, the forecasted traffic density growth in Europe and in the United States over the next fifteen years suggests that solely improving ground systems might not be sufficient to achieve the required capacity at appropriate safety levels. The development of a close co-operation between ground and airborne sides might be required to achieve these goals, and the delegation of separation assurance from controllers to pilots is one option. It takes advantage of emerging CNS/ATM technologies in pre-operational state n ADS-B along with additional avionics such as a Cockpit Display of Traffic Information (CDTI) providing the pilot with a picture of surrounding traffic.

Vision for ATM

The ATMC Panel has agreed on the following vision of the ATM Operational Concept:

“To achieve an interoperable Global ATM system for all users during all phases of flight that meets agreed level of safety; provides for optimum economic operations; is environmentally sustainable, and meets national security requirements.”

Definition of Co-operative Separation

The ATMCP has made an Air Traffic Management concept. In this concept co-operative separation is described as follows:

“Co-operative separation is when the role of separator is delegated. This delegation is considered temporary and the condition that terminates the delegation will be known. The delegation can be for types of hazards or from specified hazards. If the delegation is accepted, then the accepting agent is responsible for compliance with the delegation, using appropriate separation modes.”

Note : Participation in separation provision does not necessarily mean co-operative separation. Co-operative Separation refers to the delegation of the role of separator, not simple compliance with instructions or suggestions.

Transfer of Separation Responsibility (A Eurocontrol Development)

Definition of responsibility

As a legal foundation, the controller is responsible for maintaining the separation between aircraft. This responsibility regarding separation between two aircraft in a “station-keeping link” can under certain conditions be delegated to the commander of the trailing aircraft in that link. Such delegated separation responsibility is called “assigned separation”. The legal responsibility for maintaining the assigned separation between aircraft in a link executing station keeping rests solely with the Commander of the trailing aircraft.

Levels of delegation

Eurocontrol has defined three levels of delegation of separation assurance. In an increasing order these can be defined as limited, extended and full delegation.

Limited delegation: The controller is in charge of both identification of a situation and management of a solution to a situation. Only implementation of solution and monitoring is delegated to the crew.

Extended delegation: The controller is in charge of identification of a situation and delegate to the crew, identification and implementation of a solution and the monitoring.

Full delegation: The crew is responsible for all tasks related to separation assurance, identification of situations and solutions, implementation and monitoring.

Transfer of responsibility

The separation responsibility, when applying station keeping, is transferred from ATC to the Commander of the trailing aircraft through instruction over VHF radio. After the Commander accepts and confirms through a read back of the separation instruction, the responsibility for assigned separation rests solely with the Commander. The responsibility for assigned separation is transferred back to the controller whenever the controller through an instruction over VHF radio, with a pilot confirmation read back, resumes the separation responsibility. The crew can also surrender this responsibility to the controller for any.

Technologies being developed for use in Co-operative Separation


Distributed Air/Ground (DAG) Traffic Management (TM) is a National Airspace System (NAS) concept in which flight deck (FD) crews, air traffic service providers (ATSP) and aeronautical operational control (AOC) facilities, use distributed decision-making to enable user preferences and increase system capacity, while meeting air traffic management requirements. DAG-TM will be accomplished with a human-centred operational paradigm enabled by procedural and technological innovations. These innovations include automation aids, information sharing and Communication, Navigation, and Surveillance (CNS) / Air Traffic Management (ATM) technologies.


CE5 En route free manoeuvring

CE6 En route trajectory negotiation

CE11 Self-spacing for merging and in-trail separation


Automatic Dependent Surveillance-Broadcast (ADS-B) is designed to support aircraft separation management by providing improved surveillance information (including intent) at ranges in many cases exceeding 100 miles, allowing improved situational awareness and enhanced decision making. ADS-B is a technology that depends on non-interrogated, unsolicited, or automatic transmission of position by an aircraft. The “own-ship” position and other “surveillance” information, such as velocity, intent, flight identification, etc. are determined by or dependent of the aircraft systems (e.g., Global Navigation Satellite System (GNSS) receiver, air data computer and other equipment). The aircraft then broadcasts this information so that ATC and other aircraft, which are in range and equipped with the appropriate receivers, can receive the transmission. Pilots can utilize the data for general situational awareness and for the performance of specific operational applications.


Cockpit Display of Traffic Information (CDTI) is necessary to derive the full benefits of ADS-B technology on an aircraft. With a CDTI display, the flight crew will be able to see appropriately equipped aircraft on a cockpit display. With the CDTI, the air traffic controller and the flight crew benefit from a common picture of the aircraft traffic. This allows enhanced Air Traffic Management (ATM) because the air traffic controller can provide improved instructions to the flight crew. For example, the air traffic controller can now point out other aircraft by flight identification in addition to providing clock position, range and aircraft category. The CDTI can also display a moving electronic map of the airport surface while the aircraft is on the ground, improving situational awareness. Traffic and other (NOTAM etc.) information can be overlaid graphically on this display, leading to further situational awareness and safety improvements. In the longer term, the above-mentioned CDTI capabilities, when properly enhanced, will allow “free flight” in designated airspace.


In addition to beneficial airspace applications enabled by CDTI and improved procedures, the immediate benefit of ADS-B and CDTI is the enhanced traffic situational awareness provided to the flight crew. The enhanced situational awareness is achieved without changing procedures for pilots or air traffic controllers.


Capstone: The ongoing multi-year FAA Capstone Program’s goals are to improve aviation safety, capacity and efficiency in Alaska through the introduction of new surveillance technologies. These technologies will enhance the pilots’ abilities to cope with weather, terrain hazards and potential traffic conflicts. The FAA has provided aircraft surveillance and display equipment to several aircraft and is evaluating them for operational use by pilots. Pockets of the Alaskan air traffic control system are beginning to use the ADS-B data for control of aircraft in the airspace, thus pioneering the treatment of ADS-B data as equivalent to radar data.

European Projects; NEAN/NEAP/NUP/NUP II: These projects aim to establish a European ADS-B VDL Mode 4 network. This network is based on global standards and supports certified applications and equipment in synergy with the evolving European Air Traffic Management (ATM) concepts. The advances in aircraft surveillance provided will result in significant benefits to ATM stakeholders.

ADS Mediterranean Upgrade (MEDUP): The main purpose of the project is to achieve the integration of ADS information into a network of Air Traffic Control (ATC) centers to allow experimentation and preliminary evaluation of new, optimized procedures and ATM functions. Assessing the operational requirements to overcome existing and expected limitations of capacity and to access flexibility in the Mediterranean airspace is a high priority. The trial will use a VDL Mode 4-based network infrastructure enabling the pre-operational evaluation of new ATM procedures based on ADS-B and data link communications in the Mediterranean airspace.

ADS Mediterranean Free Flight (MFF) Program: MFF is a pre-operational program aimed at defining and validating four selected airborne separation assurance applications, and free routes, within Managed Airspace and Free Flight Airspace and analyzing the transition between them. The main objectives of MFF are to provide technical and operational evaluation of integration, interoperability, and safe use of CNS/ATM technologies and applications for future Mediterranean ATM including procedures for controllers and pilots.

More Autonomous Aircraft for the Future ATM System (MA-AFAS): The purpose of MA-AFAS is to assess enhanced Surveillance and Separation Assurance techniques using CDTI within the Airborne Separation Assurance System. This will include the domains of Partial Delegation, Station Keeping, Passing and Crossing Operations and overall Autonomous Operation. As part of these initiatives enhanced 4D flight operations and the use of space based navigation aids are being analyzed.

South European Pre-Implementation Program (SEAP): This program intends to create a South Western European ADS-B/TIS-B terrestrial network interconnected with the developing ADS-B networks of MFF/MEDUP to the East and NUP to the North. It will also include equipping a number of aircraft to evaluate a number of ADS-B/ADS-C services that are considered to bring highest returns in terms of cost, safety and capacity. This effort is expected to validate the system in terms of performance and benefits.

Additional Programs: There are other programs that are ongoing in different parts of the world. One such example is the current evaluation by Air Services Australia of the 1090 MHz-based ADS-B system for receiving data broadcast from the aircraft and using it for providing air traffic services. They are considering mandating the carriage of ADS-B transmitters in the future. The Russian and Mongolian aviation authorities have also conducted trials and evaluations of advanced surveillance based on ADS-B technology.

Human Factors Challenges

How does increasing airspace capacity affect the role of the controller in achieving optimum safety?

When unforeseen situations cause a breakdown in separation, (e.g. clear air turbulence, quickly formed weather fronts, loss of GNSS information either by ground or aircraft, or both) is the controller able to regain control of the situation easily?

Is the pilot able to maintain a constant watch of other aircraft continuously, thus ensuring separation, in addition to his/her other flight deck duties?

In addition to providing separation, between aircraft, how do the other responsibilities of the controller (i.e. monitoring weather, providing traffic information, managing the airspace, coordinating traffic with other sectors or ACC’s) affect his/her situational awareness?

Additional factors that affect controller situational awareness, i.e. route configuration and complexity, airspace classification, sectorization, special use airspace (SUA), airline schedules, aircraft mix, etc.

Who is liable, (pilot, controller, service provider, or airspace manager) when a delegated separation eventually leads to an operational loss of separation?


These Human Factors challenges mentioned above in 2.6, are the very obvious ones. Yet, the authorities have not been able to address these fully, as far as identifying solutions.

The issues of particular concern to controllers are as follows:

The issue of legal liability in cases of “shared separation responsibilities”. It is unacceptable for controllers to be responsible for decisions they did not make, or instructions that they did not issue.

The mistaken belief that controllers can somehow resume responsibility for separation at any time in cases where responsibility for separation provision has been delegated to another.

The loss of situational awareness after delegating responsibility for separation to the cockpit.

“IFATCA has considerable disquiet at the volume of issues that need to be considered as a result of the concept extending the number of agents who can potentially influence executive control of an aircraft. Any limitations of separation provision need to be established, and clearly enshrined in procedures and followed accordingly by actors. The operational concept does recognize this need and refers to “separation modes”. IFATCA posits that the amount of work required in developing and implementing these defined separation modes is considerable. Note also, that in extending the conflict management philosophy, particularly the separation provision function, there may be times when capacity will not be optimized because the needs for safety will subsume these.” (AN-Conf/11-WP/93)

SC4 believes that the concept is still immature, however if the concept does mature in the near future, IFATCA would be prepared to look at it, with the following polices in place to protect the controller.


It is recommended that paragraph 2.7 on page 4 1 2 5 be titled as Co-operative Separation. And that the following be added:

2.7.2.  Delegation of separation shall be thoroughly described and defined in ATC and Aircrew procedures.

2.7.3.  Airspace within which Co-operative Separation is used must be so designated.

2.7.4.  Before establishing a single airspace continuum over different States, all legal issues regarding liability and protection of staff should be addressed.

2.7.5.  ATC and Aircraft utilizing such delegated separation airspace shall be certified with minimum equipment.

2.7.6.  ControllersandAircrewshallbeprovidedwithspecialtrainingandcertificationtooperate in delegated separation airspace.

2.7.7.  The “delegation of separation” clearance shall be of a temporary nature, and shall be terminated either at a fix, a specified level, a specified time, or when standard ATC separation has been re-established or when one of the aircraft has landed.

2.7.8.  All aircraft and controller functions in Co-operative Separation shall be synchronized to the same time reference.

2.7.9.  “Loss of separation” warning systems shall be incorporated in the application at ATC facilities and on aircraft

2.7.10. Standard escape procedures shall be established for aircraft not being able to maintain separation assurance.


Scenarios for evolution of air traffic control. “R-2698-FAA.” Robert Wesson, Kenneth Solomon, Randall Steeb, Perry Thorndyke, and Keith Wescourt.

IFATCA Manual 2003.

The Future of Separation, The Controller’s Viewpoint – Martin Cole.

AN-Conf/11-WP/34 – The role of collision avoidance in future ATM systems.

AN-Conf/11-WP/43 – Airspace organization and management.

AN-Conf/11-WP/44 – ATM safety management systems (SMS).

AN-Conf/11-WP/86 – initial ground surveillance and airborne surveillance applications enabled by ADS-B.

AN-Conf/11-WP/93 – Conflict management.

Last Update: September 29, 2020  

March 22, 2020   777   Jean-Francois Lepage    2004    

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