Study Air Traffic Flow Management

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Study Air Traffic Flow Management

50TH ANNUAL CONFERENCE, Amman, Jordan, 11-15 April 2011

WP No. 96

Study Air Traffic Flow Management

Presented by TOC


This paper provides an overview and evaluation of current IFATCA policy regarding Air Traffic Flow Management (ATFM). Current evolution of ATFM in those regions that are fortunate enough to have a system in place is addressed, as are shortcomings and positive points. This paper compares existing legacy ATFM systems to those more advanced and those of the future. A brief insight into the considerations required of a future ATFM system is provided, and in particular the beneficial role of CDM (Collaborative Decision Making).

The paper identifies the need to create IFATCA policy by addressing the requirements of an ATFM system, present and future.


1.1 Air Traffic Controllers are able to safely manage a finite number of flights during any particular period of time depending on the capacity of a sector, the ability of an airport to accept the flights, and external factors such as weather, technical equipment availability and staffing. To exceed that number could potentially lead to an unsafe situation. An Air Traffic Flow Management Unit (ATFMU) aims to protect controllers from encountering an overload situation by making sure that only a manageable number of aircraft are flying in a particular sector at any one time.

1.2 An ATFMU acts as an interface between airspace users, those who file flight plans, the airports and ANSPs who provide air traffic control and other related services. Each provider decides how much traffic its centres / airports can safely handle after careful analysis and subject to the external influences mentioned above; the ATFMU then uses this information in its planning and the execution of flow management. Each sector, airport and ANSP will have different declared standard sector capacities calculated on its complexities, external influences (i.e. terrain) and traffic flows.

1.3 The goal of ATFM is to provide a seamless and continuous improvement in the management of air traffic capacity versus the demand. This process is completed by the exchange of accurate air traffic management data and the sharing of information among all stakeholders overseen by a regulatory framework. The associated functional requirements are identified and are sorted out into operational improvements in order to allow a smooth transition from strategy to implementation through appropriate development.

1.4 Basic versions of these systems have been adopted in the ECAC (European Civil Aviation Conference) region, the USA, Canada, South Africa (put in place prior to the Soccer World Cup 2010) and Australia. Each individual system, however does not communicate with another on a continuous basis either due to geographical position or software incompatibility.

1.5 Some of these systems could be described as adequate, some (including the European CFMU) have reached their design limits and fail to deliver. Future generations, however, require further refinement and the inclusion of external factors that are constantly updated and are communicated from one system to another.


2.1 ICAO

2.1.1 In recognition of the increasing impact of ATFM on the global aviation industry, ICAO is creating an ATFM Reference Manual to be released in 2011. The existing ICAO references for ATFM are:

2.1.2 ICAO Doc 4444 Procedures for Air Navigation Services, Air Traffic Management, Chapter 1 – Definitions:

“Air Traffic Flow Management (ATFM). A service established with the objective of contributing to a safe, orderly and expeditious flow of air traffic by ensuring that Air Traffic Control (ATC capacity is utilised to the maximum extent possible, and that the traffic volume is compatible with the capacities declared by the appropriate Air Traffic Service (ATS) authority.”

2.1.3 ICAO Annex 11 Air Traffic Services, Chapter 3 – Air Traffic Control Service, & ICAO Doc 4444 Procedures for Air Navigation Services, Air Traffic Management, Chapter 3,

“ATFM shall be implemented for airspace where the traffic demand at times exceeds, or is expected to exceed, the declared capacity of the air traffic control services concerned.”

2.1.4 ICAO Annex 11 Air Traffic Services, Chapter 3 – Air Traffic Control Service, & ICAO Doc 4444 Procedures for Air Navigation Services, Air Traffic Management, Chapter 3,

“ATFM should be implemented on the basis of regional air navigation agreements or, if appropriate, through multilateral agreements. Such agreements should make provision for common procedures and common methods of capacity determination.”


2.2.1 IFATCA does not have policy on the generic concept of ATFM. We do, however, have some policy that deals with various detailed aspects:

ATS 3.6 ATFM – Adherence to Slot Times (Technical and Professional Manual 3236)

“IFATCA recognises the potentially dangerous situations that can arise when slot times are not adhered to. In the EUR region ATFM utilises departure slot times as a means of regulating air traffic and that when a departure slot time is used, the time should be passed to the ATC unit at the departure airfield. It is the responsibility of the aircraft operator to be ready for departure to meet the assigned ATFM departure slot. Civil aviation administrations (shall) pursue with utmost vigour those operators who consistently fail to comply with ATFM measures.”

ATS 3.7 – Sector Capacity Values (Technical and Professional Manual 3237)

“Operational controllers should always be involved in determining capacity values.”

TRNG 4.3 Air Traffic Flow Management

“ATFM staff not performing clerical or administrative functions, so called ATFM controllers, must be qualified controllers with recent experience on control duties on entry to ATFM services. The responsibility for aircraft in flight remains solely with ATC and any subsequent ATFM involvement shall be at the request of ATC only. An ATFM controller must hold an ATFM rating. Such a rating will require the ATFM controller to demonstrate a comprehensive knowledge, skill and experience of all relevant ATC procedures and ATFM duties. ATFM controllers should be obliged to familiarise themselves with major changes in relation to ATFM in their region.”


2.3 Standard ATFM of Today

2.3.1 Planning for the introduction of some form of ATFM should take place when demand is exceeding capacity on a regular basis. If in addition, future traffic forecasts indicate active growth, action should be taken. Even seasonal events such as weather (CBs, snow etc.) can cause huge disruption and the demand through certain sectors can only be effectively managed through the implementation of an ATFM process. The time taken to install such a system depends essentially on its complexity and rather than creating a new off the shelf complete package, it may be more cost beneficial to an ANSP to introduce small packages addressing individual priorities to take advantage of “low hanging fruit” rewards.

2.3.2 ATFM as a concept is not in place throughout the world, let alone the future development. During the research of this paper, the author attended a webinar organised by ATC Global, the industry trade fair. This organisation holds regular web based discussions on various different themes related to Air Traffic Control. This event highlighted the lack of any ATFM in some areas in the world and the potential benefits that it can bring. These are stated to include:

  • Efficiency versus cost benefit (a maximisation of flights through a particular ANSP’s airspace generates revenue).
  • Safety Improvements.
  • Improvement of predictability from gate to gate.
  • Reduction of aviation’s carbon footprint, therefore supporting environmental sustainability goals.
  • Decreases in delays for passengers.

2.3.3 There is a perceived difficulty in implementing ATFM in regions by the airspace users, as this type of organisation has been wrongly seen to be a generator of delays, rather than an optimiser of ATC capacity management. This wrong perception can be overcome by constructive dialogue between the interested parties, and the adoption of a good CDM (Collaborative Decision Making) process.

2.3.4 Generally there is a defining moment for every ATS region that acts as a wake up call. This was for the USA, the sacking of 11000 controllers after the 1981 strike where capacity was far outweighed by demand. The airport capacity at Mexico City airport in the 1990s was the Mexican ATM provider’s defining moment, where they called for the help of the FAA in installing a basic ATFM system. In Trinidad and Tobago, the Cricket World Cup was also the occasion to set up an ATFM system. More recently the soccer World Cup in South Africa in 2010 was a catalyst in implementing an effective, semi real-time ATFM system that makes extensive use of the CDM process mentioned above.

2.3.5 The European region was one of the first to adopt an ATFM strategy, and has been continuously increasing its effectiveness, however the system has a variety of limitations that will not allow it to adapt to the future industry requirements that SESAR will bring. Some of the limitations include the inflexibility of the system. If an aircraft wants to change its flight level en-route, then the system does not provide information on its initial calculations, i.e. restrictions up-route (level availability, military training area availability), in order to make the decision transparent to its users. Likewise the information of newly vacated military training areas, available flight levels, is not passed to subsequent users. If an aircraft deviates significantly around weather, its trajectory will now take it into other sectors in which it was not initially counted in the statistical planning. It is also solely reliant on a single take-off time, which then does not cater for any later or earlier arrival into another sector’s airspace due to a headwind/tailwind etc. This late arrival may then push the sector into saturation.

2.3.6 As a case study, the European Central Flow Management Unit based in Brussels, Belgium, acts as the focal point for all the Flow Management Positions in all the ECAC member state’s ACCs. It is also the repository for all the repetitive flight plans, and all ad-hoc flight plans must pass through this organ. Calculated Take-Off Times (CTOTs) are then calculated and issued to operators and ANSPs. Central Flow Managers then continuously monitor sector load capacity and delays, bunching of traffic, unused slots and the equity of their attribution. Prior to the calculation of CTOTs, and assessment of capacity versus delay, a flight plan has 3 stages. The first “Strategic”, observing demand several months before a flight, the second “Pre-tactical”, comparing existing data with traffic forecasts and information from individual ACCs. The final stage “Tactical” is the work carried out on the actual day of the flight. At this moment CTOTs are issued, re-routings organised to avoid bottlenecks, and the analysis of different flight profiles. After this stage, statistics are compared and published to the various ACCs, which can then reflect on their performance.

2.3.7 Limitations that have been experienced by existing ATFM procedures include:

2.3.8 Lack of communication of one system to another, for example the systems in Canada and the USA do not communicate with the Eurocontrol CFMU autonomously. A telephone conference held twice a day is required between operators of each system to identify possible bottlenecks over the Atlantic. Likewise the FAA is obliged to speak directly with the Mexican ATFM authorities to flag capacity issues between North and Central America.

2.3.9 Introduction of tools such as arrival sequencers for major airports (Heathrow (AMAN), Schipol, Frankfurt) can also provide limitations to the overall ATFM system. These tools do not liaise with a central CFMU, but rather work with varied success on a real time presentation of inbound traffic. These tools provide a second level of traffic flow regulation, which arguably provides double restrictions into an airport. The Heathrow and Gatwick arrival managers (AMAN) also increase workload as they have to be constantly monitored and the approach sequence that they calculate adapted to take into account wake turbulence categories and the effect of other traffic.

2.3.10 No real time dynamic uplink of external factors is present such as en-route and airport weather, equipment shortfalls or restrictions, and sector staffing at individual ANSPs.

2.3.11 Inefficiencies exist due to the structure of current ATFM, which is perceived as a mechanism for slot allocation, and its meaning should be extended to capacity and flow management.

2.3.12 Current implementation of flow policies is extremely complex. There is a need, therefore, to simplify the processes of building a route network in an efficient and continuous way.

2.3.13 The success of the organisation of the network between ATFMU, FMPs and ACCs, depends to a large extent on the interpersonal skills of the individuals concerned, as well as the accuracy of the information that is shared between the different parties.

2.3.14 Although in the EUR region the automation of communication processes between an individual ANSP and the CFMU has rendered tasks easier, the discussion directly between ANSPs remains limited.

2.3.15 Focus on the performance of individual ACC performance (the UK ANSP NATS, provides a television update around its centres of its performance compared to other ANSPs in the EUR region) tends to diminish the awareness of the overall ATFM system performance in a geographical region.

2.3.16 Inconsistent flight profiles (non-adherence to FPLs and slots and the late implementation of restrictions) can corrupt the data and create “over deliveries” increasing complexity in en-route sectors and the effective bunching of traffic.

2.3.17 ICAO has been developing a reference manual to be published in 2011, which will provide guidance for ANSPs and airlines around the world to serve as a basis to the development of an ATFM system.

2.3.18 ATFM is building momentum among ANSPs with continued innovation for early adopters, and increased deployment by ANSPs around the globe. While ATFM is proven to enhance safety and provide measurable efficiency gains; it is also viewed as a “transformational” technology that introduces new levels of CDM and offers potential for harmonising airspace. This harmonisation of airspace concept has been called “seamless”.

2.3.19 The European CFMU is holding “Adherence Days”, where all operators and controllers will be requested to respect the levels, speeds and routings that are present on the initial flight plan of an aircraft. This highlights the fallibility of the overall system. The system cannot adapt to the realtime needs of its users, but rather its users must adapt their real-time optimum way of operating to the system’s demands and limitations.

2.4 Implementing Future ATFM solutions today

2.4.1 In the world several examples of future ATFM solutions exist and are currently being used. The freight company UPS employs a 4D trajectory style ATFM package for its aircraft inbound to their US central hub at Louisville Standiford Airport.

2.4.2 In the Bay of Bengal in the ICAO Asia/Pacific region, South Asia and Pakistan airspace providers have implemented an automated ATFM service under the auspices of the ICAO Bay of Bengal ATS Coordination Group – ATFM taskforce. This ATFM service provides the regulation into Kabul FIR, an area that has become a hotspot for air traffic congestion.

2.4.3 The Bay of Bengal (BOB) ATFMU is situated in Bangkok at the ACC and run by AEROTHAI (Aeronautical Radio of Thailand). They calculate, promulgate and manage mandatory Allocated Wheels Up Times (AWUT), Kabul FIR entry fix times and flight levels, and ATS routes for each affected flight. In turn Singapore ATC is responsible for the tactical management of flights that are subject to ATFM. They also manage non-ATFM flights (humanitarian etc) through delayed startups, non-preferred routes, en-route holding or diversion around Kabul airspace.

2.4.4 In practise the procedure necessitates coordination between the aircraft operator, flight crew, ANSPs and the Bangkok ATFMU. At start up the flight crew state the AWUT in the initial ATC clearance request transmission. The crew are then responsible for adjusting their flight profile to arrive at the Kabul FIR entry fix at the designated time and level, by adjusting the aircraft’s speed en-route with ATC acceptance. The crew/ ATC are advised to request another FIR slot time should there be any doubt about the flight’s ability to arrive as per the initial AWUT. The initial slot for that flight may then be passed to another if it reduces the overall delay.

2.4.5 The BOB ATFM system is a development of the restricted European CFMU system, but is not as far reaching as the solution proposed by UPS, which has gained interest among many airline including Ryanair for use into Stansted Airport in the UK and extension by association to its other bases around Europe.

2.4.6 One of the World’s largest freight haulage companies UPS, together with NASA and the FAA have simulated, trialled and put into operation a future style 4D trajectory management system for its aircraft fleet that operate into its major US hub at Louisville International Airport Standiford.

2.4.7 Air Traffic simulations of “Trajectory Orientated Operations With Limited Delegation” (TOOWiLD) were carried out at NASA’s Ames research centre in 2006. These simulations were aimed to investigate the ability of a system that would integrate an arrival management system scheduling aircraft along CDAs through data linked arrival information to individual aircraft. Crossing traffic also featured to increase realism.

2.4.8 The arrival system was planned, and now has been implemented by UPS to generate a runway schedule for each aircraft. Scheduled Times of Arrival (STAs) are determined from ETAs as are the minimum required wake vortex spacing at the runway threshold. ETAs are based on the aircraft’s flight plan routing, a charted CDA, ADS-B reported position information and a company cost index.

2.4.9 When an aircraft reaches 300nm from the airfield, the arrival management system computes a cruise descent profile using the STA, that gets the aircraft to the runway at its allotted time. At the same time it generates an arrival message to all other participating aircraft within the 300nm range.

2.4.10 Some of the UPS fleet are suitably equipped to conduct merging and spacing operations, this is also recognised by the arrival management system, and uplinked to other similarly equipped aircraft.

2.4.11 Merging and spacing operations have two phases:

  • A strategic set-up by a ground operator (not ATC);
  • A tactical Flight Deck Based Merging and Spacing (FDMS).

2.4.12 Both indicate speeds that the aircraft must fly to achieve the required spacing during the descent. Approaching the merge fix, the ground based unit will uplink via ACARS an advisory that includes as a minimum, the Traffic To Follow (TTF) flight identification, the spacing interval in seconds, and the common merge waypoint for the aircraft systems. The FDMS phase then takes over. Onboard equipment calculates and displays information that allows flight crew to manage their speed to achieve a desired spacing interval at a common merge fix. Pairs of compatibly equipped aircraft can be formed into linked chains that allow the second aircraft to become the TTF for a subsequent third aircraft. These procedures require less controller input and workload, less fuel burn and increase capacity.

Fig. 1 Future ATFM solution

2.4.13 Although dispensations have been granted by the FAA for the implementation of this unique procedure, the design of CDAs and their interaction on the surrounding route structure can be important.

2.4.14 ATCOs retain the same responsibilities as they have today. It has been experienced, however, that their role is different with regard to managing arrivals. While they are expected to control nonparticipating arrivals, they are actively encouraged to allow participating arrivals to “do their own thing”. Non-participating traffic, crossing traffic, and transitioning traffic have become a greater challenge to integrate into the approach flow.

2.4.15 Taking into consideration that this system is by far the most advanced in the world today, it still has some limitations. The most restrictive limitations including that it only works for one company into one airport with aircraft that have identical equipment levels. It should be seen, however, as a great stepping-stone, and learning exercise for the NextGen and SESAR concepts mentioned in the next chapter.

2.5 Investigating the ATFM of the Future

2.5.1 The next generation of ATFM is relatively forward thinking and will enable the harmonisation of a global network of systems. The evolution of ATFM has been named I-ATM (Integrated ATFM), or ATFCM (Air Traffic Flow and Capacity Management). It aims to serve:

  • Across operational domains – surface, departure, en route and on arrival; across FIR boundaries.
  • Across planning time frames – scheduling, strategic planning, pre-tactical, tactical and post-operations.
  • Between service provider and flight operator – coordinating efforts and aligning objectives for mutual benefit.
  • Across international boundaries – data exchange and strategic control.

2.5.2 On the 25th March 2010, the European Commission adopted rules on how to manage the flow of traffic in order to optimise available capacity in the use of airspace. Presently there is no legislation concerning ATFM at a European level. The introduction of legislation is seen as an important step in implementing the EU network manager foreseen in the second package of measures of the Single European Sky. At the moment the cooperation between the Eurocontrol CFMU and with local flow management units at Member States level is based on voluntary procedures.

2.5.3 This new legislation will eventually lead to the introduction in Europe of SWIM (System Wide Information Management). This system will build on experience gained in the previously mentioned countries and will connect all ATM stakeholders. This is an “ATM Intranet” whose aim is to allow the transfer of information for improved decision making. Through SWIM, all air traffic stakeholders will simultaneously have access to all relevant information to cater to their own business needs for planning and execution. Controllers, pilots, dispatchers will share the same information in near real time. A reduction of congestion, not only in the air, but also on the ground is expected to have numerous benefits to the environment.

2.5.4 In order to achieve the future ATFM process, several areas have been highlighted to receive particular attention:

  • Improving Traffic Flow and Capacity Management through proactively optimising ATM/airport capacity whilst at the same time balancing it with demand, and identifying and exploiting other available capacity.
  • Improving traffic flow management by developing the variety of flow measures and procedures with ATC to best manage expected traffic.
  • Ensuring quality of service, by continuously assessing through the use of Key Performance Indicators (KPIs) the efficiency of the service to respond to the airspace users needs.

2.5.5 Another main point to address is the collaboration with ATM partners through the exchange of accurate ATM data (FPL, airspace, crisis decision etc) within a regulatory framework between all the relevant stakeholders:

  • Ensuring Flight Plan Data Consistency – Each airspace user has different requirements, however a common basis should exist in order that each stakeholder is able to complete their task in an effective and efficient manner.
  • Optimising the Interface with Airspace Management – Increased capacity relies on airspace usage and therefore there is a need to optimise the interface with airspace management and to consider the users’ requirements, for example the military.
  • Collaborating with Airport Operations – The airport has to be seen as an integral part of the ATM system in a “gate to gate” concept. The on time delivery of arriving aircraft at gate must be seen as a prerequisite to an on time departure. All the data required by the airport, in order that it runs smoothly should be available to all stakeholders in order to ensure an effective CDM process.
  • Managing Critical Events – although sometimes unplanned, the reaction by the ATFM system should mitigate their impact by sharing information in real time.
  • Creating a Regulatory Process – to ensure that there is equality between all partners and compliance to the rules.

2.5.5 Collaborative Decision Making (CDM)

CDM is the key process in ATFM that allows decisions about events (e.g. snow, runway closure etc) to be taken by those best positioned to make them using comprehensive and up to date quality information. This will allow decisions about a particular flight to be made according to the latest information available, thereby enabling flights to be dynamically optimised to reflect near or real time events.

2.5.6 This CDM process is an enabler of ATFM strategy allowing the sharing of all relevant information between the parties involved in making decisions and supporting a dialogue between all stakeholders (airspace users, FMPs, ATFMUs, ANSPs, airport, military organisations etc) throughout all the different phases of flight as highlighted in figure 2. This will allow all organisations to update each other on events that are taking place from the strategic level to real time.

Fig. 2 Different phases of flight

2.5.7 In order to be efficient and to meet the required objectives, CDM should have the following characteristics:

  • An inclusive process
  • A transparent process
  • A process that builds trust between the stakeholders

2.5.8 Safety is obviously the highest priority in aviation. The main purpose of ATM services is to “ensure the safe separation of aircraft in the air and on the ground”. Eurocontrol have added an interesting new caveat to the above statement. Eurocontrol require the above, whilst at the same time the “maintaining of the most efficient operational and economic conditions”, they go on “In the future it is foreseen that separation assurance will be enforced by a regulatory aspect and complemented by ATM data (e.g. FPL data, airspace status etc). Its implementation is performed by a safety planning process, initially through airspace management, flow and capacity management; then separation by ATC, and finally collision avoidance through cockpit tasks”. This, of course, describes 4D trajectories.

2.5.9 A 4D trajectory is the ultimate application of ATFM to future ATS routes. Both NextGen and SESAR will use this concept.

2.5.10 The 4D concept relies on a RBT (Reference Business Trajectory) which the airspace user agrees to fly and the ANSP agrees to make available. Contrary to existing ATFM systems, it implies a target time of arrival over a waypoint of the trajectory, e.g. the initial approach fix, within a time window tolerance. At busy airports, or during peak hours, the RBT time window tolerance (currently -2min; 3min) may not be accurate enough to ensure an efficient pre-regulation of traffic and to optimise runway capacity. The aircraft, in this case, could be tasked to achieve a Controlled Time of Arrival (CTA) at the IAF with a certain time tolerance.

2.5.11 The 4D concept can be combined with ASAS (Airborne Separation Assistance Systems) limited delegation clearances concept. Trajectory based operations are first used to precondition the flow, sufficient to avoid overloading local airspace sectors. Subsequently, Air Traffic Controllers issue limited delegation clearances to aircraft to cross behind, merge with, or follow aircraft in the proximity.


3.1 Existing ATFM procedures have many limitations.

3.2 Users are required to adapt their real-time operations to limitations of ATFM systems.

3.3 IFATCA has policy designed to address certain small aspects of current ATFM concepts, but not as a whole. A high level statement should therefore be defined to promote the adoption of ATFM around the world.

3.4 ATFM will have an important role in the ATM solutions of the future; however ATFM is expected to evolve into 4D Trajectory Management.

3.5 ATFM has been implemented differently on a regional basis. No clear guidance exists from ICAO to provide an international standard.

3.6 Tactical sector capacity should be determined by an ATCO/FMC.

3.7 Current ATFM restrictions are not transparent to all users.


It is recommended that:

4.1 IFATCA Policy is;

IFATCA encourages the implementation of ATFM processes provided that:

  • The process achieves an optimum overall performance.
  • Air Traffic Controllers and Flow Management Controllers are involved in the design of their local procedures and the determination of capacity values.
  • The communication between and the compatibility of regional systems is established.
  • The tactical capacity is managed on an operational level.
  • The process, including restrictions, is transparent to all users.

and is included in the IFATCA Technical and Professional Manual.

4.2 IFATCA Policy, on page 3237 of the IFATCA Technical and Professional Manual:

“Operational controllers should always be involved in determining capacity values.”

is deleted.


ATC Global Webinar on the Future of ATFM.

Individual interviews with Flow Mangement Professionals.

Questionnaire to the proposed ATFM Stakeholders (airlines, ANSP etc.).

Eurocontrol study on “Air Traffic Control & Capacity Management for the ECAC states”.

Eurocontrol – 4D Trajectory Management, An Initial Pilot Perspective.

“Metron Aviation” publicity and case studies.

Air Traffic Concept Utilising 4D Trajectories and ASAS, NASA Ames Research Centre.

“Air/Ground Simulation of Trajectory Oriented Operations with Limited Delegation” Paper presented by NASA to the 7th Europe/USA Air Traffic Management and Research Seminar.

Last Update: September 30, 2020  

April 18, 2020   961   Jean-Francois Lepage    2011    

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