Complexity of Multiple Delay Absorption Programmes

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Complexity of Multiple Delay Absorption Programmes

56TH ANNUAL CONFERENCE, Toronto, Canada, 15-19 May 2017

WP No. 90

Complexity of Multiple Delay Absorption Programmes

Presented by TOC


As air traffic counts around the world increase, the demand on airports is growing exponentially. In order to facilitate safe and orderly operations at aerodromes, ANSPs are applying spacing methods deeper and deeper into the en route environment. The application of these methods is increasing the burden on en route controllers. In areas with a high density of different ANSPs there is concern over the increasing potential demand and complexity on the controller of running many unique delay absorption programs.


1.1 As traffic increases globally, airports and traffic managers are reaching out further and further to start spacing earlier. Some ANSPs and airports are developing their own methods of issuing delays to individual aircraft, and this creates potential for multiple methods of delay absorption to be running in a sector. Each method may be unique in its functionality, methods and displays. Running one delay program on its own can be complex but multiple systems can lead to an exponential increase in complexity and workload.

1.2 There are many factors that can affect sector complexity. Each sector has inherent complexity before air traffic is even considered. Once traffic is added, there are new facets from the standard flow complexity to abnormal situations. Some of the many factors effecting complexity include:

  • airspace geometry
  • crossing and transitioning traffic
  • special use of airspace
  • flow control demands
  • terrain
  • international boundaries
  • language issues
  • radio quality
  • weather
  • lack of automation tools
  • conservation regulations
  • temporary requirements
  • special events
  • control room distractions
  • and more

Some controllers additionally have the added complexity of complying with delay absorption programs. Each program may require different actions from the controller which can add in varying amounts to their workload.

1.3 With the potential addition of a variety of delay absorption programs there should still be no degradation of safety. Placing additional demands on controllers who already face complex environments can be problematic. This paper seeks to determine the best way to introduce such changes into the operational environment safely.



2.1 What is complexity? Complexity is hard to define and even harder to understand. Even the Merriam-Webster definition of complexity “something complex” or “the quality or state of being complex” doesn’t explain very much. Thankfully the same page provides a simplified definition as well, “the quality or state of not being simple” or “a part of something that is complicated or hard to understand.” (

2.2 IFATCA policy originating from 2012 ATS 3.33:

IFATCA encourages the development of sequencing and merging tools, provided that:

  • They provide controllers with reliable and effective information.
  • Local airspace structure, complexity and traffic density are taken into account.
  • Integration with other systems and adjacent units is possible.


2.3 There is a system-wide goal to maximize capacity while not overloading the controller. When determining how far complexity can be increased, PANS-ATM (Doc 4444) suggests six factors directly related to capacity in the assessment. Especially pertinent are the sections on structural complexity and controller workload.

3.1.2 Capacity assessment

In assessing capacity values, factors to be taken into account should include, inter alia:

a) the level and type of ATS provided;

b) the structural complexity of the control area, the control sector or the aerodrome concerned;

c) controller workload, including control and coordination tasks to be performed;

d) the types of communications, navigation and surveillance systems in use, their degree of technical reliability and availability as well as the availability of backup systems and/or procedures;

e) availability of ATC systems providing controller support and alert functions; and

f) any other factor or element deemed relevant to controller workload.


2.3.1 Air traffic complexity is multifaceted, relying on a combination of both static and dynamic factors.

2.3.2 Static complexity of a sector comprises the permanent factors of a sector. This includes the shape of the sector, terrain, NAVAIDS and airways.

2.3.3 Dynamic complexity contends with the changing elements of a sector including the number of and mix of aircraft, weather, and separation standards. Sectors are built to handle standard traffic flows. When atypical situations arise, extra attention should be paid. It is under flow complexity that delay absorption programs can be found.

2.3.4 Air traffic complexity in the simplest sense is based on a mix of sector complexity coupled with the traffic being worked. Human factors researcher Schmidt wrote that a gage of control difficulty was “related to the frequency of occurrence of events which require decisions to be made and actions be taken by the controller team.” Whenever air traffic complexity is calculated it is important to consider the inherent level of entropy from the fluid environment that controllers work in. Above and beyond standard traffic there may be emergencies, equipment malfunctions, coordination, pilot requests, military activities, mixed type air craft and merging or sequencing procedures all to contend with while not mitigating safety ( Complexity levels can be calculated in various ways. In some ANSPs there is a simple formula to determine the number of aircraft within sector boundaries at a set time. Other ANSPs use more in-depth formulas to calculate complexity that factor in the necessary interactions with the pilots in their jurisdiction. Using this methodology, factors like transitioning an aircraft to a different altitude would count as more complex and having many transitioning aircraft would be exponentially more so. Using Schmidt’s gage, the type of delay absorption interaction would factor in heavily. If the controller just had to issue a speed, there would be a single interaction and only one chance for error. If the program requires extensive vectoring or changes in speeds then that multiplies the complexity and thus the possibility of error. Sridhar explains a different expression of complexity, dynamic density, as including, “the number of aircraft, but their relation to each other, airspace geometry and varying traffic flow conditions. “ A measure for dynamic density was created utilizing controller input regarding factors impacting their performance. Factors commonly cited included the number of aircraft, aircraft on headings and aircraft requiring close monitoring for separation purposes ( For human factors researcher Mogford et al, “There is evidence that the processing of air traffic information changes as ATC complexity increases. With increased complexity, controllers use more economical control procedures to regulate their workload. Finally, literature relating to individual differences (such as age and skill level) and their relationships to ATC performance and sector complexity are reviewed.”

2.3.5 Complexity can be increased by weather, depending on the type of weather, it may cause an impact in different ways. With weather like thunderstorms, that require deviations, the additional coordination may use vital time and may leave a controller unable to comply with delay programs. Every aircraft that is off their planned route requires extra monitoring with the ambiguity to their routing. Depending on the system in use, there may be an added challenge in the loss of long range conflict tools. Whereas a very turbulent day may cause an entirely different type of complexity, it may include frequencies being tied up with transmissions. There is also the additional complexity of possible reduction of usable altitudes. Extra vigilance is required in these situations to continuously reassess potential conflicts as pilots’ intentions change, and these situations can rapidly become very complex and potentially physically exhausting. High variability in winds between altitudes can make a big difference in speed assignments, especially pertinent when trying to obtain set distances between aircraft. A controller could be contending with upwards of 100 knots of variation. Additional consideration is the expectation to accommodate mixed-mode operations. Including aircraft with a more diverse range of avionics and equipage, such as a variety of RNP and APCH capabilities. Yet again this adds to potential difficulty.

Delay Absorption

2.4 Delay absorption comprises many programs that allow for air traffic spacing to be done proactively for an airport or several similarly located airports. Delay absorption utilizes several different traffic management initiatives and can be used as both a proactive and reactive approach depending on the situation.

2.4.1 Reactive traffic management initiatives stem from events that could not have been predicted, such as an emergency at an aerodrome or unexpected weather situations. While these situations certainly impact traffic management, these are not the scope of this paper.

2.4.2 Proactive traffic management is the focus of this paper and more specifically the delays taken en route. These are the types of delays where plans can be put into place early to prevent reaching a sector overload situation.

2.5 There are several different delay absorption programs around the world. With these programs, which have different names and specifications, there seem to be relatively few categorizations of what would be expected by the controller utilizing the programs. Each program requires the use of: miles in trail, speeds applied or time to gain or lose.

2.5.1 Within Document 9971 (Manual on Collaborative Air Traffic Flow Management) there is a listing of three different methodologies to sequencing. Sequencing methods

Except in very low traffic density situations, some type of sequencing is usually required in order to maintain optimum landing rate. The following three sequencing methods can be applied to both types of CDO:

a) Automated sequencing methods — Use of automated sequencing systems such as required time of arrival, traffic management advisory displays and relative position indicators. Such systems provide for efficient planning adjustments to be made to aircraft trajectory prior to beginning a CDO procedure. Automated sequencing methods are rapidly evolving and will increasingly play an important role.

b) Speed — Speed control is most effective when a small correction is made early in a procedure and given time to take effect, or, when speeds are a part of the procedure. Speed control allows for predictable performance and is done to establish and maintain separation, and ensure consistent performance between different aircraft. Small speed adjustments can allow the aircraft to stay on a predefined closed path. Large speed adjustments may be counterproductive when following aircraft also need to be slowed to match and will require the aircraft to depart from efficient aircraft flight configurations.

c) Vectoring — Vectoring is the most flexible way to sequence arriving traffic and maintain capacity. It is also the most frequently used method. Vectoring, however, provides the least advanced predictability to pilots with respect to flight path distances and may require pilots to respond to a situation rather than to plan ahead. Providing the pilot with information on estimated distance-to-go can help to mitigate uncertainty. Aircraft may be on a planned open path vectoring procedure or may be vectored off a closed path procedure in order to establish or maintain sequencing and spacing. In closed path CDO small speed adjustments should be considered first, preferably before the aircraft is vectored off the procedure. Remaining on the procedure will allow the FMS to maintain distance calculations.


2.5.2 Miles in trail is the easiest traffic initiative to understand and needs the least explaining. The controller would be expected to use mechanisms to get aircraft on a set route or to a set airport a determined distance apart.

2.5.3 Some programs enlist the controller to issue a specified assigned speed. These speeds can be determined by a computer or by a controller working the traffic, depending on the program.

2.5.4 Arrival Manager (AMAN) and Extended Arrival Manager (XMAN) programs are used through much of the world. These programs allow for increased TMA capacity and predictability. As a by-product of the aforementioned, there is a reduced need for holding and workload, that remains more within an acceptable range. With advancements in delay absorption there has been an extension of the range of the programs from 100NM to over 200NM. This distribution of the workload further out allows the work to be shared among more controllers. An example is spacing into EGLL, which reaches out as far as 350NM ( These programs can either give a speed to assign, as in the 250 seen in the datablock above, or an amount of speed to reduce, example – reduce by M.04. The numbers are presented in as discreet a manner as possible. The programs may be applied to a single aircraft or to a whole stream for one airport.

2.5.5 Time to gain or lose programs such as Time Based Flow Management (TBFM) is defined as, “the technology and method used for adjusting capacity/demand imbalances at select airports, departure fixes, arrival fixes and en route points.” ( In the US version, it presents to the controller as a number in the datablock. The number is representative of a time to lose or gain, shown as the 0 beside the data tag in the picture. As a higher number appears, it may require controller action to “lose the time” back to an acceptable range, this could be done via speed, vectors or holding. It can be complex as there is no additional tool to help with the time to lose, rather experience would tell the controller which method to use.

2.5.6 Ground Based Interval Management-Spacing, GIM-S, is the next incarnation of delay absorption in the United States. GIM-S features freeze horizons adaptable to over 700NM but the challenge is to find the distance from the aerodrome that will accomplish the spacing without degrading the spacing, typically 250-300NM. There are three levels to GIM-S which result in speed assignments, these speeds are denoted to the controller by a star in the datablock. Once clicked on, a suggested speed is shown. The speed can be deferred on by the controller if they are unable to issue the speed. There is a nuance though, that the calculations are for the metering point which may result in a drastic speed reduction if the speeds are not applied in a timely manner. Pilots may not be able to comply with the drastic speed reduction and it could result in needing to reorder the arrival lists (

2.6 Local circumstances sometimes cause the development of unique systems. Programs that perform well in one state may not perform the same elsewhere and thus not be the right choices for all states. As pointed out in the TOC paper from 2010 B.7.4 (WP-L007):

It is also assumed that somehow a down-scaled NextGen or SESAR system will meet the needs of the rest of the global ATM community – but such an assumption has not been validated and will most probably prove wrong.


2.6.1 For the introduction of each new system, training is necessary for both traffic managers and controllers involved. Also, each combination of systems that can be run concurrently should be trained on. Each different ANSP may have unique hurdles to overcome in their training and implementation.

2.6.2 It is important to consider that traffic density can affect different sectors, and combinations thereof, in different ways. What would be complex in one sector can be multiplied when working several combined. Traffic counts, both predicted and real, should be considered in running a delay program. When considering adding extra complexity to a sector, such as running one or more delay absorption programs, how heavy traffic is in the sector should be considered. It may be undue added stress to add on to a potentially overloading sector and there may be an advantage to applying the program in a sector that is less saturated with traffic. Complexity can also be effected by supervisor actions such as determining which sectors are capable of handling which programs, if upstream sectors need to provide any assistance and whether to combine or de-combine sectors.

2.6.3 In order to provide better information for the delay programs, as well as complexity planning and traffic counts, accurate flight plan information is necessary. As technology advances, with the rise of SWIM technology, Trajectory Based Operations and CDM, there can be more precise planning. This more accurate planning allows for better pre-planning on departures and less need for controller intervention.

2.6.4 Differing levels of technology and tools on hand allow for different implementations and on varying scales depending on the ANSP. Appropriate tools can help reduce safety hazards, such as speed overtakes and other conflicts. The equipment available at the ANSP must be considered when running any delay absorption program, not every program will be suitable for every ANSP. When using delay absorption programs conflict alert systems should remain functional. If speeds are altered there should be automation to ensure medium and long term conflict alerts can perform their respective functions. If aircraft are on vectors some systems, such as Enroute Decision Support Tool (EDST – US), stop probing the medium-term conflict alert, in part due to the unknown duration of the vectors.

2.6.5 Mixed mode operations can add greatly to the complexity of any situation. As time goes on, the likelihood of a bigger variety of equipage and types seems highly possible. In some cases, this variety equipage can mean vast differences in speed performance and navigation. One challenge in recent years has been the great increase in very light jets which can reach high altitudes but only fly at a slow speed. Policy per IFATCA Paper on Mixed Mode Operations from 2009 ATS 3.13:

Mixed mode operations are defined as ATM Operations that require different procedures due to variances in airspace users’ characteristics and/or ATM design within the same area of controller responsibility.

Efforts should be undertaken to reduce existing Mixed Mode Operations by creating intrinsically safe solutions.

Introductions of new Mixed Mode Operations should be avoided by creating intrinsically safe solutions.

When the safety of a Mixed Mode Operation cannot be completely managed at an intrinsic level, an assessment must take place that the change in the ATM system does not increase controller workload to an unacceptable level.


2.6.6 Different ANSPs with many airports within a small geographic area, such as Europe and Asia require more cooperation and coordination with each other. It is easy to see how any one airport developing a system could easily affect several neighbouring ANSPs. As shown, there is potential for some facilities to face upwards of five different ANSPs which could result in multiple different programs to contend with. With the addition of more programs, in both quantity and in unique aspects of each program, there is potential great growth of complexity. As of the writing of this paper, when Eurocontrol’s Muenster/Ruhr sectors are combined, controllers must contend with three different airports using delay absorption programs: London, Amsterdam and Frankfurt. There are talks about other nearby airports coming online with delay absorption programs in the near future such as Paris and Munich. When each ANSP develops their own unique programs, it can create new and complicated challenges. In smaller ANSPs, it could reduce the reach of the programs.

2.6.7 In some programs the delay absorption information is sent out once and utilized, in others it is fluid, requiring further complexity to follow along with the changing situation. Workload can increase for the controller and any unique challenges may be exacerbated, along with the potential for compounded complexity.

2.6.8 Also to consider is the existing policy on Controlled Time of Arrival (CTA) in ATS 3.32:

  • Arrival Manager (AMAN) is available to define reliable CTA times.
  • RTA equipage level of aircraft is sufficient to support CTA operations.
  • Procedures and controller tools are available to integrate RTA equipped and nonequipped aircraft in the same traffic stream.
  • Tactical ATC interventions are always possible.
  • Accurate wind and temperature data is available.
  • Means to communicate the CTA contract with aircraft are available (preferably data link).


2.6.9 Different ANSPs have different sets of regulations between sectors and states. The agreements between the above should set forth any necessary rules. The priority of completing the delay program may fall at different levels of priority. In the XMAN/AMAN programs, the priority for issuance is below other air traffic duties. When compliance cannot be complied with, coordination is accomplished rather than hand off refusal.

2.6.10 Safety still remains paramount in air traffic control. There have been several studies, but only one from 1989 cited a controller survey wherein sequencing and delay was a contributing factor to safety. More studies are yet needed to show a stronger correlation.


3.1. Complexity is a mix of static factors, such as sector size and terrain, as well as dynamic ones, such as weather and delay absorption programs. These matters are augmented by differences in ATCOs such as age, skill level, and training received.

3.2. There are several methods to compute complexity. Some utilize aircraft count from the simple number of aircraft to weighted counts using aircraft actions. There are alternative methods including frequency use time and amount of actions taken.

3.3. There are several different methods of delay absorption programs which may require different controller actions. Some require a onetime issuance of a speed while others may require a more complex series of vectoring and speed control.


4.1. It is recommended that:

IFATCA Policy is:

Multiple delay absorption programs applying to the same area of jurisdiction can increase complexity so as to reduce safety and efficiency. The implementation of multiple delay absorption programs must be balanced against core controller tasks.

And is included in the IFATCA Technical and Professional Manual.


Last Update: October 1, 2020  

January 3, 2020   475   Jean-Francois Lepage    2017    

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