56TH ANNUAL CONFERENCE, Toronto, Canada, 15-19 May 2017
WP No. 92
Review ATS 3.36 – Recategorization of Aircraft for Wake Turbulence
Presented by TOC
This working paper reviews the current provisional policy on Recategorization of Aircraft for Wake Turbulence adopted at the 2013 IFATCA Annual Conference in Bali, Indonesia. In recent years, there have been a number of initiatives to review and update the wake turbulence categories defined by ICAO and the corresponding state variations. The main aim has been to either reduce the wake turbulence separations or make them dependent on the wind conditions in order to increase the capacity, efficiency and resilience of airport operations. The policy adopted by IFATCA in 2013 highlights the effect those initiatives could have on controllers’ workload. This paper analyses the relevance of said policy given the evidence currently available on the implementation of some of those recategorization initiatives.
1.1 In the context of aviation, wake turbulence refers to the effect of the rotating air masses generated behind the wing tips of large jet aircraft . In extreme cases, that turbulence can create an uncommanded roll on the aircraft behind beyond the ability of that aircraft or crew to counteract such effect. This is particularly relevant when aircraft are in trail or closely spaced, which happens mostly in the vicinity of aerodromes, either on departure or on final approach.
1.2 In order to protect aircraft against wake turbulence, ICAO, more than forty years ago, grouped aircraft types in three categories according to their maximum certificated takeoff mass: Heavy (H), Medium (M) and Light (L). It then defined distance and time-based wake turbulence separation minima between those aircraft categories to be applied in the approach and departure phases of flight . States adopted those categories and separation minima, with some states modifying them to suit operations in their country. Recently, a new category, Super Heavy (J), was recommended by ICAO through a number of state letters (the last one in 2008 ) when the A380-800 aircraft came into operation, given its stronger wake vortices compared to existing aircraft.
1.3 Prompted by the deeper understanding of wake vortex behaviour, the development of measuring technology and the data available resulting from the characterisation of the wake vortices of the A380-800, a number of institutions started working on assessing whether it was possible to update ICAO’s wake turbulence categories and separation minima and develop new wake turbulence-related procedures. IFATCA decided, in 2012, to look at the effect those initiatives could have on the role of the air traffic controller, adopting provisional policy at the 2013 IFATCA Annual Conference in Bali, Indonesia .
1.4 This paper revisits IFATCA’s policy on the recategorization of aircraft for wake turbulence. It looks at the evidence made available in the last four years by all the initiatives that are looking at redefining ICAO’s wake turbulence categories and separation minima and their implementation results. Based on that information, this paper recommends new policy to better reflect the current situation with respect to wake turbulence separation and procedures.
2.1 The current provisional IFATCA policy on the recategorization of aircraft for wake turbulence reads :
|IFATCA recognizes the efforts to change current wake turbulence categories and the corresponding separation standards. Certain concepts have the potential to lead to an increase in complexity for the controller and therefore also an increase in controller workload. These concepts can only be supported provided that this increase in controller workload does not exceed acceptable levels.|
2.2 The above policy was a result of the exhaustive study in  of the different initiatives looking at redefining wake turbulence categories and separation minima, with the aim to improve airport capacity and resilience. The study concentrated on the approach phase of flight even though it touched upon departure-related initiatives. The policy was proposed at a time when no implementation of those new wake turbulence categories was available and, as such, it was trying to anticipate where the problems could arise from a controllers’ point of view. The main concerns in  were the potential increase in complexity due to the new categories and the need for specific automated support tools to help the controller deal with any workload increase, which is reflected in the policy.
2.3 Progress in the last 4 years has been slow, with most initiatives suffering unexpected delays, mostly due to lack of worldwide standardization and adoption. Therefore, implementation is not as widespread as it was forecasted in 2013. In order to analyse the validity of current policy, this section looks at the current state in the main wake turbulence recategorization initiatives, identifying if the concerns addressed in the policy have materialized or have the potential to do so.
2.4 In the last 10-15 years, knowledge about wake turbulence behaviour has increased due to an improved understanding of the physical processes associated to wave vortices and the availability of measured data from flight test and flight simulation wake turbulence encounters . The wake turbulence assessment of single new aircraft, like the A380-800 or the B747-800, also indicated that weight might not be the only parameter to be considered for wake turbulence category allocation (other parameters like wing span and geometry should also be considered).
2.5 In light of the above, in 2005, ICAO requested the FAA and Eurocontrol to lead an effort, known as RECAT, to harmonize wake turbulence separation standards for all aircraft for both departure and arrival operations. That initiative was split in three phases :
a) RECAT-1: it consisted on the optimization of the ICAO wake turbulence separation categories, with up to six categories. This initial work aimed at understanding the fleet mix at the American and European airports and at establishing the relationship between the number of categories and the operational benefits.
b) RECAT-2: the idea of this second phase is to replace the above categories with static pair-wise wake turbulence separations, so that each aircraft pair has its appropriate wake turbulence separation minima.
c) RECAT-3: the final phase aims to define dynamic pair-wise wake turbulence separation minima where other factors like aircraft mass and atmospheric/meteorological conditions are take into account when defining the separation minima.
2.6 One of the aims of RECAT-1 was to propose a new wake turbulence categorization (six categories, A to F) to ICAO so it could be included in ICAO PANS-ATM Doc 4444 . However, the differences in fleet mix between the North American and European airports and the different points of view about some of the wake turbulence categories prevented the proposal to ICAO from happening. In fact, RECAT-1 was further split in two implementations, RECAT-US and RECAT-EU, tailored to the needs and characteristics of their respective airports and fleet mix.
2.7 The first implementation of RECAT-US  went live at Memphis International Airport in November 2012 where FedEx, the main operator in and out of that airport, benefited from peak capacity increases of up to 13% . The FedEx fleet, with a large percentage of Heavy aircraft, made use of the split between “upper Heavy” (category B) and “lower Heavy” (category C) aircraft which reduces the wake turbulence separation minima when an “upper Heavy” follows a “lower Heavy”.
2.8 The US implementation of RECAT is in operation at 17 airports, which have experienced different levels of capacity increases, depending on the specific mix of aircraft at each specific airport but always below the 10% mark . Analysis of the operation at those airports showed that the level of capacity increase was highly dependent on the airport having a high percentage of aircraft within the categories which experience a reduction in separation compared to ICAO’s wake turbulence separation minima.
2.9 On the European front, RECAT-EU implementation is somewhat behind that of the US. RECAT-EU had to be approved by the corresponding safety regulator, the European Aviation Safety Agency (EASA), delaying its implementation. It was only in October 2014 that EASA published a letter confirming that the safety case had been approved and that Member States could implement RECAT-EU at their airports . It is expected RECAT-EU to deliver capacity increases of up to 5% during peak periods, again depending on the individual airport traffic mix, in any case, slightly below the capacity increases observed in the US.
2.10 At the moment, RECAT-EU is only operational at three airports in France: Paris Charles De Gaulle, Paris Le Bourget and Pontoise-Cormeilles-En-Vexin . Eurocontrol has identified other congested airports in Europe which could benefit from RECAT-EU implementation, with four to six airports likely to introduce those new procedures within the next five years.
2.11 It can be seen that the implementation of RECAT-1 is in a much earlier stage than it was anticipated it to be in . Only a small number of airports have implemented RECAT-1 in the world and the information regarding the effect such change has had on the air traffic controllers is limited. The introduction of RECAT-1 implies that air traffic controllers have to work with six wake turbulence categories instead of the more common four, which has been the case at some non-RECAT-1 airports for a number of years . Both in RECAT-US and RECAT-EU, it has been reported it took air traffic controllers only a few months to adapt to the new procedures requiring only a minor technological change to show the aircraft wake turbulence category on the strips and radar displays .
2.12 In order to avoid the lack of agreement for RECAT-1, a new working arrangement between the FAA and EASA had been setup so that RECAT-2 efforts can result in a common RECAT-2 wake turbulence separation minima proposal for adoption by ICAO . RECAT-2, supported by Next Generation Air Transportation System (NextGen) in the US and Single European Sky ATM Research (SESAR) in Europe, aims to define static par-wise wake turbulence separation minima between aircraft so that each possible combination of two aircraft has a corresponding separation minima . The main problem of this approach is that ICAO recognises some 1,200 different aircraft types , so a matrix providing every possible pair-wise wake turbulence separation minima would need to have in excess of 1,440,000 entries. Such figure would pose a considerable problem from an implementation point of view.
2.13 In order to overcome the above problem, recent developments have concentrated on trying to include only the aircraft types that would contain at least 99% of all the aircraft in operation today. This has resulted in about 100-125 aircraft types, considerably reducing the number of pair-wise wake turbulence separation minima. In addition, that figure could be reduced even further if we only considered a specific region or airport (Europe would only need 96 aircraft types ). An additional level of simplification could be achieved by grouping the least common aircraft types in categories, as in RECAT-1, and use pair-wise wake turbulence separation minima only for the most common aircraft types .
2.14 It can be seen, from the figures mentioned, that the amount of information and data is considerably larger compared to RECAT-1. That increase in information is expected to increase the complexity of the air traffic controller’s job and, consequently, the workload he or she experiences. In addition, the underlying aim of RECAT is to get aircraft closer together on final approach, which would result in a higher RT load while dealing with a higher number of specific separation minima between aircraft. As such, it is imperative that appropriate automated support tools are provided, so that air traffic controllers can carry out their duties providing the required level of service without experiencing unacceptably high workload.
2.15 Finally, RECAT-3 will build on RECAT-2 implementations to include dynamic information, for example aircraft mass and meteorological conditions, to adjust the pair wise wake turbulence separation minima dynamically . Although RECAT-1 and RECAT-2 look at increasing the capacity and efficiency of airport operations by reducing the wake turbulence separation minima were possible, RECAT-3 will take into consideration factors like the wind conditions to add flexibility to the operation, concentrating more on improving the level of resilience at the airports than their capacity. In any case, the amount of information in RECAT-2 and the dynamic nature of RECAT-3 indicate that more advanced support tools will be needed to help the air traffic controller.
2.16 Although we seem to be far from a widespread RECAT-2 or RECAT-3 implementation, a great deal of effort has gone on the technical side of both projects to make sure that challenges are resolved and procedures simplified so that implementation is feasible. It is also important that a similar level of effort goes into understanding the human factor element of those initiatives. It is necessary to assess how the air traffic controller can be affected by the new procedures, tools and methods of operations in order to make sure that the overall performance of the ATM system is not degraded.
Time-Based Separation (TBS)
2.17 The other main initiative to improve the capacity, efficiency and resilience of airport operations has been Time-Based Separation (TBS). TBS has been developed by Lockheed Martin and NATS (UK) within the SESAR framework. It provides a new method for separating arriving aircraft by time instead of distance. In strong headwind conditions aircraft’s ground speed on final approach is reduced, reducing the landing rate and causing delays and, potentially, flight cancellations. TBS mitigates that problem by reducing the distance between aircraft in those conditions, so that the time interval between them is the same as in a reference wind scenario . A safety case was presented to the regulator showing that TBS is, at least, as secure as current distance-based wake turbulence separations. In tailwind conditions, TBS increases the spacing between aircraft due to the fact that the wake vortices decay less quickly than in headwind conditions.
2.18 TBS is only in operation at London Heathrow Airport (UK) where it started operating in May 2015. NATS claim that up to 80,000 minutes of delay will be saved per year at London Heathrow using TBS . This is particularly important in strong headwind conditions, where using the six UK wake turbulence categories (stemming from the four ICAO wake turbulence categories) and distance-based separation normally results in cancellations and flow control measures to be put in place.
2.19 Given the dynamic nature of TBS, where the distance between aircraft changes as the wind conditions on final approach change, it was clear from the outset that air traffic controllers were going to need support tools to be able to apply TBS and that extensive training was going to be needed for the new method of operation. The information needed to apply TBS is shown in two ways. A table shows the distance-based equivalent of the time-based separation to be applied, using the same six UK wake turbulence categories used in distance-based operation. At the same time, indicators are shown along the final approach track behind every aircraft where wake turbulence separation applies, so that air traffic controllers know how close the follower aircraft can be. After a few months of getting used to the new method of operation, air traffic controllers are comfortable applying TBS and using the tools provided for it.
2.20 Looking ahead, TBS is trying to use the results from RECAT-2 in order to provide pairwise wake turbulence separation minima that can also be converted to time-based separation according to the wind conditions. As mentioned above, due to the amount of information to be assimilated by the air traffic controllers, it is even more important that the appropriate support tools are provided. In a way, that evolved TBS system would be equivalent to a RECAT-3 system with dynamic pair-wise wake turbulence separation minima.
2.21 In particular, due to the potentially high number of pair-wise wake turbulence separation minima and their potential change with wind conditions, it is necessary that human factors teams within ANSPs and air traffic controllers carry out a detailed assessment of how the sheer volume of information can affect the way air traffic controllers operate and their workload. It can be argued that, for the approach air traffic controller, having to separate aircraft on final approach by four to six discreet values can become second nature and be done consistently with appropriate training. However, having to separate aircraft by a varying large number of discreet values can have a very negative effect on the ability of the air traffic controller to achieve consistent accurate spacing. This could degrade the performance of that new system below the performance of the original system it was trying to replace.
3.1. This paper has looked at recent developments of the two-main wake turbulence recategorization initiatives, RECAT and TBS, against IFATCA’s provisional policy on the matter, adopted at the 2013 IFATCA annual conference. The aim has been to assess the validity of said policy considering the level of worldwide implementation of RECAT and TBS.
3.2 The current level of implementation of the different variations of RECAT and TBS is considerably below what, in 2013, was expected for 2017. The reasons behind that slow development can be found in the difficulty to get all the stakeholders of a project to agree on a course of action but also in the inherent complexity of the systems which are being developed. That could give the impression that current IFATCA’s policy is still valid and no change is needed.
3.3 However, even though implementation is not as widespread as expected, both RECAT and TBS are already looking at future iterations of their systems. In both cases, those new iterations have in common the need for advanced tools to support the controllers when carrying out their duties and the vast amount of information that would need to be assimilated by the controllers.
3.4 In light of the above, this paper proposes new policy which emphasizes the fact that any implementation of a new wake turbulence recategorization system must take into account the effect its new procedures and associated tools have on the air traffic controllers’ duties and their workload. It is particularly important that the performance of the new ATM system is not degraded compared to that of the original one and that, at the same time, the task of the controllers is not negatively affected by the new wake turbulence recategorization implementation.
4.1. It is recommended that the IFATCA provisional policy on page 3 2 3 40 of the Technical and Professional Manual:
IFATCA recognizes the efforts to change current wake turbulence categories and the corresponding separation standards. Certain concepts have the potential to lead to an increase in complexity for the controller and therefore also an increase in controller workload. These concepts can only be supported provided that this increase in controller workload does not exceed acceptable levels.
be replaced by the following full policy:
Any aircraft wake turbulence recategorisation, be it distance-based or timebased, for the purpose of increasing runway capacity, must:
– Conduct appropriate safety assessments, including a thorough understanding of the human factor element.
– Design clear procedures for the application of the new wake turbulence categories.
– Provide adequate tools to support the controller when applying those procedures.
– Incorporate contingency procedures for cases where support tools are unavailable and/or the new wake turbulence categories cannot be applied.
– Ensure that the new system does not have a negative effect on the efficiency of the overall ATM system.
References Procedures for Air Navigation Services – Air Traffic Management; PANS-ATM Doc 4444, ICAO; 2007.  Guidance on A380-800 Wake Vortex Aspects; ICAO State Letter; ICAO; July 2008.  Study Recategorization of Aircraft for Wake Turbulence; B.5.6 WP90; IFATCA Annual Conference; April 2013.  IFATCA Technical and Professional Manual; ATS 3.36; IFATCA; 2016.  Redefinition of ICAO Categories for Wake Turbulence (RECAT); 12th Air Navigation Conference; ICAO; October 2012.  Wake Turbulence Recategorization; Order JO 7110.659C; US Department of Transportation, Federal Aviation Administration; February 2016.  NextGEN Operational Performance Assessment; US Department of Transportation, Federal Aviation Administration; September 2015.  RECAT-EU European Wake Turbulence Categorisation and Separation Minima on Approach and Departure; Eurocontrol; July 2015.  Implementation of the RECAT-EU Wake Turbulence Separation Scheme at Paris Charles De Gaulle, Paris-Le Bourget and Pontoise – Cormeilles-En-Vexin Airports from March 22nd 2016; AIRAC AIC FRANCE A 03/16; Direction des Opérations, Service de l’Information Aéronautique, DSNA; March 2016.  RECAT 1: Lessons Learned from MEM; WakeNet-Europe Workshop 2013; May 2013.  More Efficient Runway Throughput with RECAT-EU; Airport Business, Winter 2015.  EASA’s Role in Wake Turbulence Separation Standards; Skyway; Eurocontrol; Winter 2015.  Aircraft Type Designators; ICAO Doc 8643; ICAO; March 2016.  RECAT-2 EU Development, Consultation and Review; Eurocontrol.  Wake Turbulence; Aeronautical Information Circular P 001/2015; CAA – NATS; January 2015.  Time Based Separation – Heathrow Concept of Operation; World ATM Congress 2014; SESAR Joint Undertaking; March 2014.
Last Update: October 1, 2020