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Revising wake turbulence categories to gain capacity (RECAT)

RECAT is a EUROCONTROL-FAA initiative whose objective is to review the existing ICAO wake turbulence categories and associated pair-wise separation minima for both departure and arrival operations. The aim is to use up-to-date knowledge about the wake vortex phenomenon in order to propose new categories that will be more adapted to today’s aircraft and that will allow gains in runway capacity.


Today, three wake turbulence categories exist: Heavy, Medium, and Light that are identified from the maximum take-off weight (MTOW) of the aircraft. As an example, for arrivals, the distance-based separation minima between successive aircraft of given categories are as follows:
ICAO distance-based separation minima for the approach
ICAO distance-based separation minima for the approach
The radar separation minimum applies wherever the wake turbulence of the Leader aircraft is not a risk for the Follower. The graphs below illustrate the basic methodology which will be followed within the RECAT project. These graphs propose a simplified representation, as a function of aircraft weight, of:
  1. the maximum strength of the wake vortex (WV) generated by an aircraft at a distance that corresponds to the ICAO wake turbulence separation minimum, i.e. it corresponds to the maximum wake vortex its Follower may encounter.
  2. the resistance of an aircraft to an encounter with any wake vortex, when it is separated by the required ICAO wake turbulence separation minimum from its Leader.
To simplify the process, it has been assumed that the resistance to the wake vortex and the strength of the wake vortex as a generator can be taken to be equivalent as they follow similar trends: the heavier an aircraft is, the stronger the wake it generates at a given distance, but also the more resistant it is to the wake of another aircraft.

The basic rationale behind the RECAT project is to gather the aircraft types that are close to the boundaries of adjoining categories to build a new category. This decreases the separation behind aircraft near the “light” boundary of one category to recover some capacity; and similarly increases separation behind aircraft near the “heavy” boundary of the other category, where necessary, to increase safety. The objective within such a process is, in particular, to take into account aircraft that did not exist at the time the current ICAO categories were defined. Note that it is also possible that the current wake turbulence separation minima could be modified based on improved knowledge of the wake vortex phenomenon and that an existing category is split.
Before RECAT
Before RECAT
Before RECAT
Before RECAT

An Example

To estimate possible capacity gains resulting from a modification of the wake vortex categories, three different scenarios have been tested and compared with the current ICAO categories and separation minima. In each scenario, a different set of wake turbulence separation minima is applied to the wake vortex categories. The new categories and separation minima were proposed by a group of experts for the purpose of the exercise but have not been subject to any safety assessment, nor built to optimise the capacity of the runway. In order to do this, a very busy 24-hour traffic sample was generated and the runway throughput estimated for each scenario using a TAAM fast-time simulation. The graph below shows the runway throughputs which were achieved.
Runway throughput (number of aircraft that land per hour) over the day from 0=[0am to 1am]  to 23=[11pm to 0am]
Runway throughput (number of aircraft that land per hour) over the day from 0=[0am to 1am[ to 23=[11pm to 0am[
The “Demand” scenario illustrates how busy the traffic sample was. The “ICAO” scenario represents the runway throughput with the current ICAO wake turbulence categories and separation minima. As the demand was very high, it was possible to saturate the runway and estimate its capacity for each scenario. These capacities were then compared with the one achieved with the ICAO standards. The results ranged from a reduction of capacity of around 3% for scenario 1 to an increase of capacity of 9% for scenario 3.

The exercise has thus illustrated the possibility of increasing the runway capacity under certain scenarios but it has also warned of some limitations of RECAT. In fact, some of these scenarios considered not only a change of the wake categories and of the associated wake turbulence separation minima, but also took into account the possibility of reducing the radar separation minimum that is applied between all aircraft pairs for which no wake turbulence separation minimum exists. The reduction of the radar separation minimum is a problem that is independent from the RECAT objectives. In fact, without a reduction of the radar separation minimum, which was the case in scenario 1, it is not obvious whether the capacity increase with RECAT will be significant. If the pair-wise separations will be reduced in some cases, they will be increased in others to improve the safety of the operations. Furthermore, if capacity gains can reasonably be expected with carefully-chosen categories and separation minima, it is nonetheless possible that the impact will be negligible at some airports as the impact on capacity will depend on the traffic-mix operating there.
  Acrobat Further details of the fast-time simulations

The Proposed Methodology

The objective of the methodology used in RECAT is to identify new categories that optimise capacity gains while adapting pair-wise separations such that operations will be at least as safe as they are today. These new separations will be assessed using the latest models for wake behaviour, both transport and decay models, as well as models that estimate the impact of the wake on the Following aircraft. This may necessitate further work to establish a severity criterion for wake encounters as well as to define what an “acceptable” encounter is. Finally, the wake turbulence category of an aircraft may no more be identified through a simple criterion, such as the maximum take-off weight (MTOW), as other aircraft characteristics such as the wing span, inertia, or approach speed will be taken into consideration.

The methodology includes the following stages:
  1. Identify the most frequent aircraft types operating both in the ECAC area and in the USA. Typical traffic samples representative of the traffic-mix in Europe and in the USA will also be built at this stage of the process.
  2. For each pair of aircraft types identified in Stage1, estimate the minimum safe separation between the 2 aircraft based on the wake characteristics of the generator and of the worst possible impact of its wake encounter on the Follower.
  3. Identify the boundaries of the new categories and the associated pair-wise separations that will optimise the capacity gains for the traffic samples built during Stage 1. The associated pair-wise minimum separations will correspond to the greatest minimum separation amongst those estimated during Stage 2 for all the pairs of aircraft to be taken into account within the considered pair-wise category.
  4. Confirm capacity benefits for specific major airports in Europe and in the USA.
  5. Confirm operational acceptability and identify training and technical adaptations needed for a future implementation.
  6. Finalise the safety case, and all legal and technical documentation for a submission to ICAO.
Stages 3 to 5 may be repeated until a final list of categories has been established, which can be implemented operationally in a similar way to today’s procedures. This implementation, in particular, must not be dependent on the implementation of new assistance tools for air traffic controllers and pilots. The objective is to propose these new categories to ICAO by 2011.
  Acrobat RECAT project presentation

Next steps

The first proposal of new categories should be available in 2009 for further validation and approval by ANSPs. Once this first modification of the wake vortex categories has been proposed to ICAO, the objective is to further develop the RECAT project towards a dynamic categorisation of aircraft and separation minimum estimation that would cope both with local conditions at a given airport and with weather conditions, in order to increase capacity and safety further. This RECAT version will require the implementation of new operational systems and will be tailored to the air traffic controllers’ and pilots’ tools and working methods that will be defined by SESAR and implemented by 2020. In that respect, these new tools may also integrate other wake vortex related solutions such as Time-Based Separations (TBS) or Crosswind Reduced separations for Departure OperationS (CREDOS).


  Acrobat Further details of the fast-time simulations
  Acrobat RECAT project presentation


The RECAT project is sponsored by DAP/APN, Directorate of ATM Programmes - Airspace Network Planning and Navigation Division, of EUROCONTROL.

For further information, please contact:
Catalin Lepadatu
Project manager
Elsa Freville
Validation manager
  Last validation: 21/11/2008