RECAT - Wake Turbulence Re-categorisation

RECAT - Wake Turbulence Re-categorisation

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Definition

Wake Turbulence Re-categorisation, or Wake RECAT, is the safe decrease in separation standards between certain aircraft.

Description

The existing International Civil Aviation Organisation (ICAO) wake vortex separation rules are based solely upon aircraft weight, categorised as Heavy, Medium or Light. While safe, in some respects, they are now outdated and lead to over-separation in many instances. This, in turn, diminishes airport capacity and causes unnecessary traffic delays increasing costs, fuel burn and emissions. As an example, both the Boeing 747 and the Boeing 767 are categorised by ICAO as "Heavy" aircraft. Whilst the 4nm traditional separation distance, applied between two "heavies", is appropriate when the 767 is following the 747, in the reciprocal case, standard separation is excessive. To safely reduce the separation minima between a given aircraft pair, whether on departure or on final approach, both the wake vortex generated by the leader and the following aircraft’s resistance to it have to be taken into consideration.

Background

The ICAO categorisation of aircraft into three weight dependent groups has, for some time, been considered inadequate by many National Aviation Authorities and this has led to some regional variation in the categories and separation standards. The introduction of the Airbus A380 and the associated wake vortex turbulence concerns lead to an ICAO supervised joint study with experts from Airbus, the FAA, EUROCONTROL and JAA/EASA. The primary focus of the study was the A380 and the resulting Airbus A380 Wake Vortex Guidance was published in 2008. However, the supporting programme of over 180 hours of innovative flight testing including back to back comparative testing of different aircraft, cruise wake encounter tests, and ground and airborne LIDAR wake measurements, also led to the introduction of the RECAT programme.

Re-categorisation

After years of extensive, collaborative research between EUROCONTROL, the FAA, their research facilities and the aviation industry, the experts concluded that the required separation between certain aircraft could be safely decreased. Research had proven that, in addition to weight, other aircraft characteristics – such as speed and wingspan – also affect the strength of the wake generated as well as the following aircraft’s reaction to that wake. Using that information, under the RECAT-EU 6-category scheme, aircraft were re-assigned to one of six new categories (A through F) which were derived by redefining the transition weight between the old categories, adding a Super category and splitting each of the Medium and Heavy categories into two new ones. The resulting categorisation is as follows:

  • CAT A - "Super Heavy"
  • CAT B - "Upper Heavy"
  • CAT C - "Lower Heavy"
  • CAT D - "Upper Medium"
  • CAT E - "Lower Medium"
  • CAT F - "Light"

The RECAT-EU distance-based minima and time-based minima have been approved by EASA and are published as AMC7 to the EU 2020/469 ATS.TR.220 Application of wake turbulence separation, and available for deployment.
RECAT-EU is also integrated in the recently published ICAO enhanced wake separation minima based on wake turbulence group (Amendment 9 to Doc 4444 PANS-ATM edition from Nov. 2020).

All aircraft types certificated prior to 01 January 2013 have been assigned to one of the new categories, with examples provided in the table below:

Super Heavy Upper Heavy Lower Heavy Upper Medium Lower Medium Light
CAT-A CAT-B CAT-C CAT-D CAT-E CAT-F
AIRBUS A-380-800 AIRBUS A-330-200 AIRBUS A-300-600 AIRBUS A-318 ATR ATR-42-300/320 DASSAULT Falcon 10
ANTONOV An-124 Ruslan AIRBUS A-330-300 AIRBUS A-300 AIRBUS A-319 ATR ATR-42-500 DASSAULT Falcon 20
ANTONOV An-225 Mriya AIRBUS A-340-300 AIRBUS A-310 AIRBUS A-320 ATR ATR-72-201 FAIRCHILD DORNIER 328
(...) AIRBUS A-340-500 BOEING 707-300 AIRBUS A-321 BOEING 717-200 EMBRAER Brasilia
  AIRBUS A-340-600 BOEING 757-200 ANTONOV An-12 BOEING 737-200 BEECH 400 Beechjet
  AIRBUS A350-900 BOEING 757-300 BOEING 737-600 BOEING 737-300 RAYTHEON BAe-125-700/800
  BOEING 747-400 (international, winglets) BOEING 767-200ER BOEING 737-700 BOEING 737-400 BRITISH AEROSPACE Jetstream 32
  BOEING 747-8 BOEING 767-300ER BOEING 737-800 BOEING 737-500 BRITISH AEROSPACE Jetstream 41
  BOEING 777-200 / 777-200ER BOEING 767-400 BOEING 737-900 BOMBARDIER Challenger 600 GATES LEARJET Learjet 35
  BOEING 777-300 BOEING C-135 Stratolifter LOCKHEED C-130 BOMBARDIER Regional Jet CRJ-100 LEARJET Learjet 60
  BOEING 777-200LR and 777-F MCDONNELL DOUGLAS DC-10 MCDONNELL DOUGLAS MD-81 BOMBARDIER Regional Jet CRJ-700 SAAB 340
  BOEING 787-8 Dreamliner MCDONNELL DOUGLAS MD-11 MCDONNELL DOUGLAS MD-90 BOMBARDIER Dash 8 Q400 PIAGGIO P-180 Avanti
  ILYUSHIN Il-96 ILYUSHIN Il-76 TUPOLEV Tu-204 EMBRAER ERJ-135 CESSNA 650 Citation 3/6/7
  (...) TUPOLEV TU-95 (...) EMBRAER ERJ-145 CESSNA 525 CitationJet
    (...)   EMBRAER ERJ 170-100 C180
        EMBRAER ERJ 190-100 CESSNA 152 Aerobat
        GULFSTREAM AEROSPACE Gulfstream 4 (...)
        (...)  

Separation Criteria

Under standard ICAO criteria, wake turbulence separation, for arrival or departure, can be charted as follows:

Preceding Aircraft Following Aircraft Minimum Separation
HEAVY HEAVY 4.0 NM
HEAVY MEDIUM 5.0 NM
HEAVY LIGHT 6.0 NM
MEDIUM LIGHT 5.0 NM

Special separation standards of 6NM, 7NM and 8NM for a Heavy, Medium and Light respectively following a Airbus A380 also apply. In some regions, there are also special standards for aircraft following a Boeing 757.

Under the RECAT programme, the required separation is as depicted in the following table.

An Empty Field Indicates Minimum Radar Separation
Leader/Follower CAT A CAT B CAT C CAT D CAT E CAT F
CAT A 3 NM 4 NM 5 NM 5 NM 6 NM 8 NM
CAT B   3 NM 4 NM 4 NM 5 NM 7 NM
CAT C     3 NM 3 NM 4 NM 6 NM
CAT D           5 NM
CAT E           4 NM
CAT F           3 NM

For time based separation on departure, the following values, in seconds, apply.

Leader/Follower CAT A CAT B CAT C CAT D CAT E CAT F
CAT A   100s 120s 140s 160s 180s
CAT B       100s 120s 140s
CAT C       80s 100s 120s
CAT D           120s
CAT E           100s
CAT F           80s

Benefits

Immediate benefits, in terms of runway capacity and operational efficiencies, will result from implementation of RECAT protocols. These benefits include:

  • Peak period runway throughput can increase by 5% or more depending on airport traffic mix.
  • For an equivalent volume of traffic, RECAT spacing results in a reduction of the overall flight time for each affected aircraft reducing fuel burn, emissions and operating costs.
  • Due to more efficient departure and arrival spacing, RECAT allows a more rapid recovery from adverse conditions or a runway change.
  • In airspace trending towards saturation, such is the case in Europe, the projected fleet renewal is trending towards a greater percentage of larger aircraft. Under RECAT, this evolution will actually further enhance runway capacity.

Implementation

The first implementation of RECAT separation standards, based on the FAA RECAT 1.5 scheme, occurred in the United States at Memphis, Tennessee in November 2012. FedEx, the major carrier at Memphis, has reported substantial efficiencies and savings due to the RECAT programme. Since, all major US TRACON facilities have transitioned to RECAT

The first European implementation of the RECAT-EU separation standards occured at Paris/Charles de Gaulle Airport in 2016. Since, it is already in daily operations at Paris CDG/LFPG, London Heathrow/EGLL (since 2018), Vienna/LOWW (since February 2020), and Barcelona / LEBL (since May 2022)
Partial application is operated at Leipzig (between freighter aircraft type – B757, B767, A300), Koln-Bonn (same as Leipzig) and Toulouse (between AIRBUS test flights).

Further optimisation with Pair-Wise Separation (S-PWS)

An additional optimisation is obtained by the specification of wake turbulence minima on an aircraft by aircraft basis, with the characterisation of wake turbulence generation and of encounter resistance per aircraft type, and related risk assessment, as refinement from RECAT-EU. This led to the establishment of static Pair-Wise Separation (S-PWS) Minima (also called RECAT-EU-PWS) for 103x 103 aircraft types, representing more than 95% of the European traffic at major airports, together with RECAT 20-Category to complement for the whole fleet.

The development of ‘S-PWS’ solution for Approach and Departure, was led by EUROCONTROL under the SESAR2020 programme, referred as PJ02.01.04 (S-PWS-A) and PJ02.01.06 (S-PWS-D) solutions.

The minima are derived on the basis of the same methodology as used in RECAT-EU, however aligning the wake turbulence encounter risk on a pair-wise basis, delivering further separation reduction of 0.5NM or 1NM for some frequent aircraft pairs. On this, EUROCONTROL has produced a generic Safety Case, submitted to EASA for review and recommendations, in support of regulatory acceptance of local deployments.

There are several ways to operate the RECAT-EU-PWS scheme: on a pair-wise basis (with ATC separation delivery tool support such as used for enabling Time-Based Separation - TBS), on a categorical system basis (with/without ATC separation delivery tool support), or on a procedural basis for selected aircraft type pairs in view of upgrading the applied wake categorisation scheme and bringing quick wins.

A presentation of the RECAT-EU-PWS scheme is available on the EUROCONTROL Airport page.

Further development of dynamic PWS minima (D-PWS), based on dynamic evolution of generated wake turbulence as a function of the prevailing weather conditions, are also under development under SESAR industrial research programme.

Further Reading

EUROCONTROL

FAA

US Department of Transport

Airbus

  • Wake Votrices, C. Lelaie, Airbus Safety First Magazine No. 21, pp. 42-50, January 2016.
Categories
Wake Vortex Turbulence

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