RECAT - Wake Turbulence Re-categorisation

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 While safe, in some respects, they are now outdated and lead to over-separation in many instances.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 FAAEUROCONTROL 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, 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"

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 HeavyUpper HeavyLower HeavyUpper MediumLower MediumLight
CAT-ACAT-BCAT-CCAT-DCAT-ECAT-F
AIRBUS A-380-800AIRBUS A-330-200AIRBUS A-300-600AIRBUS A-318ATR ATR-42-300/320DASSAULT Falcon 10
ANTONOV An-124 RuslanAIRBUS A-330-300AIRBUS A-300AIRBUS A-319ATR ATR-42-500DASSAULT Falcon 20
ANTONOV An-225 MriyaAIRBUS A-340-300AIRBUS A-310AIRBUS A-320ATR ATR-72-201FAIRCHILD DORNIER 328
(...)AIRBUS A-340-500BOEING 707-300AIRBUS A-321BOEING 717-200EMBRAER Brasilia
 AIRBUS A-340-600BOEING 757-200ANTONOV An-12BOEING 737-200BEECH 400 Beechjet
 AIRBUS A350-900BOEING 757-300BOEING 737-600BOEING 737-300RAYTHEON BAe-125-700/800
 BOEING 747-400 (international, winglets)BOEING 767-200ERBOEING 737-700BOEING 737-400BRITISH AEROSPACE Jetstream 32
 BOEING 747-8BOEING 767-300ERBOEING 737-800BOEING 737-500BRITISH AEROSPACE Jetstream 41
 BOEING 777-200 / 777-200ERBOEING 767-400BOEING 737-900BOMBARDIER Challenger 600GATES LEARJET Learjet 35
 BOEING 777-300BOEING C-135 StratolifterLOCKHEED C-130BOMBARDIER Regional Jet CRJ-100LEARJET Learjet 60
 BOEING 777-200LR and 777-FMCDONNELL DOUGLAS DC-10MCDONNELL DOUGLAS MD-81BOMBARDIER Regional Jet CRJ-700SAAB 340
 BOEING 787-8 DreamlinerMCDONNELL DOUGLAS MD-11MCDONNELL DOUGLAS MD-90BOMBARDIER Dash 8 Q400PIAGGIO P-180 Avanti
 ILYUSHIN Il-96ILYUSHIN Il-76TUPOLEV Tu-204EMBRAER ERJ-135CESSNA 650 Citation 3/6/7
 (...)TUPOLEV TU-95(...)EMBRAER ERJ-145CESSNA 525 CitationJet
  (...) EMBRAER ERJ 170-100C180
    EMBRAER ERJ 190-100CESSNA 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 AircraftFollowing AircraftMinimum Separation
HEAVYHEAVY4.0 NM
HEAVYMEDIUM5.0 NM
HEAVYLIGHT6.0 NM
MEDIUMLIGHT5.0 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/FollowerCAT ACAT BCAT CCAT DCAT ECAT F
CAT A3 NM4 NM5 NM5 NM6 NM8 NM
CAT B 3 NM4 NM4 NM5 NM7 NM
CAT C  3 NM3 NM4 NM6 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/FollowerCAT ACAT BCAT CCAT DCAT ECAT F
CAT A 100s120s140s160s180s
CAT B   100s120s140s
CAT C   80s100s120s
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 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. The bulk of the FedEx fleet serving Memphis is made up of CAT-C (old ICAO "heavy") aircraft. The new separation standards result in reducing the old 4 mile requirement to Minimum Radar Separation of 2.5 to 3 miles and save the Company both time and fuel. The FAA has reported an airfield capacity increase of as much as 15 percent at peak hours and FedEx has demonstrated an average sector time reduction of about 5 minutes and a fuel savings approaching $1.8 million per month. Since 2012, RECAT procedures have been also introduced at several other major US airports including Louisville International AirportCincinnati North KentuckyAtlanta/Hartsfield-Jackson InternationalHouston Intercontinental and most of the New York City area airports. A diagram depicting implementation can be found here by scrolling to the bottom of the page and selecting "Wake Re-Categorization Phase 1" in the "Capabilities" box.

The first European implementation of the RECAT separation standards is scheduled for Paris/Charles de Gaulle Airport at the end of 2015.

Future Development

Plans for the immediate future in both Europe and United States revolve around expansion of the number of airports using the new RECAT separation standards. However, further enhancement of that capability is also envisioned.

Under RECAT-2, separation standards will be refined on an aircraft by aircraft basis with a planned 115 x 115 matrix of aircraft types. Although this will require an appropriate software application to implement, it is envisioned that it will result in a tool that covers as much as 95% of the potential leader/follower pairs for existing aircraft types and will provide the controller with the optimum separation for each pair.

RECAT-3 will further enhance the pair-wise separation of RECAT-2 by incorporating meteorological data from ground and airborne sensors to identify when separation standards could or should be further refined.

Related Articles

Further Reading

EUROCONTROL

FAA

US Department of Transport

Airbus

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