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Wake Vortex Turbulence is defined as turbulence which is generated by the passage of an aircraft in flight. It will be generated from the point when the nose landing gear of an aircraft leaves the ground on take-off and will cease to be generated when the nose landing gear touches the ground during landing. Where another aircraft encounters such turbulence, a Wake Vortex Encounter (WVE) is said to have occurred.
Wake vortex generated by aircraft on departure or final approach is one of the main factors defining safe separation minima between two aircraft. Existing ICAO Wake Vortex separation rules (based upon the Heavy, Medium and Light categorisation) were implemented over 40 years ago and have in some respect become outdated, resulting in States introducing their own local amendments.
The reason ICAO rules have not previously been updated is that the means to do this was not available. In particular, any change would require the completion of a full safety case to demonstrate that the change was safe to implement.
Due to the lack of measuring technology it was not possible to support the necessary safety arguments with data and therefore the safety of proposed changes could not be proven. However, with the development of measuring technology, e.g. LIDAR and an increased understanding of the physics of wake behaviour it is now possible to update the ICAO wake vortex provisions and also develop new advanced wake turbulence related procedures. These will have a positive effect on safety and capacity and could significantly reduce airport delays.
EUROCONTROL has been involved in these developments since 2002. During this period, together with its partners, EUROCONTROL has developed the largest global wake database ever collected from Frankfurt, Paris and London. This, together with the better knowledge of wake behaviour, has enabled new metrics, taking into account both the strength of generated wake and an airframe’s ability to resist it. The proposals to implement these metrics, and associated standards, are termed RECAT (Re-categorisation of Wake Turbulence Separation Minima).
With the introduction of the Airbus A380 one of the main challenges for ATM was to develop associated wake turbulence separations before it entered service. The issue was the lack of a proven methodology available so, in July 2003 a European led A380 Steering Group was established with representatives from EUROCONTROL, JAA (EASA), AIRBUS, FAA, NASA and ICAO.
The tasks of the group were to:
- Select methodologies for evaluation of A380 wake turbulence;
- Assess A380 wake turbulence in comparison to other aircraft in the Heavy category;
- Establish recommendation for A380 wake turbulence separation to ICAO.
An initial but conservative approach resulted in an interim ICAO State Letter requiring a 10NM separation minimum for all aircraft following the A380.
A second State Letter, this time based on a Safety Case and Safety Assessment Report produced by the Working Group in accordance to EUROCONTROL Safety Regulatory Requirement 4 (ESARR 4), was issued by ICAO in October 2006 with reduced interim separations; a third State Letter was then issued July 2008 with further reduced separations enabled by additional data.
Work was then extended with the analysis of Airbus flight test data to determine if further separation reductions were possible. At this stage it was agreed with EASA and Airbus that the A380 work be incorporated into the wider wake turbulence re-categorisation for Europe (known as RECAT-EU).
Re-categorisation of the ICAO Wake Turbulence Separation Minima (RECAT)
The aim of RECAT is to safely increase capacity at airports by redefining wake turbulence categories and their associated separation minimum.
Today’s ICAO separations based solely on Maximum Take Off Weight (MTOW) and aircraft are allocated into one of three categories (i.e. Heavy, Medium or Light). It is commonly recognised that the current Medium category as defined by ICAO is too large, encompassing aircraft from the size of the Boeing 757 down to aircraft types such as the Beech 1900. Because the separations are defined based on the worst case in each category, this leads to over separation in many instances.
In Europe, RECAT-EU divides the ICAO Medium and Heavy categories each into two categories and also creates a Super Heavy category for the Airbus A380. In October 2014, following a thorough stakeholder consultation and detailed review of the safety assessment, EASA confirmed that the RECAT-EU safety case provides the assurance necessary for deployment. The first deployment is now planned for Paris-CDG in 2015.
The future will see the development of RECAT-2 and RECAT-3. The former, which is currently being prepared, will define separations for each pair of aircraft from a matrix of 115 by 115 aircraft. This will cover separations for more than 95% of the most common global aircraft types. A series of consultations are ongoing, with the intention that an agreed US/Europe RECAT-2 package be put before ICAO for incorporation into PANS-ATM (Doc. 4444). The 115 by 115 separation matrix could be used both for a customised categorisation as a function of a local traffic mix or for applying pairwise separations. It is however anticipated that to apply pairwise separations a dedicated ATCO support tool would be needed.
RECAT-3 will create a dynamic pair-wise separation scheme, which gives an extra level of flexibility in applying separations by using meteorological data from ground and air system sensors to identify when separations can be further refined.
Separation delivery tool
Pairwise, time based or weather dependant separations will be challenging for the ATCO to apply without a support tool. In parallel to the safety assessment of the different separation reductions, EUROCONTROL is working with the industry to develop a separation delivery support tool that integrates the different separation minima, runway configuration and meteorological conditions to advise the ATCO of the appropriate separation to be applied between two aircraft.
The initial phase of this work was an extensive review of the best practices in terms of separation delivery in Europe. Site visits and controller interviews were organised at medium and major European airports, radar tracks were analysed and local procedures reviewed.
In parallel, a model that predicts aircraft behaviour and speeding management on the glide slope is being developed and benchmarked against collected radar tracks. The purpose of this model is to provide the ATCO with guidance to help apply separation that takes into account both the wake vortex minima and a buffer for the compression effect on short final (i.e. inside 4 miles, when ATC speed control no longer applies). To be effective, such a model would need to consider aircraft specific performance characteristic (deceleration, final approach speed, etc. …) and actual weather conditions (headwind, wind gradient, etc…)
A first version of this tool and associated HMI jointly produced with Thales has been tested using the EUROCONTROL real time simulation facilities at Brétigny in October 2014. As a result an upgraded version, calibrated against several airport radar databases and associated weather measurements, is now under development.
Wind dependent separations
The Time Based Separation (TBS) - Transitional Step
Strong headwinds bring difficulties to airports by reducing aircraft ground speed during approach, which decreases the landing rate, resulting in delays and even cancellations.
The key principle of the TBS concept is to define the minimum in-trail distance separation to apply, as a function of headwind, to maintain a constant time separation, as observed in low headwind conditions, consequently maintaining the landing rate.
Although the TBS concept does not reduce the time separation with reference to the ground (i.e. the ‘zero wind’ time between aircraft is maintained), the reduction of the actual distance between aircraft in the air results in the potential for encounters with wake turbulence that has had less time to decay. However, extensive measurement of wake for a wide range of aircraft under various wind conditions has demonstrated that there is always a greater rate of wake decay in such windy conditions and that this is a reliable mitigation to the potential encounter of a younger wake.
The use of a Separation Delivery Tool will be required to accurately apply the separation and also to manage the compression effect, which varies according to the wind. The compression effect results in reduction of the observed distance separation and takes place when leading aircraft decelerate to final landing speed while the second aircraft is still maintaining its approach speed (i.e. during the final phase of the approach when ATC speed control is no longer applied).
A TBS system that provides in real-time the separation to apply between two aircraft needs to be fed by:
- the aircraft sequence to anticipate aircraft specific speed management and to define the time separation required for a given wake category pair, and;
- the wind profile, approximately 10 minutes before landing, to define the separation on final approach.
These are the two main technical challenges of the TBS system development that require respectively the development of an easily usable sequencing tool and a now casting technology based upon merging wind profile measurement and heuristic technics.
Note that a simplified version of the TBS system could be derived from the full concept. This consists of applying a uniform reduction for all aircraft pairs when the headwind is above a given threshold. The result would be a partial recovery of the lost capacity. Such a simplified concept could be applied using only procedural changes that do not require a system upgrade.
The first deployment of full TBS is planned at London-LHR in 2015.