Background

As they pass through the air, aircrafts generate strong rotating air turbulence with vortices in the wingtips, which can last for several minutes and therefore can be encountered at considerable distances from the generating aircrafts, until their circulation strength decays and the turbulence disappears. These wake vortex turbulences can be hazardous to other aircrafts if encountered too near and in a bad combination of several factors, being a possible cause of discomfort, incidents, or even accidents. The main protection mechanism to avoid that hazard is to maintain a minima separation to the generating aircrafts that takes into account the wake vortex hazards.

The dangers of wake vortex encounters (WVE) in en-route airspace operations have been traditionally considered negligible, with a very low probability of a severe wake vortex encounter. However, a few significant accidents have occurred in the recent years in en-route airspace, thus showing that the level of risk is actually higher than initially thought, e.g., some wake encounters have been reported as severe up to 25NM behind big generating aircraft.

In the meanwhile, the SESAR program is looking for increasing sector capacities in en-route sectors to allocate the high-dense air traffic levels that are expected for the next decades. One way to do that is by reducing the separation minima between flights in en-route operations since, due to the technological advances in the navigation and surveillance systems, current separation schemes could be considered over-conservative in some cases for some aircraft categories. 5NM in horizontal and 1000 feet in vertical is applied today in en-route operations for all aircraft types and conditions.

On top of that, the ATM system is expected to experiment several changes in the next decades that will cause the increase of both the probability of encounters and their severity. For instance, the traffic densities are expected to be much denser than today in certain areas; the aircraft mix is going to be more heterogeneous than today too, with larger aircraft sharing airspace with smaller aircraft; and new concepts of operations such as free-routing, or continuous climbing and descending operations for all the flight phases may increase the risks of the WVEs hazards in en-route operations.

Therefore, a deep understanding of the WVE hazards by characterizing and quantifying the risks is highly needed to improve the safety and efficiency of current and future operations.

Objectives and Approach

Given the above background context, the R-WAKE project (“Wake Vortex Simulation and analysis to enhance en-route separation management in Europe”) was set with the main objectives of, first, investigating the risks and hazards of wake vortex encounters in the en-route airspace, and second, identifying potential enhancements to the current separation schemes in order to allow aircrafts to be more closely spaced while keeping or improving the current safety levels, thus aiming at expected benefits in the safety, airspace capacity, and flight efficiency of the European Air Traffic Management System.

The R-WAKE project is funded by the SESAR Joint Undertaking under the European Union’s Horizon 2020 research and innovation program, addressing the SESAR 2020 Exploratory Research program topic ER-07-2015 separation management and separation standards, in the area of advanced air traffic services, as an application-oriented research (i.e., aiming to cross the exploratory to industrial (ER/IR) maturity gate.

The R-WAKE project specific research question is formulated as: ‘What separation minima reductions can be applied in specific and clearly defined operational conditions to increase airspace capacity while keeping or enhancing the current safety levels taking into account the risk of en-route WVE hazards?’

To enable the investigation, the project proposed the development of a fast-time simulation-based system for dynamic risk modelling, referred as the R-WAKE System to become a key research tool and concept validation means, covering the simulation of weather, traffic, wake vortex, wake vortex interaction and prediction, and safety and cost-benefit analysis methods and tools. The R-WAKE System has been used to simulate and characterize the WVE hazard, available in form of simulation databases and concept study scenarios, and used as evidence body to identify and support the targeted new separation enhancement proposals, referred as the R-WAKE Concept.

The main result achieved is an evidence based proposal of wake separation minima adjustments to improve the current en-route separation schemas that takes into account aircraft category and encounter geometry, referred to as the R-WAKE-1 concept, which, following a positive Exploratory to Industrial Research (ER/IR) maturity assessment gate has been considered to be near TRL-2 level and ready for transition to Industrial Research, and has been recommended as a new operational improvement step SESAR Solution to be included in the SESAR programme ATM Master Plan.

The methodology and concepts developed in R-WAKE can therefore be considered as paramount for a short-term revision of the current separation standards, while also a key enabler of the future SESAR 2020+ ATM system. Future Trajectory Based Operations (TBO) could take full advantage of the enhanced knowledge generated by the R-WAKE project to dynamically adapt the separations to the different atmospheric and flight conditions to enhance the safety, capacity and efficiency of the operations.

Solution

The project faced a complex development relying on two full iterations, one per year, of the three closely-coupled project work areas, that is: [1] the target concept research scoping and study approach, i.e. defining the target-concept to be assessed, its study scenarios, and the requirements for the safety study terms and the simulation system, [2] the research system development, i.e., the integrated simulation toolset and methodology, and [3] the research realization, i.e., execution of the simulations, data analysis, concept development, safety & robustness analysis, and results assessment.

Research Approach

The scoping of the target concept has been a major topic addressed in the project, due to the novelty of the topic and the risk of potentially ending up with an application-oriented research that could be not pragmatic enough for the purpose of the ER/IR gate. Following the example of the RECAT program (minima wake separation optimization in airports TMA/approach), the R-WAKE Concept was formulated with a step-wise approach, as an evolutionary roadmap of changes starting by the simplest “static distance-based category-wise separation” concept, and planning to move forward towards the overall R-WAKE concept idea defined as “Dynamic Pair Wise Separation (D-PWS) Management to prevent WVE hazards during En-Route Operations”, which is targeting scenarios in the long-term ATM (SESAR 2020+ target concept).

The concepts being explored (research scoping) have been iteratively clarified, specified and narrowed along the project. Following the scoping advises from SJU, especially those stated during the interim mid-term review, the concept could be finally developed satisfactorily in the second half of the project: the project would address a first meaningful short-term oriented step of the R-WAKE Concept, labelled as R-WAKE-1 Concept, which consisted in the development of a new separation scheme, as simplified as possible, in order to minimize the ‘impact’ –i.e., need for changes– on current ATM systems and methods, and to facilitate as much as possible the potential short-term implementation of the concept and the its smoothest integration into the current system.

The research approach was structured in two incremental steps, aligned with the hazard risk assessment terms, i.e., severity and frequency (see Figure 1). Different simulation models were used at each of the steps. In more detail:

  • Research Step-1, refers to the study of upsets and severity levels for a variety of vortex behaviors, aircraft types, and encountering conditions in the upper flight level. The use of high fidelity micro-level models was used to simulate the expected of behavior/upsets of flights in the presence of a WVE, and an absolute severity baseline was developed from the simulation results through a qualitative assessment made by professional pilots and air traffic controllers.
  • Research Step-2, refers to the study of the hazard risks in terms of frequency or repeatability of the hazardous events, thus assessing the profile of encounter occurrences per severity level. The absolute severity baseline resulted from step 1 was used to categorize each of the WVE found by their severity. To model the uncertainties and to better account for the potential risks, some macro-level models were introduced in the simulations.

 

The assessment of risks –frequency or repeatability study- (research step 2) is divided into two additional steps:

  • Research Step 2.1: focused on the assessment of Conditioned Individual Risk (CIR), that is, the study of the potential risk level supported by a flight that is forced to have a –simulated– WVE under many different conditions that could potentially occur in the real ATM system, in terms of separation distance, encounter geometry, aircraft condition, and atmospheric condition. This approach led to the characterization of the Suspected Hazard Areas (SHA) per each pair of aircraft types.
  • Research Step 2.2: focused on the risk assessment at system level, that is the study of the frequency profile in different airspace regions arising from simulating different traffic patterns. In this case the encounters are not forced as in Step 2.1, but they emerge as a consequence of the (lack of) effectiveness of the risk mitigation layers in the ATM system. The R-WAKE system can measure the risk at European airspace level (ECAC), named System ECAC-wide Risk (SER), and also the risk in particular airspace regions of the system, called Segregated Airspace Risk (SAR). The systemic risk is important to compare the effectiveness of different separation schemes understood as risk mitigation elements of the system. In particular the new R-WAKE concept separation schemes could be compared against the current schemes.

 

R-WAKE_final_summary_fig1 (research approach)

Figure 1: R-WAKE Research Approach Steps

 

System Development

The R-WAKE research system has been architected as a framework of two elements: i) the safety and robustness analysis (SRA) methodology, and ii) the integrated simulation platform (Figure 2). The entire framework is fully oriented at the task of providing quantitative and qualitative evidence to support the development of the safety and business cases.

The system can generate a body of quantitative evidence by implementing the concept of dynamic risk modelling, widely accepted and used today in the ATM research and practitioner community. This includes the simulation of trajectories, wake vortex encounters and, when necessary, the application of some ATM constraints. The qualitative evidence, including the interpretation of the simulation results, is part of the R-WAKE safety and robustness analysis (SRA) methodology, which is thoroughly aligned with the applicable references from SESAR and EUROCONTROL, i.e.: the SESAR Safety Reference Material (SRM), EUROCONTROL ESARR4 (Risk Assessment and Mitigation in ATM), and EUROCONTROL SAM (Safety Assessment Methodology).

 

R-WAKE_final_summary_fig2 (system frmk)

Figure 2: R-WAKE System: a framework of safety and robustness analysis method and simulation tools

 

The integrated R-WAKE system simulation platform includes (Figure 3 shows the general architecture):

  • a Weather Simulation (WXS) based on historical real met data;
  • an ATM traffic and trajectory simulator (TRS), which embeds a high fidelity Wake Interaction Assessment Model (WIAM) and a Wake Encounter Severity Prediction System (WEPS);
  • a Wake Vortex Simulator (WVS); and
  • a Safety and Robustness Analysis (SRA) (including brand-new risk models developed in the project) and methods and tools for a cost-benefit analysis (CBA).

 

R-WAKE_final_summary_fig3 (system integrated simulator arch)

Figure 3: R-WAKE System – integrated simulation framework – high level architecture

 

The R-WAKE simulation platform can be set up in three different configurations, according to three different types of safety studies planned in the research approach:

  • R-WAKE System configuration 1: Micro-simulator of upset for the severity baseline study (Step 1); based on the new implementation of a Wake Interaction Assessment Model (WIAM);
  • R-WAKE System configuration 2: Macro-simulator of conditioned individual encounters (Step 2.1) (see Figure 4), supporting the study of the suspected hazard area (SHA) for a given pair of aircraft under different flight conditions.
  • R-WAKE System configuration 3: Macro-simulator of the ATM systemic traffic scenarios (Step 2.2). This uses the full integrated system, in which the traffic simulator (TRS) reproduces ATM traffic scenarios covering specific airspace sectors or even the full European area (ECAC), allowing to study of the risk in the system by assessing the frequencies of each type of WVE (dynamic risk modelling approach).

 

R-WAKE_final_summary_fig4 (system setup for CIR study)

Figure 4: R-WAKE system setup to study Conditional Individual Risks (CIR) / Suspected Hazard Areas (SHA)

 

Simulation Analysis

The design of experiments aimed at generating the knowledge and evidence required in the project while minimizing the massive combinatorial problem of exploring all the parameters.

The realization of research step 1 (upset and severity baseline with WIAM tools) produced an upset characterization database with 23.328 scenarios, after combining study parameters of the vortex objects and the encountering aircraft. A representative subset of 12 upset scenarios was selected and assessed by experienced pilots and air traffic controllers, leading to the establishment of the R-WAKE absolute severity baseline in the form of a severity matrix of upset parameter thresholds (labelled SMUP). With the SMUP any WVE encounter can be classified into 5 severity levels (1-no-safety-effect, 2-minor, 3-major, 4-hazardous, and 5-castastrophic) as a function of the maximum upset generated in the following parameters: 1/ bank roll, 2/ altitude loss, 3/vertical speed change, 4/true airspeed change, and 5/ load factor. The resulting SMUP represents a tangible research outcome of the project, and is also the kernel model integrated in the macro-simulator to classify the WV encounters detected according to their severity (absolute safety criteria).

The realization of research step 2.1 (CIR/SHA database with TRS-WVS-WEPS tools) simulated 583.632 scenarios and delivered the corresponding database of upsets classified by their severity class. The scenarios considered different aircrafts types, geometries, and atmospheric conditions, and the upsets were calculated within a longitudinal-vertical grid of separation points covering the full wake vortex hazard area. Such CIR database represents a 3D map (4D indeed, since time-based is also considered) of the WVE suspected hazard areas (SHAs) –see Figure 5–, which enable the condition-wise identification of currently over-protected cases (opportunities for minima reduction), and the discovery of under-protected cases with respect the current separation standards that may need of a separation minima increase. An outcome data sample is shown in Figure 6 with a screenshot of the R-WAKE simulation database CIR/SHA viewer tool. Such tool has been developed and made public to facilitate the exploration and analysis of the Suspected Hazard Areas database. Figure 7 shows another way to visualize a SHA, formatted to explore the longitudinal-vertical Minimum Wake Separation (MWS) in a category-wise matrix, and highlighting the opportunities found for separation reduction (green-shadowed cells) or the needs detected for separation increase (red-shadowed cells).

 

R-WAKE_final_summary_fig5 (SHA concept)

Figure 5: Risk Maps of Reasonable Worst Case (RWC) – Suspected Hazard Area (SHA) per Severity Class

 

R-WAKE_final_summary_fig6 (CIR-SHA viewer screenshot)

Figure 6: R-WAKE simulation results database: CIR/SHA data exploration tool

 

R-WAKE_final_summary_fig7 (simulation results SHA sample)

Figure 7: R-WAKE Simulation results database: Sample of a SHA table with longitudinal-vertical Minimum Wake Separation (MWS) per category-pairs, highlighting detected over-protections (opportunities for separation reduction (green shadow), and under-protections (needs for separation increase (red shadow).

 

The R-WAKE-1 concept development (identification of specific separation schemes improvements) was elaborated on the basis of the generated CIR/SHA database (research step 2.1), looking at the opportunities and needs in the three separation dimensions (longitudinal, vertical, lateral).  The resulting R-WAKE-1 concept developed consists in four new static separation schemes and two new dynamic wind dependent separation schemes, labelled as En-Route Minimum Wake Separation (RMWS), designed to provide the required safety protection and looking for airspace capacity increases. The schemes are:

  • RMWS-1: Static MWS-lateral-orthogonal for followers in parallel tracks for any generator
  • RMWS-2: Static MWS-vertical for all followers in any relative headings per generator category.
  • RMWS-3: Static MWS-longitudinal for followers levelled in-trail or crossing behind per generators category
  • RMWS-4: Static MWS-longitudinal for follower climbing/descending behind or crossing in FL below a generator’s trajectory (and not separated vertically) per Generators (RA, RB, RC)
  • RMWS-5: Dynamic Wind-Dependent MWS-lateral-orthogonal for parallel tracks
  • RMWS-6: Dynamic Wind-Dependent MWS-combined-vertical-lateral-orthogonal in parallel tracks

The proposed R-WAKE-1 concept schemes are complemented with a set of application ideas (operational strategies) that aim to illustrate how the expected benefits in airspace capacity and safety could potentially materialize. The applications are:

  • APP-1: Lateral Separation reduction for parallel tracks
  • APP-2: Vertical separation reductions to increase vertical physical airspace capacity
  • APP-3: Longitudinal separation reduction for in-trail traffic at the same flight level as the generator aircraft
  • APP-4: Combination of vertical separation reduction and lateral separation to compress the traffic vertically
  • APP-5: Increase safety during climbing/descending operations with traffic crossing below
  • APP-6: Cooperative subliminal offsets based on ADS-B
  • APP-7: Wake encounter risk advisory service;

An initial Cost Benefit Analysis of the R-WAKE-1 concept and potential applications concluded that R-WAKE-1 separation schemes could result, if implemented, in a good balance between: a) increased safety and potential reduced capacity under some conditions (in which currents standards do not protect the flights enough), and b) increased capacity and unchanged safety under other conditions.

Maturity Assessment and further research needs

The maturity achieved by the proposed R-WAKE-1 concept and its justification material was assessed in close collaboration with the SJU against the Exploratory to Industrial Research (ER/IR) gate criteria, concluding that all the ER/IR criteria is satisfied fully or partially-non-blocking, and therefore the concept was considered near TRL-2 level, and ready for transition into industrial research and development phase.

A comprehensive list of further research needs (next steps) is provided derived from the assumptions and limitations in the performed approach, in order to move forward the development and validation of both, 1st) the R-WAKE system (simulation models and safety and robustness analysis method required to support R-WAKE concept development), and 2nd) the R-WAKE-1 concept (safety and robustness analysis (safety case) and business case requirements, and concept operational assumptions validation).

A main cornerstone in the project approach is the development and validation of the Aircraft Wake Interaction Assessment Model (WIAM) that enables high-fidelity micro-simulation of wake encounter upsets, and the development of the hazard absolute severity baseline. The limited real life data that has been available to validate this core micro-simulation models represents a main limiting factor in the results so far. Therefore, among the research next steps it is particularly important to plan for large data collection campaigns with real aircraft flight data to enable to enable further validation of the micro simulation wake interaction assessment models, in order to increase the confidence in the hazard severity baseline.

Impact

(including the socio-economic impact and the wider societal implications of the project so far)

The R-WAKE project has clearly identified and preliminary quantified with enough level of confidence the need and opportunity in the ATM system for enhancing the current separation minima schemes provisioned in en-route operations taking into account the generated knowledge on Wake Vortex hazard assessment applying a high-fidelity simulation based approach. The project tangible outcomes include:

  • A WVE hazard absolute severity baseline, defined as a matrix of five upset parameters thresholds per severity class, developed with contributions of experienced pilots and air traffic controllers.
  • An ATM traffic simulator that includes high-fidelity WVE dynamic risk models, referred as the R-WAKE system, and consisting of an integrated framework of methods and tools that support the safety and robustness assessment of new separation schemes.
  • A public database of simulation results, which constitutes an evidence body to support the new separation scheme proposed in the project. The database contains the upset and severity class computed for a large number of encounter scenarios, including different aircrafts types, geometries, separation distances and atmospheric conditions. The database is complemented with a tool to visualize and analyse the 3D/4D map of the WVE suspected hazard areas (SHA). The public access to the database and the SHA visualizer tool can support other future R&I activities.
  • An evidence-based proposal to improve the current en-route separation scheme, referred as the R-WAKE-1 concept, which consists of six new separation schemes designed to enhance safety against WVE hazards and to increase airspace capacity. Flight efficiency could potentially be also optimised under certain conditions.
  • And an assessment of the feasibility and impact of the proposed concept on ATM, including an initial validation strategy and outlining an implementation plan. The assessment concluded that there is enough justification for proposing R-WAKE-1 as a new SESAR Solution in the ATM Master Plan pipeline. A roadmap of incremental evolutions (stepped-approach) has been also identified towards a long term R-WAKE concept for optimizing en-route separation minima provision in future SESAR ATM 2020+.

 

Linking with the SESAR programme

‘Optimised En-Route separation minima resulting from static Distance-Based Geometry-Based Category-Wise assessment of WVE hazards’,

which includes associated candidate Enablers: of type Standard (the separation schemes as such), of type Procedures (to manage the new separation schemes), and of type Systems (introduction of ATC tools to support the assessment, application, monitoring and management of the new schemes).

The R-WAKE-1 SESAR solution can be developed in phases according to the different and specific possible applications of the new R-WAKE-1 separation schemes. It must be remarked that the suggested seven applications (APP-x) are just ideas that require further research, development and validation, and they have been elaborated in this project with the only purpose of fostering discussion among experts and a deeper understanding about how the R-WAKE-1 separation schemes and their potential applications could improve safety, capacity and efficiency in en-route airspaces.

As shortest-term opportunity, among the suggested applications it is worthy to highlight the APP-7: WVE risk advisory service. It is indeed not an application of the new separation schemes proposal, but it responds to the idea that a new ATM decision support tool can be developed in a short term to provide ATC controllers with an early warning of potential WVE encounters in the on-going traffic situation, supporting as well in the identification by ATC of the most appropriate risk mitigation instruction to be applied, facilitating for example the application of the EASA recommendations published in the Safety Information Bulletin SIB-2017-10. This WVE-related ATM service tool could act as an R-WAKE-1 precursor system, thus paving the way towards subsequent WAKE-1 Concept applications development.

In addition of the R-WAKE-1 Concept as a SESAR Operational Improvement step, the R-WAKE system could evolve as well to become a SESAR Technological Solution (Enabler) in order to assist opportunities like: a) offline post-operations and safety assessments for sectors or for the entire ECAC; b) simulation-based WV Encounter Prediction (WEPS) integrated in the avionics to support the identification of WVE hazards on-board and even to enable cooperative subliminal offsets with ADS-B (APP-6); and c) as an enabler of the WVE risk advisory service (APP-7) mentioned above.

Regarding Stakeholder Expectations, although a formal stakeholder analysis has not been performed, it is possible to identify that the R-WAKE concept is relevant to most of the ATM stakeholders: Regulatory (for the formulation of modified separation schemes for En-Route airspace that maintain or improve current safety levels); ANSPs (for the changes to separation services provided resulting in increased En-Route airspace capacity and a reduction in the number of WVE related safety incidents;  Airspace Users (for the changes to separation procedures resulting in the opportunity to plan and operate more efficient routes); Network Manager (changed separation schemes resulting in the opportunity to modify the design of En-Route airspace enabling greater capacity and offering Airspace Users more efficient routes), and Aeronautics Industry (possible introduction of new controller tools and avionics, possible exploitation of WVE data and improved accuracy (e.g. MET) in AC-to-AC and AC-to-Ground data exchange).

In terms of potential cooperation and relationship with ongoing SESAR projects, the opportunities to further develop R-WAKE concept and research system can be related mainly to the SESAR 2020 project PJ10 Advanced Air Traffic Services – Separation management for En-route and TMA – PROSA, Controller Tools and Team Organization for the Provision of Separation in Air Traffic Management. It is also relevant to highlight the relation with SESAR 2020 PJ02 project EARTH (Increase Runway and Airport Throughput – High Performance Airport-TMA Operations), in particular Pj.02-01 on ‘Wake Turbulence Separation Optimization’ (related to the RECAT evolution), in terms of relevant ATM community research resources. This illustrated in Figure 9.

 

R-WAKE_final_summary_fig8 (SESAR context and relations)

Figure 9: SESAR Programmatic Context