The CREDOS Project. Safe separation distances for take-off and departure: Evaluation of Monte Carlo based safety assessment results D3-10.

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1 The CREDOS Project Safe separation distances for take-off and departure: Evaluation of Monte Carlo based safety assessment results D3- Abstract: This document contains the results of a safety assessment with regard to wake vortex encounter risk during departure using the combination of the tools WakeScene-D and VESA-D. The encounter probability and risk is assessed for several scenarios in comparison taking into account the effect on the encountering aircraft. Based on the relative encounter risk for different crosswinds and separations a crosswind threshold is derived which could allow a safe reduction of separations during departure. Contract Number: AST5-CT Proposal Number: 3837 Project Acronym: CREDOS Project Co-ordinator: EUROCONTROL Document Title: Safe separation distances for take-off and departure: Evaluation of Monte Carlo based safety assessment results Deliverable Nr: D3- Delivery Date: 9 NOV 9 Responsible: AD Nature of Deliverable: R (Report) Dissemination level: PU (Public) File Id N : Status: Approved Version: 2 Date: 5 NOV 9 Approval Status Document Manager Verification Authority Project Approval Airbus Airbus Project Management Committee Sebastian Kauertz Sebastian Kauertz PMC Members WP3 Leader WP3 Leader Status can be: draft, for approval, approved. Only approved deliverables may be sent to the EC. 2 The first version delivered to the EC should always be version (any previous versions should be a, b etc). Page

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3 Safe separation distances for take-off and departure: Evaluation of Monte Carlo based safety assessment results Sebastian Kauertz

4 Ref.: CREDOS Deliverable D3- Authors Sebastian Kauertz Airbus Deutschland GmbH Edition history Edition No. Date Author(s) Section Comment a September 4, 29 S. Kauertz All Initial version b September 7, 29 S. Kauertz All After st internal review c October 9, 29 S. Kauertz All 2nd version for approval November 5, 29 S. Kauertz All Approved version This document is produced under the EC contract AST5-CT It is the property of the CREDOS consortium and shall not be distributed or reproduced without the formal approval of the CREDOS Steering Committee.

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6 Contents Glossary V Introduction 2 VESA sensitivity and worst-case studies 3 2. Baseline scenario Circulation influence Mass influence Altitude influence Synopsis Methodology of risk assessment 3. Definitions Description of criteria used for assessment Results of risk assessment 5 4. Overview of conducted simulations Encounter probability Encounter risk Influence of crosswind direction Departures only on one route I

7 Page: II Ref.: CREDOS Deliverable D3-4.6 Departures straight out Altitude distribution Encounter angles Influence of actual wind frequencies Synopsis Sensitivity analysis Influence of turbulence Influence of flexible thrust rating Influence of ambient temperature Influence of sample size Influence of severity criterion definition Influence of increased separation Synopsis Summary Conclusions Proposal for reduction of wake turbulence related departure separations under crosswind conditions Recommendations Literature 6 A Boundaries of severity criteria envelopes 63 B Results of VESA sensitivity studies 65 B. Circulation influence B.2 Mass influence B.3 Altitude influence

8 CONTENTS Page: III C Tabular summary of encounter risk and probability 8 C. Departures on all SIDs C.2 Departures only on TOBAK2F C.3 Departures Straight-Out D Encounter probability for different severity criterion definitions 87 D. Encounter probability for SC > D.2 Encounter probability for SC = D.3 Encounter risk

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10 Glossary Latin symbols A [m 2 ] Area b [m] Span, vortex span d [m] Diameter H [m] Height above ground L [Nm] Induced rolling moment p [%] Probability r, R [m] Radius S [m 2 ] Wing area t, T [s] Time u [m/s] Flow velocity V, v [m/s] Speed x, y, z [m] Cartesian Coordinates Greek symbols [-] Difference Γ [m 2 /s] Circulation Φ [-] Bank angle Θ [ ] Pitch angle α [ ] Angle of Attack β [ ] Angle of Sideslip γ [-] Flight path angle ρ [kg/m 3 ] Air density σ [-] Standard deviation V

11 Page: VI Ref.: CREDOS Deliverable D3- Subscripts Initial value c Core, center enc Encounter Fol Following aircraft Gen Generating aircraft ind induced min, max Minimum- / maximum value Sep Separation w Wind WV Wake Vortex x, y, z In corresponding direction Superscripts Vector Mean Abbreviations AAE ACE AFE BADA CAE CREDOS DPCE DRCE D2P DLR DVM MOPS MTOW NOWVIV RWA SC SID VESA WakeScene Aircraft Attitude Envelope Attitude Control Envelope Air Flow Envelope Base of Aircraft Data Cabin Acceleration Envelope Crosswind Reduced Separations for Departure Operations Dynamic Pitch Control Envelope Dynamic Roll Control Envelope Deterministic 2-Phase wake vortex decay and transport model Deutsches Zentrum für Luft- und Raumfahrt Deterministic Wake Vortex Model Multi-Objective Parameter Synthesis Maximum Takeoff Weight Nowcasting Wake Vortex Influence Variables Relative Wind Angle Severity Criterion Standard Instrument Departure Vortex Encounter Severity Assessment Wake Vortex Scenarios Simulation Package

12 Chapter Introduction This document describes the simulations conducted within CREDOS Work Package 3 to determine safe reduced separation distances during take-off under crosswind conditions. It uses the combination of the two tools WakeScene-D (developed by DLR) and VESA-D (developed by Airbus). In deliverable D3-9 [5] already several sensitivity studies using WakeScene-D have been shown, which give a first insight into which parameters influence wake encounter probability at different separations under certain crosswind conditions. Now subsequent VESA-D simulations are used of several different possible scenarios which also take into account encounter severity in addition to the encounter probability. These simulations are aimed at determining the crosswind threshold under which a reduction of the take-off separation from the standard 2 s can be safely carried out. In addition to the evaluation of the Monte Carlo simulations a sensitivity study using only VESA-D is described in the first part of the report. It shall show sensitivity of the severity criterion implemented in VESA-D to several main influence parameters. The aim of the risk assessment is not to compute the absolute frequency of encounters that are likely to happen in real life. For this it would be necessary to further improve and validate some of the submodels employed in the simulations, and to feed several inputs with more exact airport-specific data than it was done here (e.g. exact traffic mix on the considered airport, realistic SID sequences used, fraction of time each runway is used etc.). Thus all the results shown here are based on a relative assessment, comparing different scenarios with a reference that has been carefully defined to represent the present situation as well as possible (see also [7]).

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14 Chapter 2 VESA sensitivity and worst-case studies Complementing the sensitivity studies performed with WakeScene-D some studies have also been performed with VESA-D to see the influence of different parameters on the distribution of the severity criterion used in VESA-D. Parameter variations with respect to a baseline scenario are done to find out under which conditions the highest severity ratings are to be expected. For these studies VESA-D was used independently from WakeScene-D, using parameters which are based on WakeScene-D results for the take-off scenarios. The A32 was used as a follower aircraft, encountering vortices using the Forced Encounter technique described e.g. in [2] or []. The severity criterion and its different envelopes are described in detail in [4]. The boundaries of the different envelopes are recalled in App. A. 2. Baseline scenario For generic studies, a baseline scenario has been defined which is based on typical results from the WakeScene-D operational scenarios. The input parameters of this baseline scenario are listed in Tab. 2.. The encounter altitude is just outside of the range in which the severity criterion in VESA-D is depending on altitude (see [4]). The aircraft mass is in the middle of the mass range selectable for the A32 in the WakeScene/VESA simulations. Flexible thrust rating and configuration are typical for take-off. As can be seen, only straight-out departure routes have been used. In this scenario the Flight Director commands a pitch angle of Θ = 7 just after take-off which the pilot model will follow, resulting in a flight path angle of about γ =. 3

15 Page: 4 Ref.: CREDOS Deliverable D3- Table 2.: Input parameters of baseline scenario for sensitivity studies Parameter name Value Altitude f t Aircraft mass 675 kg Thrust setting FLEX 39 Configuration SID route Straight Vortex circulation 3 m 2 /s Vortex span 4 m Vortex aimed for Nearest Wind speed 4.2 m/s Wind direction 9 ( 6 RWA) The circulation and vortex span are typical values from the WakeScene simulations for crosswindand reference scenarios, while here a case was chosen as baseline where the vortices did not yet drift away due to ground influence. "Nearest vortex aimed for" means that regardless of the horizontal encounter angle the projected flight path of the aircraft always intersected with the nearest vortex of the pair. This has been maintained for all the sensitivity studies shown here and only represents one special case. In reality the vortices are intercepted in many different distances to the two vortices, resulting in different distributions of encounter severity. Finally, the windspeed and wind angle chosen correspond to a crosswind component of about 7 kt. With the parameters in this scenario a range of horizontal and vertical encounter angles was simulated and the severity of the encounter determined using the multi-parameter envelopes in the severity criterion. The range of encounter angles chosen was Ψ = for the horizontal and γ = for the vertical encounter angle in steps of (for encounter angle definition see also Fig. 4.2). The corresponding distribution of the severity criterion value SC is shown in Fig. 2. for this baseline scenario. In Fig. 2.2 the contributions of the four different envelopes making up the severity criterion are shown seperately. Each of the four plots shows the value of the respective envelope at the time when the maximum overall Severity Criterion value was reached. This means that they do not necessarily show the maximum value that was reached in each envelope for a certain encounter angle combination. As soon as a summed severity criterion value of SC = was reached, the values of each of the four envelopes contributing to this value were recorded. The maximum criterion value over the whole encounter angle range is noted in the four envelopes. As can be seen already, the Cabin Acceleration Envelope has the main contribution to the severity rating for a large region of encounter angles, with a small contribution by the Air Flow Envelope at small encounter angles. Both the Aircraft Attitude and Aircraft Control Envelope have no contribution throughout the encounter angle range. The maximum severity rating occurs at horizontal encounter angles around Ψ = ±2 and slightly positive vertical encounter angles of around γ = +5 (a positive encounter angle meaning the aircraft approaches the wake from above). The severity then

16 2. Baseline scenario Page: 5 SC Figure 2.: Distribution of Severity Criterion SC vs. encounter angle for baseline scenario AAE AAE max = ACE ACE max = AFE AFE max = CAE CAE max = Figure 2.2: Distribution of Envelope Criteria vs. encounter angle for baseline scenario

17 Page: 6 Ref.: CREDOS Deliverable D3- somewhat decreases again for encounter angle around Ψ = ±45 and then increases again for further increasing encounter angles. The explanation for this characteristic behaviour is not clear yet. Detailed analysis of the cabin acceleration metrics calculated during these encounters however show that the biggest contribution to this characteristic distribution is by exceedance of the minimum vertical acceleration limit (g), with smaller contributions by the maximum vertical acceleration (+2g) and the lateral acceleration (±.5g symmetric around g). An asymmetry with respect to a horizontal encounter angle of can be seen, which could be expected due to the crosswind applied in all the simulations. This leads to the aircraft being aligned slightly different relative to the vortex system as the vortices are placed relative to the flight path of the aircraft, not its heading. Based on this scenario parameter variations have been done for some of the parameters given in Table 2., and for each combination of parameters the severity was determined by VESA-D for the range of encounter angles. The main influence parameters on severity besides the encounter angles were assumed to be the vortex circulation, aircraft mass and altitude. 2.2 Circulation influence Based on the baseline scenario the circulation of the vortices placed in front of the aircraft was varied according to Table 2.2. All other parameters were kept constant. Table 2.2: Input parameters for circulation study Parameter name Value Altitude ft ft ft ft ft Aircraft mass 675 kg 675 kg 675 kg 675 kg 675 kg Thrust setting FLEX 39 FLEX 39 FLEX 39 FLEX 39 FLEX 39 Configuration SID route Straight Straight Straight Straight Straight Vortex circulation 2m 2 /s 25 m 2 /s 3 m 2 /s 35 m 2 /s 4 m 2 /s Vortex span 4m 4m 4m 4m 4m Vortex aimed for Nearest Nearest Nearest Nearest Nearest Wind speed 4.2 m/s 4.2 m/s 4.2 m/s 4.2 m/s 4.2 m/s Wind direction The results are shown in App. B. containing the distribution of the four single envelopes and of the complete severity criterion consisting of the sum of these four envelopes versus the encounter angles. The CAE is dominating most of the encounter angle range here again, ranging from maximum value of CAE=.65 up to CAE=. for a circulation of higher then about 3 m 2 /s. However, in the

18 2.2 Circulation influence Page: 7 range of small horizontal encounter angle of between Ψ = ± at any vertical encounter angle the cabin acceleration does not seem to be an issue. Here the other envelopes become more important, although only at circulations of more then Γ = 3 m 2 /s. One noticeable characteristic in the Cabin Acceleration Envelope becomes visible especially for low circulation encounters, which is that it tends to lower values around a horizontal encounter angle of around Ψ = ±45. Beyond that it starts to increase again while the encounter angle approaches Ψ = 9. Overall the circulation has a strong influence in the level of the different criteria. Furthermore there is apparently a division between the CAE and the other three envelopes, the CAE being more relevant in areas of high horizontal encounter angles, while the other three envelopes show noticeable values mainly within a more limited range of encounter angles. Furthermore the AAE and ACE envelopes seem to give a noticeable contribution only for higher circulations above Γ = 3 m 2 /s. The following figures 2.3 and 2.4 show more detailed computations of the center section of Figs. B.9 and B.. The asymmetry between positive and negative horizontal encounter angles can be clearly seen here as well. SC Figure 2.3: Detailed distribution of Severity Criterion SC vs. encounter angle, circulation= 4m 2 /s

19 Page: 8 Ref.: CREDOS Deliverable D3- AAE AAE max = ACE ACE max = AFE AFE max = CAE CAE max = Figure 2.4: Detailed distribution of Envelope Criteria vs. encounter angle, circulation= 4m 2 /s

20 2.4 Altitude influence Page: Mass influence Similar to the circulation of the vortices the mass of the encountering aircraft was varied according to Table 2.3. Five equidistant points were chosen within the mass range that can be chosen in the WakeScene/VESA combined simulations. The lowest weight corresponds to an aircraft with 5% fuel and 5% payload, the highest mass to the MTOW of the aircraft. All other parameters were kept constant again, the circulation is that of the baseline scenario. Table 2.3: Input parameters for mass study Parameter name Value Altitude ft ft ft ft ft Aircraft mass 77 kg 7225 kg 675 kg 6275 kg 58 kg Thrust setting FLEX 39 FLEX 39 FLEX 39 FLEX 39 FLEX 39 Configuration SID route Straight Straight Straight Straight Straight Vortex circulation 3m 2 /s 3 m 2 /s 3 m 2 /s 3 m 2 /s 3 m 2 /s Vortex span 4m 4m 4m 4m 4m Vortex aimed for Nearest Nearest Nearest Nearest Nearest Wind speed 4.2 m/s 4.2 m/s 4.2 m/s 4.2 m/s 4.2 m/s Wind direction The results are shown in App. B.2. The mass influence on the AAE and ACE values is not very high. The cabin acceleration increases with decreasing aircraft mass. On the other hand, the Air Flow Envelope seems to yield higher values with higher aircraft mass. 2.4 Altitude influence Furthermore the altitude at which the vortex was placed in front of the aircraft was varied according to Table 2.4. Note that this altitude is not necessarily the altitude at which the actual encounter (closest approach to the vortices) takes place. Depending on the encounter angles the aircraft will climb further for a certain time until reaching the closest distance to the vortices, which can amount to up to a few hundred feet. Table 2.4: Input parameters for altitude study Parameter name Value Altitude ft 8 ft 6 ft 4 ft 2 ft Aircraft mass 675 kg 675 kg 675 kg 675 kg 675 kg Thrust setting FLEX 39 FLEX 39 FLEX 39 FLEX 39 FLEX 39

21 Page: Ref.: CREDOS Deliverable D3- Configuration SID route Straight Straight Straight Straight Straight Vortex circulation 3m 2 /s 3 m 2 /s 3 m 2 /s 3 m 2 /s 3 m 2 /s Vortex span 4m 4m 4m 4m 4m Vortex aimed for Nearest Nearest Nearest Nearest Nearest Wind speed 4.2 m/s 4.2 m/s 4.2 m/s 4.2 m/s 4.2 m/s Wind direction The results are contained in App. B.3. As seen in the study before, the AAE, ACE and AFE envelope values are constrained to a limited range of encounter angles more or less symmetrically distributed around. Obviously the AAE and ACE, which are the two envelopes taking into account an altitude dependency, only become important below encounter altitudes of approx. H=6 ft. As expected the CAE does not show a dependency on altitude as the altitude is not contained in the calculation of the metrics taken into account in the CAE. 2.5 Synopsis There is apparently a division between the CAE and the other three envelopes, the CAE being more relevant in areas of high horizontal encounter angles, while the other three envelopes show noticeable values mainly within a more limited range of encounter angles. In earlier projects, when the targeted flight phase was mainly the approach phase, the investigated encounter angles were roughly in the range of Ψ = γ = ±5, which might indeed be the most typical range for approach, where the different aircrafts tracks do not diverge that much. In take-off however more divergence must be taken into account especially once the aircraft have reached an altitude where they begin to turn. At higher encounter angles however the lateral and vertical accelerations play a more important role obviously, while the effect on attitude and controllability of the aircraft is smaller due to the shorter exposure time to the vortices. The main influence on overall severity is by the vortex circulation, while the follower aircraft mass has a moderate influence on severity. The encounter altitude, as it is implemented only in the AAE and ACE, influences the severity rating only in the limited range of encounter angles where these two envelopes are important.

22 Chapter 3 Methodology of risk assessment The methodology and tools used for the risk assessment that will be presented in the following are described here. The simulation environment is a combination of the two tools WakeScene-D and VESA-D (where "D" is for "Departure"). WakeScene-D [6] is a tool that simulates the traffic and weather situation around the airport, including generation and evolution of wake vortices shed by the aircraft. It has been adapted within CREDOS to cover the take-off and departure phase up to an altitude of approx. 3 ft, and it allows running extensive Monte-Carlo simulations varying several influence variables. Its purpose is to determine those departures out of a large number of simulated ones which lead to a potential encounter. Extensive studies have been conducted with WakeScene- D which are described in [5]. In WakeScene-D however no interaction between the vortices and an encountering aircraft is included. For the purpose of investigating this VESA-D is used [], containing a full 6-degree-of-freedom aircraft simulation with vortex influence on the aircraft as it encounters a vortex wake. VESA-D also includes a pilot model to realistically control the aircraft during take-off and during a wake encounter [3], as well as criteria to rate the severity of each simulated encounter [4]. Departures identified by WakeScene-D as leading to a potentially significant encounter are simulated in VESA-D to determine how severe they actually are. This in the end allows an assessment of the risk of wake vortex encounters for several pre-defined scenarios, e.g. comprising different combinations of crosswind and separation. In this report mainly the final outcome of the VESA-D simulations will be discussed. All risk assessments are based on the severity criterion implemented in VESA-D. This criterion is described in detail in [4]. The simulation scenarios used in the VESA-D risk assessment differ slightly from those used in the WakeScene-D studies in that they use crosswind ranges instead of crosswind thresholds to define the crosswind that is present in a certain scenario. This means e.g. that a scenario contains crosswinds between 6 and 8 kt magnitude and not "greater than 6 kt". While the crosswind threshold definition is closer to operational practice, the crosswind range method allows a more detailed analysis of how encounter risk develops with crosswind, as the influence of the wind is not averaged over a large range of crosswind speeds. Figure 3. illustrates the relation between the different scenarios.

23 Page: 2 Ref.: CREDOS Deliverable D3- The risk of wake vortex encounters is assumed to be higher on low-crosswind days then on high crosswind days. However the overall risk over a long period of time must take into account the contributions from all possible crosswind conditions and their respective frequency, represented by the area beneath the curves. Obviously, a reduction in separation without any other additional means of encounter risk mitigation will increase the risk for any given weather condition. The working hypothesis here is that separations can be reduced if the associated risk with the new separation and the corresponding crosswind is below the highest risk for current separations, which is assumed to occur at low crosswinds. CREDOS ICAO CREDOS Encounter risk Overall risk 2s sep. Overall risk 6s sep. CREDOS operation high crosswind ICAO operation low crosswind Crosswind [kt] Figure 3.: Definition of assessed scenarios As mentioned before, in each scenario a defined crosswind in a range of 2 knots was present. Suitable weather cases containing the desired crosswind near the ground have been picked out of the NOWVIV database and were used in random order for the simulations. Further parameters that were varied are Generator aircraft type Generator aircraft diversion from nominal path Generator and follower aircraft mass Generator and follower aircraft departure SID route Generator and follower aircraft thrust mode Generator and follower aircraft start point on runway (to account for rolling take-off)

24 3. Definitions Page: 3 These influence the actual trajectories of both aircraft and thus the encounter frequency. For further description of how the different parameters were varied see also [7]. 3. Definitions Throughout the evaluation of the results several terms are used that shall be explained here first to aid the understanding of the results. First of all, crosswind is defined as the component of the wind at m height at the airport which is perpendicular to the take-off runway, regardless of the direction of the crosswind (from the left or right of the runway) unless noted otherwise. The change of the wind direction aloft is implicitly taken into account by the realistic weather profiles used in the simulation (see also [6]). A scenario usually denotes a certain combination of crosswind magnitude and separation time. For specific investigations certain parameters can be varied to make up a specific scenario (for example without using cartain SID routes), which will be described when necessary. Separations used in this report are usually time separations, based on those recommended by ICAO for take-off. Within the simulation the separation time is the time between start of the take-off roll of two consecutive aircraft. It is always kept constant at the value defined for the corresponding scenario and not varied statistically. Intermediate departures are not taken into account. The reference scenario is the scenario with t Sep = 2 s separation time. Within this report a reference scenario made up of several crosswind bins of 2 kt range each is used, which has been termed the alternative reference scenario in earlier reports [7]. This allows a more detailed investigation of the crosswind-dependency of the encounter risk. As intermediate departures are not considered in the simulation, the according separation time does not apply. Severity is the impact the wake encounter has on the encountering aircraft, taking into account all potential hazards to the safety of the aircraft [4]. The term encounter probability denotes the probability of an encounter exceeding a specific level of severity, usually SC =. This filters out those cases where encountering the wake has no significant effect on the aircraft. The worst-case encounter probability is the probability of encounters giving the highest severity of SC =. Encounter risk in the context of this report is defined slightly different in that it takes into account the actual magnitudes of the severity SC. Each encounter is weighted with the severity criterion value SC, meaning that scenarios with a lot of low-severity encounters will yield a lower risk then those with a lot of high-severity encounters.

25 Page: 4 Ref.: CREDOS Deliverable D3-3.2 Description of criteria used for assessment The criteria used to assess the simulation results with respect to encounter risk shown in the following shall be recalled here. They are based on the severity criterion which was developed within CREDOS [4] and is implemented in the simulation platform. This severity criterion can yield values between SC = and SC =. A value of SC = means that there is no significant disturbance of the normal operation of the aircraft. An encounter rated with this value is not considered hazardous in any way and can be disregarded. A value of < SC < means the metrics taken into account in the criterion exceed a defined limit typical for normal operation. This does not yet necessarily pose a danger to the aircraft, but it is a deviation from normal operation and is counted as a significant wake encounter. Once the severity reaches a value of SC = the dynamic reactions of the aircraft exceed limits which are not wanted to be exceeded during normal operation. This does not automatically lead to a crash or damage to the aircraft, but it is a situation preferably to be avoided in normal operation. The limits defined for the different metrics are given again in App. A. This allows on the one hand to filter out all encounters which are not specifically dangerous to the aircraft, i.e. which are not causing a deviation from normal operation, by determining the number of encounters with a severity value of SC >. Furthermore it allows determining the number of "worst-case" encounters by only looking at the fraction of encounters with SC = (which is of course included in SC > ). Finally, by weighting each encounter with its actual SC value a more precise value for the risk can be computed which takes into account the severity of each single encounter. Each scenario used in the following comparisons contains only encounters which occured while a certain crosswind level was present (see 4.). The interpretation could therefore be worded for example as follows: If the crosswind is between 6 and 8 kt and the separation time is 6 sec., the probability of an encounter of severity SC> is x %. Note that this kind of interpretation does not take into account the frequency of this specific crosswind condition during e.g. one year. Also, to determine the encounter probability for e.g. a crosswind threshold of >6 kt the probabilities of all corresponding ranges have to be averaged weighted with the frequency of the respective crosswind. An evaluation taking into account crosswind frequencies will be presented in a separate chapter (see ch. 4.9).

26 Chapter 4 Results of risk assessment 4. Overview of conducted simulations Table 4. shows which combinations of crosswind levels and separation times have been simulated for the risk assessment. Each field corresponds to one simulation scenario with 5. departures simulated in WakeScene-D and subsequently in VESA-D. A crosswind denoted as "2-4 kt" means the crosswind in this scenario was always between 2 and 4 kt magnitude. A crosswind denoted as "> kt" means the crosswind magnitude in this scenario was always above kt. These combinations were simulated using all SID routes available for the simulated runway 25R in Frankfurt. Additionally the scenarios with 2s and 6s separation were simulated only using departure route TOBAK2F as well as without any specific departure route, that means continuing straight in runway direction. In total 32 different scenarios or 6 Million departures were simulated for the basic risk assessment. Table 4.: Simulations conducted for risk assessment, separations and crosswinds Separation time 6 s 9 s 2 s kt 2-4 kt 2-4 kt 2-4 kt 4-6 kt 4-6 kt 4-6 kt 6-8 kt 6-8 kt 6-8 kt 8 - kt 8 - kt 8 - kt > kt - - Only the A32 has been used as a follower aircraft as opposed to the four different follower aircraft used in [5]. The aircraft flew in take-off configuration (Configuration +F, Flaps and Slats extended) and with a take-off mass and thrust setting varied according to the description in [7], Appendix A. Contrary to the simulations in [5] no flight path deviations have been applied, as these are not exactly 5

27 Page: 6 Ref.: CREDOS Deliverable D3- reproducible in VESA-D. The sensitivity studies in [5] have however shown that their impact on wake encounter probability is very small. 4.2 Encounter probability The simulations conducted with WakeScene-D and consecutively with VESA-D have shown that close to the ground the encounter probability decreases rapidly with crosswind, while encounters above a certain altitude seem to become more important (see also [5]). The boundary between these two domains was chosen to be 3 ft, as this is approximately the height up to which the vortices are in or near the ground effect that prevents them to descent further and causes them to move sideways or even rebound. Evaluations of the simulation results have shown that the relation between encounter probability and crosswind is basically different at lower and higher altitudes. The following Fig. 4. illustrates this by comparing the encounter probability for severity class SC > for different crosswind levels and separations split into low and high encounter altitude Encounter probability vs. crosswind high/low, only SC> Altitude above 3 ft 2s separation 9s separation 6s separation.5 Probability SC> [%].5.5 Altitude below 3 ft.5 2 cw= 2kt cw=2 4kt cw=4 6kt cw=6 8kt cw=8 kt cw>kt Figure 4.: Encounter probability by altitude and crosswind for SC>, 2s, 9s and 6s separation The upper half of the plot represents the encounters that took place above 3 ft, the lower half those below 3 ft. Each bar (top and bottom half) represents a simulation with 5. departures containing only crosswinds of the indicated range. The 2s scenario is thus the one called the alternative reference scenario in [7, 5]. Solely the case with 6s separation and a crosswind of greater than kt is different in that it contains also winds above 2 kt. Those are not very numerous

28 4.2 Encounter probability Page: 7 however. For crosswind of -2 kt and 6s or 9s separation no simulation results are available yet, and also for crosswind greater than kt only a simulation with 6s separation has been performed. For all other scenarios a non-visible bar means there are correspondingly little or even no encounters. What can be seen in the plot is that for low altitudes the crosswind very effectively reduces the encounter probability, so that for a crosswind of 6 kt or above and 6s separation the probability is clearly below that for low crosswinds at any separation. Notable is also the considerably lower encounter risk at low altitude for 9s separation compared to 6s already at a crosswind of 4-6 kt. Furthermore it is obvious that the behaviour at higher altitude is fundamentally different. The encounter risk for 2s separation first increases with increasing crosswind before dropping slightly again for high crosswinds above 6 kt. The curves for a separation of 6s and 9s seem to behave similar, only on a higher level. Also, while the encounter probability for the 2s scenario seems to be about equal at low and high altitudes for very low crosswinds below 2 kt, it is already a factor of 5 higher at altitude for crosswinds between 2 and 4 kt. For 4 kt crosswind and more almost all encounters take place at altitudes above 3 ft for the 2s reference scenario. For none of the cases with 6s separation does the encounter probability at higher altitude drop below that of the 2s scenario, regardless of which level is chosen as a reference. For a separation of 9s it could be stated that above 8 kt crosswind a level of probability comparable to the maximum level at 2s separation is reached. The situation is similar for a severity rating of SC =, as shown in Fig. 4.2, which shows the probability of those encounters with a severity rating of SC = versus crosswind. These are included in the encounters of class SC > shown before. The probabilities are about one order of magnitude lower than for the more conservative severity class of SC >. The trend with increasing crosswind however is comparable. In fact, for the cases with a crosswind of 6 kt and above no encounters remain below 3 ft altitude out of the 5. departures simulated, neither for 6s nor for 9s or 2s separation. Here the encounter probability with suspended wake turbulence separation is falling slightly below the reference level at 2s separation already at 4-6 kt crosswind (.62% vs..68%). Analogous to SC > the probability of wake encounters of this severity is several times higher at high altitudes than it is at low altitudes when a certain crosswind is present.

29 Page: 8 Ref.: CREDOS Deliverable D s separation 9s separation 6s separation Altitude above 3 ft Probability SC= [%]..5.5 Altitude below 3 ft. cw= 2kt cw=2 4kt cw=4 6kt cw=6 8kt cw=8 kt cw>kt Figure 4.2: Encounter probability by altitude and crosswind for SC=, 2s, 9s and 6s separation 4.3 Encounter risk So far only the probability of exceeding a certain severity was assessed. Now the severities of each single encounter shall be taken into account as well, i.e. each encounter is weighted with its severity rating SC. This kind of computation also accounts for effects like scenarios with primarily low-severity or high-severity encounters, which would be rated the same as long as the number of encounters is the same when only the probability as discussed in the previous chapter is regarded. Although the risk is still shown as a percentage value, no probability of an encounter can be inferred from this representation due to the type of computation. No discrimination into worst-case encounters with SC = ratings is made here, as this would lead to the plot shown in Fig The following Fig. 4.3 shows the so-computed encounter risk for the three separation distances versus crosswind. The overall values are lower as now each encounter is weighted with a value between and. The plot shows the same characteristics than the encounter probability in the previous chapter. The encounter risk for 9s separation at 8- kt crosswind however is not so clearly below the maximum reference level anymore than the encounter probability. These findings support the conclusions already drawn from the WakeScene-D sensitivity studies Mathematically, the risk is the sum of the severity values of each encounter divided by the total number of simulated encounters.

30 4.3 Encounter risk Page: 9 Encounter risk high/low 2s separation 9s separation 6s separation Altitude above 3 ft.5 Risk [%].5 Altitude below 3 ft cw= 2kt cw=2 4kt cw=4 6kt cw=6 8kt cw=8 kt cw>kt Figure 4.3: Encounter risk by altitude and crosswind, 2s, 9s and 6s separation (see [5], ch. 2.3), which identified the encounters at higher altitude to become more prominent with increasing crosswind. The altitude domain above 3 ft up to the maximum simulated altitude of 3 ft contains in particular the region where aircraft reduce climb angle to gain speed as well as perform the first turn defined in the departure route (at least for the Frankfurt scenario chosen for the investigations). In this region the routes of consecutive aircraft start to diverge, but are not yet sufficiently spaced to be safe from wake vortex encounters. Wakes coming from generator aircraft can be transported into the way of the follower aircraft, which would otherwise (without significant wind influence) have stayed clear of the followers flight path.

31 Page: 2 Ref.: CREDOS Deliverable D3-4.4 Influence of crosswind direction The sensitivity studies with WakeScene-D have already revealed that the encounter probability differs significantly depending on the direction the crosswind is coming from. This applies equally to the encounter risk determined in VESA-D. Figures 4.4 and 4.5 show the encounter risk as in Fig. 4.3, but divided into those encounters occuring with a crosswind from the port (left-hand) side on ground and those with a crosswind from the starboard (right-hand) side. They clearly show the higher risk when the wind is coming from the port side. The increase in encounter risk with crosswind magnitude is visible in both plots, although it is very shallow at 2s separation and starboard crosswinds. Also, the difference in encounter risk between port and starboard crosswinds is generally greater at 2s separation. Encounter risk high/low, only port crosswind 2s separation 6s separation.5 Altitude above 3 ft Risk [%].5 Altitude below 3 ft cw= 2kt cw=2 4kt cw=4 6kt cw=6 8kt cw=8 kt cw>kt Figure 4.4: Encounter risk by altitude and crosswind, only crosswind from port side These differences could be explained by the so-called Ekman-spiral, i.e. the winds turning in clockwise direction with altitude (on the northern hemisphere) when viewed from above, caused by friction in the atmospheric boundary layer [9]. This effect is included in the weather database used in WakeScene-D (see [8]) which contains realistic wind profiles generated by a sophisticated weather prediction model. The effect can add a headwind component to the wind when it comes from the left or a tailwind component when it comes from the right of the departure direction. This component can either partly counteract the downward sinking of the vortices due to their mutual induction, increasing encounter probability, or move the vortices further away, reducing the chance to encounter a vortex. A wind coming mainly from a headwind direction on the ground, with little or no crosswind component,

32 4.4 Influence of crosswind direction Page: 2 Encounter risk high/low, only starboard crosswind 2s separation 6s separation.5 Altitude above 3 ft Risk [%].5 Altitude below 3 ft cw= 2kt cw=2 4kt cw=4 6kt cw=6 8kt cw=8 kt cw>kt Figure 4.5: Encounter risk by altitude and crosswind, only crosswind from starboard side thus will turn by up to 45 to the right within the altitude domain considered in the simulations, introducing a crosswind component which reduces encounter risk at higher altitude. Crosswinds from the left, or port, direction however turn into a headwind, while crosswinds from the right turn further to the right or even receive a tailwind component, explaining the imbalance in encounter risk between port and starboard side. This explains the generally increasing encounter risk with crosswind at higher altitude, which is dominated by the cases with crosswinds from the port side. Finally, near the ground, below appr. 3 ft, these effects are not yet significant and thus the crosswind component remains effective in transporting the vortices clear of the runway. However there is a further factor that could cause an increase in encounter risk due to crosswind, which is the routing of the SIDs used in the simulations. Three of the five departures routes in the Frankfurt scenario used for the assessment turn to the south while the two others continue roughly in runway direction (see Fig. 4.6). For these southerly routes a port-side crosswind with respect to the departure runway has a considerable headwind component. This wind component leads to the vortices generated on the southerly routes being transported towards the follower aircraft, whether it is flying on a northerly route or following on the same southerly route. WakeScene-D studies also indicate that certain SID route combinations exhibit higher encounter frequencies than others [5]. Both of these factors would fit well with the fact that the risk increase with crosswind is only seen above 3 ft altitude. To confirm these conclusions, a simulation using only one departure route for all generating and following aircraft has been conducted, which is shown in the next section.

33 Page: 22 Ref.: CREDOS Deliverable D3-4.5 Departures only on one route As mentioned in the previous section, the fact that encounter probability and risk are significantly higher when crosswind is coming from the port side with respect to the departure runway could be influenced also by an effect caused by the combination of departures routes used in the simulations. To investigate this, a series of simulations similar to those described before has been conducted, but using only one northerly departure SID route for all generating and following aircraft, whereas in the former simulations northerly and southerly routes were mixed randomly. Figure 4.6: SID route structure used in WakeScene/VESA risk assessment The SID routes available in the WakeScene/VESA risk assessment simulations are shown in Fig For this investigation only route TOBAK2F was used for all departing aircraft, which is turning to the right by only about Ψ = 25 shortly after the runway end. Note that the simulated departures were usually terminated before the second turn defined in the SID as the aircraft had then already reached the altitude limit of 3 ft. All other parameters have been kept the same as in the previously shown simulation scenarios. It shall however be recalled here that the generator aircrafts flight paths contain small random deviations from the nominal flight path given in the SID route description (according to the trajectory model described in []), so that the actual route of the following aircraft can in fact be slightly downwind as well as upwind from the leading aircraft, even if all aircraft fly on the same nominal route. The results are shown in the following figures 4.7 to 4.9 for a separation of 6s compared to 2s. There is still the trend to increasing encounter probability and risk with increasing crosswind up to a

34 4.5 Departures only on one route Page: 23 magnitude of about 4-6 kt. However especially the behaviour at higher crosswinds is different from the cases with all SID routes. The decrease in risk with increasing crosswind at higher altitudes is much more like that at lower altitudes, with the risk (and probability) at 6s separation dropping below the level for low crosswinds at 2s for crosswinds of 8 kt or higher. As in these scenarios no southerly SIDs are contained, no vortices from these routes can be blown into the followers path, causing the encounter risk to remain low at higher crosswind magnitudes Encounter probability vs. crosswind high/low, only SC> Altitude above 3 ft 2s separation 6s separation.5 Probability SC> [%] Altitude below 3 ft 2 cw= 2kt cw=2 4kt cw=4 6kt cw=6 8kt cw=8 kt cw>kt Figure 4.7: Encounter probability for SC>, departure only on TOBAK2F Figures 4. and 4. finally show the influence of crosswind direction on encounter risk in these simulations. The characteristic increase in encounter risk up to around 4-6 kt crosswind at altitude is here also visible in both plots, and is also less prominent at 2s and starboard winds, just like in the simulations with all SIDs. This suggests that this behaviour is mainly due to the veering wind or Ekman-spiral effect explained earlier. The generally larger difference in encounter risk between port and starboard crosswind at 2s separation could be explained by the fact that the vortices have more time to descent and to be transported by the wind the higher the separation time is.

35 Page: 24 Ref.: CREDOS Deliverable D3-.2 2s separation 6s separation.5 Altitude above 3 ft Probability SC= [%]..5.5 Altitude below 3 ft. cw= 2kt cw=2 4kt cw=4 6kt cw=6 8kt cw=8 kt cw>kt Figure 4.8: Encounter probability for SC=, departure only on TOBAK2F Encounter risk high/low 2s separation 6s separation.5 Altitude above 3 ft Risk [%].5 Altitude below 3 ft cw= 2kt cw=2 4kt cw=4 6kt cw=6 8kt cw=8 kt Figure 4.9: Encounter risk, departure only on TOBAK2F

36 4.5 Departures only on one route Page: 25 Encounter risk high/low, only port crosswind 2s separation 6s separation.5 Altitude above 3 ft Risk [%].5 Altitude below 3 ft cw= 2kt cw=2 4kt cw=4 6kt cw=6 8kt cw=8 kt Figure 4.: Encounter risk, departure only on TOBAK2F, only crosswind from port side Encounter risk high/low, only starboard crosswind 2s separation 6s separation.5 Altitude above 3 ft Risk [%].5 Altitude below 3 ft cw= 2kt cw=2 4kt cw=4 6kt cw=6 8kt cw=8 kt Figure 4.: Encounter risk, departure only on TOBAK2F, only crosswind from starboard side

37 Page: 26 Ref.: CREDOS Deliverable D3-4.6 Departures straight out Finally, the same simulations have been repeated without flying a specific SID but only continuing straight out in runway direction. This way also any influence of a curvature of the tracks can be excluded. The results are shown in Fig. 4.2 and 4.3 for the encounter probability and Fig. 4.4 for the encounter risk. The trends are the same as in the scenario before with only the TOBAK2F route used. The influence of the curvature of this departure therefore seems to be very small. The remaining characteristics must be mainly due to the wind effects. Fig. 4.5 and 4.6 show the influence of the crosswind direction, where the difference between port and starboard crosswind is even clearer than in the TOBAK2F scenario. Especially at 2s separation the encounter risk is very small when the wind is coming from the right Encounter probability vs. crosswind high/low, only SC> Altitude above 3 ft 2s separation 6s separation.5 Probability SC> [%] Altitude below 3 ft 2 cw= 2kt cw=2 4kt cw=4 6kt cw=6 8kt cw=8 kt cw>kt Figure 4.2: Encounter probability for SC>, departures straight out

38 4.6 Departures straight out Page: s separation 6s separation.5 Altitude above 3 ft Probability SC= [%]..5.5 Altitude below 3 ft. cw= 2kt cw=2 4kt cw=4 6kt cw=6 8kt cw=8 kt cw>kt Figure 4.3: Encounter probability for SC=, departures straight out Encounter risk high/low 2s separation 6s separation Altitude above 3 ft.5 Risk [%].5 Altitude below 3 ft cw= 2kt cw=2 4kt cw=4 6kt cw=6 8kt cw=8 kt Figure 4.4: Encounter risk, departure straight out

39 Page: 28 Ref.: CREDOS Deliverable D3- Encounter risk high/low, only port crosswind 2s separation 6s separation.5 Altitude above 3 ft Risk [%].5 Altitude below 3 ft cw= 2kt cw=2 4kt cw=4 6kt cw=6 8kt cw=8 kt Figure 4.5: Encounter risk, departures straight out, only crosswind from port side Encounter risk high/low, only starboard crosswind 2s separation 6s separation.5 Altitude above 3 ft Risk [%].5 Altitude below 3 ft cw= 2kt cw=2 4kt cw=4 6kt cw=6 8kt cw=8 kt Figure 4.6: Encounter risk, departures straight out, only crosswind from starboard side

40 4.6 Departures straight out Page: 29 These last two simulations, compared with the simulations including all SIDs, support the earlier conclusions that there is a strong influence of the departure route layout on encounter risk. Summarizing the simulations the following can be concluded. For both types of scenarios, those using all SIDs and those using only the northerly SIDs, the encounter risk can be reduced below the maximum risk at 2s separation averaged for all crosswinds (which occurs around 4-6 kt crosswind magnitude, Fig. 4.3) by ensuring a crosswind of at least 6 kt coming only from the starboard side at 6s separation. Of course the risk will then be even more below that for 2s separation and port-side crosswinds. Without constraint on the crosswind direction however a crosswind component of 8 kt or more is sufficient if only the northerly routes are used for all departures. In that case the encounter risk as well as the probability of worst-case encounters (SC = ) is below the total one (above and below 3ft) at 2s separation and low crosswind, which is the most frequent weather condition. As the frequency of crosswinds with more than 8 kt magnitude is much lower, the impact on overall safety is considered to be limited (see also ch. 4.9). The same holds true if only straight out departures are conducted, which is however the less operationally relevant case. These conclusions confirm the results shown in [5].

41 Page: 3 Ref.: CREDOS Deliverable D3-4.7 Altitude distribution The following sections in this chapter give some supporting evaluations of the simulations with all SID routes included. This section describes the altitude distribution of the encounters in the scenarios discussed above in more detail. In Fig. 4.7 the altitude distributions of all those encounters with a severity rating of SC > are shown for the 2s separation reference case with different magnitudes of crosswind. Each of these plots is based on a simulation of 5. departures, so the absolute frequencies of encounters can be directly compared. The relative frequency is the number of encounters related to the total number of departures. As can be seen, with increasing crosswind the encounters near the ground quickly disappear, while more encounters are added in the region between -2 ft above ground. Fig. 4.8 shows the distributions for the available simulations with 6s separation, which show the same general behaviour. For 4-6 kt crosswind there are still more encounters at low altitudes, as was already seen in Fig. 4.. There the lower vortex age tends to increase the encounter severity. The following plots 4.9 and 4.2 additionally take the severity rating SC into account by showing the distribution of severity ( < SC <= ) versus encounter altitude. They visualize the results already discussed in the previous sections for 2s separation on the left (blue) and 6s separation on the right (red). Especially for cases with reduced separation the larger number of considerable encounters at low altitude compared to the 2s separation case at crosswinds between 4 and 6 kt is obvious. It is also visible how these are reduced with increasing crosswind. Finally the larger number of encounters with a rating of SC = for reduced separation is visible in the plots (along the SC= line). These plots suggest that even a boundary between low and high altitude set at 5 or 6 ft, instead of the 3 ft that were chosen, would not change the qualitative conclusions drawn from the data with respect to the two altitude domains.

42 4.7 Altitude distribution Page: Abs. frequency Rel. frequency [%] Abs. frequency Rel. frequency [%] Encounter Height above ground [ft] 2 3 Encounter Height above ground [ft] Crosswind -2 kt Crosswind 2-4 kt Abs. frequency Rel. frequency [%] Abs. frequency Rel. frequency [%] Encounter Height above ground [ft] Crosswind 4-6 kt 2 3 Encounter Height above ground [ft] Crosswind 6-8 kt Figure 4.7: Distribution of encounter altitudes for different crosswinds at 2s separation

43 Page: 32 Ref.: CREDOS Deliverable D Abs. frequency Rel. frequency [%] Abs. frequency Rel. frequency [%] Encounter Height above ground [ft] Crosswind 2-4 kt 2 3 Encounter Height above ground [ft] Crosswind 4-6 kt Abs. frequency Rel. frequency [%] Abs. frequency Rel. frequency [%] Encounter Height above ground [ft] 2 3 Encounter Height above ground [ft] Crosswind 6-8 kt Crosswind 8- kt Figure 4.8: Distribution of encounter altitudes for different crosswinds at 6s separation

44 4.7 Altitude distribution Page: 33 Crosswind -2 kt Crosswind 2-4 kt Figure 4.9: Distribution of severity vs. encounter altitude for different crosswinds

45 Page: 34 Ref.: CREDOS Deliverable D3- Crosswind 4-6 kt Crosswind 6-8 kt Crosswind 8- kt Figure 4.2: Distribution of severity vs. encounter altitude for different crosswinds