CUSTOMER: NLR. NLR-TR May 2016

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1 CUSTOMER: NLR NLR-TR May 2016

2 Netherlands Aerospace Centre NLR is a leading international research centre for aerospace. Bolstered by its multidisciplinary expertise and unrivalled research facilities, NLR provides innovative and integral solutions for the complex challenges in the aerospace sector. NLR s activities span the full spectrum of Research Development Test & Evaluation (RDT & E). Given NLR s specialist knowledge and facilities, companies turn to NLR for validation, verification, qualification, simulation and evaluation. NLR thereby bridges the gap between research and practical applications, while working for both government and industry at home and abroad. NLR stands for practical and innovative solutions, technical expertise and a long-term design vision. This allows NLR s cutting edge technology to find its way into successful aerospace programs of OEMs, including Airbus, Embraer and Pilatus. NLR contributes to (military) programs, such as ESA s IXV re-entry vehicle, the F-35, the Apache helicopter, and European programs, including SESAR and Clean Sky 2. Founded in 1919, and employing some 650 people, NLR achieved a turnover of 73 million euros in 2014, of which three-quarters derived from contract research, and the remaining from government funds. For more information visit:

3 NLR-TR May 2016 Wind Turbines near Airports Problems and solutions for wind turbine siting in the vicinity of airports CUSTOMER: NLR AUTHOR(S): P.J. van der Geest NLR NLR - Netherlands Aerospace Centre

4 May 2016 NLR-TR No part of this report may be reproduced and/or disclosed, in any form or by any means without the prior written permission of NLR. CUSTOMER NLR CONTRACT NUMBER OWNER NLR DIVISION NLR Aerospace Operations DISTRIBUTION Limited CLASSIFICATION OF TITLE UNCLASSIFIED APPROVED BY : AUTHOR REVIEWER MANAGING DEPARTMENT DATE DATE P.J. van der Geest DATE 2

5 NLR-TR May 2016 Contents 1 Introduction 5 2 Rules, regulations and guidelines Collision risk Aircraft operations Radar and communication disturbance Wind hindrance 7 3 Safety assessment in Aviation 8 4 Example study: Wind turbine park near Teuge airport Wind turbine wake effects on air traffic Collision risk 13 5 Conclusions 15 6 References 16 3

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7 NLR-TR May Introduction Erecting tall wind turbines (150 meter or more above ground level) near airports presents special siting challenges to wind energy programs and is considered a growing problem by airport authorities. At the same time, in the search for suitable terrain to erect new wind turbines, the area around airports is scoring high in many cases. Usually this area is relatively unpopulated due to noise hindrance constraints and is mostly also free of other large obstacles that could cause wind blockage for the wind turbines. The terrain around airports is thus often targeted by companies developing wind turbine parks as a preferable option for their plans. However, the erection of such wind turbines may intrude the airspace that is required to ensure safe and regular aircraft operations at the airport. This may result in a conflict of interest of two parties competing for the same volume of airspace. The paper will go into some detail concerning the safety requirements that apply around airports and how safety assessment methods can be used to walk the fine line between the interest of the aviation community for a safe and regular operation and the spatial needs of wind energy development companies. The issues that may arise are further illustrated for a wind turbine park that is planned near Teuge airport in the Netherlands. The assessment of potential wake effects on air traffic and the assessment of collision risk of aircraft with the turbines is described. 5

8 May 2016 NLR-TR Rules, regulations and guidelines When erecting (large) wind turbines near an airport in general four main issues occur that may affect the safety of aircraft operations, i.e.: 1. Collision risk 2. Wind Hindrance 3. Aircraft operational impact 4. Disturbances of radar, navigation aids and communication aids In order to control the risks involved with these issues a set of international and national rules, regulations and guidelines are in place. The applicable regulatory framework for each of the mentioned safety aspects is shortly introduced. 2.1 Collision risk This issue is mainly dealt with by International Civil Aviation Organization Annex 14, Aerodromes [ICAO Annex 14], and sometimes some local regulations. The safety issue is in general that the wind turbine or other obstacle protrudes one of the defined Obstacle Limitation Surfaces (OLS). The OLS define the maximum height that an obstacle near an airport is allowed to have. Usually the so-called Inner Horizontal Surface (IHS) or Conical Surface are of most interest, when it comes to erecting wind turbines. The IHS is a circular surface a height of 45m with a radius ranging from 2 to 4 km, depending on the runway reference length. The Conical Surface slopes outward from the IHS with a slope of 5%, up to a height of 80 to 145m, also depending on the runway reference length. In case one of these surfaces is protruded an aeronautical study is required to demonstrate that the obstacle does not affect the safety and regularity of the operations to/from the airport. 2.2 Aircraft operations. Due to the erection of wind turbines flight procedures may need to be changed, or aircraft performance requirements may be affected (for instance one-engine out performance). The impact of obstacles on procedure design is mainly covered by ICAO Doc 8168 (the so-called PANS- OPS), [ICAO Doc 8168]. 2.3 Radar and communication disturbance The disturbance of radar, navigation aids and communications systems are mainly addressed by ICAO Annex 10 (Volumes 1-5). Within Europe further guidance material is provided by ICAO EUR Doc 15 on managing building restricted areas. Furthermore in the Netherlands a requirement exists that any wind 6

9 NLR-TR May 2016 turbine within 15 km from the instrumented range of a primary radar needs to be assessed for potential disturbances. 2.4 Wind hindrance Due to the large dimension of today s wind turbines the wake and turbulence may become hazardous for aircraft passing downwind behind the wind turbine. However, no specific international regulations do exist that protect aircraft from entering the potential hazardous wake or induced turbulence behind a large wind turbine. In addition to the basic regulations some new developments have to be mentioned here. In the Netherlands it is felt that the increase of the number of large wind turbines, in particular in the proximity of smaller airports, may become hazardous to general aviation, and that this issue is currently insufficiently covered by international regulations. For this reason the Inspectorate of Transport and Waterworks has amended the national regulatory framework for civil airports ( Regeling houdende regels voor burgerluchthavens ). Due to this amendment the Inner Horizontal and Conical Surface have been extended with an Outer Horizontal Surface, that for airports with runways shorter than 1200 meter, protects the airspace from obstacles larger than 100 meter up to 5100 meter from the airport. Wind hindrance at airports due to large obstacles, such as wind turbines, is currently not regulated. This issue has been recognized by EC and EASA. In 2009 the authority of EASA has been extended to include aerodromes. In this context the basic regulation EC No 216/2008, defining the common rules in the field of aviation, has been amended. In this amendment the essential requirements for aerodromes have been specified. It is interesting to note that the issue of wind hindrance has been recognized in this amendment. It is specified that the airspace around aerodromes shall be safeguarded from obstacles, and that therefore obstacle monitoring surfaces shall be developed, implemented and monitored. The possibility of obstacle-induced turbulence is specifically mentioned as a hazard that needs to be monitored, assessed and mitigated as appropriate. No further guidance is further given on how this can be achieved. However, in the Netherlands a wellaccepted criterion is the so-called 7 knots criterion that implies that for commercial aircraft an obstacle may not cause a reduction of the undisturbed wind velocity with more than 7 knots at any point of the aircraft trajectory. Based on this criterion NLR-ATSI (Nieuwpoort et al, 2006) developed appropriate turbulence monitoring surfaces. 7

10 May 2016 NLR-TR Safety assessment in Aviation In aviation the demonstration that a vehicle, system or operation is safe can basically be performed in two ways: By means of compliance; in aviation the standards and recommended practices have been laid down in an extensive international and national regulatory framework, in which the collective experience concerning safe operations is reflected. Such standards are for instance the ICAO Annexes (1 through 18) and the Certification Specifications of EASA (e.g. CS-25). When an applicant shows that his design or operation is compliant with the appropriate regulations, it is inherently assumed that the design is safe. By means of safety assessment; because it is impossible to have appropriate safety regulations in place for any imaginable operation or innovative design, safety may also be demonstrated by assessing safety in an absolute or relative way, and compare the result with an overall target level of safety. It should be noted that even in the case that appropriate regulatory requirements do exist the applicant may choose to deviate from those requirements by providing an analysis that shows an equivalent level of safety. In this case also a safety assessment is required to support the claim of equivalent safety. In some cases the regulatory framework explicitly includes such an option. For instance In ICAO Annex 14 it is specifically mentioned that New objects or extensions of existing objects should not be permitted [.]above the inner horizontal surface except when, [...] after aeronautical study it is determined that the object would not adversely affect the safety or significantly affect the regularity of operations of aeroplanes. In the context of erecting wind turbines near airports this is an important observation, because it means that wind turbines under some conditions are allowed to protrude in the otherwise obstacle free airspace around an airport, provided that it can be shown that safety and operations are not adversely affected. Although safety assessments in aviation can have various forms and can use various methods, the structure is basically in all cases similar. The safety assessment generally consists of the following steps, as illustrated in Figure 1: 8

11 NLR-TR May 2016 Determine the operation/system Identify the hazards: What can go wrong? Assess frequency: How often does it occur? Assess severity: What are the consequences? Assess risk tolerability: Is the risk the combination of frequency and severityacceptable or not? Identify risk mitigation measures: What can be done to reduce all risk, as far as practicable, to the acceptable level? severity determine operation/ system identify hazards how often does it occur? assess frequency acceptable unacceptable tolerable frequency assess risk tolerability identify risk mitigation measures Figure 1: Generic safety assessment process what can go wrong? what are the consequences? assess severity is the risk acceptable or not? what can be done to reduce the risk? 9

12 2500 m May 2016 NLR-TR Example study: Wind turbine park near Teuge airport Teuge airport is a small airport near Apeldoorn in the Netherlands. It is a so-called VFR airport, which means that the airport is only used by aircraft navigating using visual references on the ground. The airport mainly uses a single concrete runway, with a length of 1199 meters. An energy company planned to erect 5 large wind turbines (tip height 150 meter above ground and rotor diameter 90 m) in a business park (Ecofactorij) near the airport. The planned turbines are located around 5 km from the airport and at 1250 meter from the SIERRA reporting point where approaching aircraft converge to proceed to the landing circuit. The situation is visualized on the map below. Inner Horizontal Surface (45 m above ground) 3600 m Conical Surface Outer Horizontal Surface (100 m above ground) 1200 m 5100 m Ecofactorij wind turbines Figure 2: Part of the aeronautical chart for Teuge airport, including edges of the applicable Obstacle Limitation Surfaces and the location of the planned wind turbines 10

13 NLR-TR May 2016 As shown in Figure 2 the planned Ecofactorij wind turbines are located beyond the edge of the appropriate Conical Surface. This means that from the perspective of the governing regulation (ICAO Annex 14) there is in principle no restriction to erect the planned wind turbines. It is also shown that the Ecofactorij wind turbine park is located partially within the boundaries of the Outer Horizontal Surface. This aspect will be discussed later. 4.1 Wind turbine wake effects on air traffic The primary concern was initially that the wind turbine park was located fairly close to the reporting point SIERRA. The reporting point SIERRA is a visual reference point, located at the crossing of the highway A1 and the Groote Wetering (i.e. a watercourse running northbound to Teuge airport). All air traffic arriving at Teuge airport for approach and landing on runway 09 or 27 is required to pass over point SIERRA and fly North along the Groote Wetering to enter the Circuit Area South. The point SIERRA is nominally to be overflown at 700 ft (~210 m) above aerodrome elevation (i.e. with around 200 ft vertical clearance relative to the maximum height of the wind turbines). The proximity of the wind turbine park to point SIERRA was identified as a potential hazard to the air traffic over SIERRA, in particular due to induced wake and turbulence effects. Because, as mentioned in paragraph 2, no specific regulations exist with respect to obstacle induced wake and turbulence effects, NLR-ATSI was asked to perform a safety assessment to ensure that no unacceptable risks were introduced. For this safety assessment the following steps were performed: 1. Calculation of the flow field behind the wind turbine (i.e. the potential hazard). This was done using an empirical method, as described by Vermeer, et al. This method provides accurate results in the mid to far wake area (roughly 5 to 10 diameters behind the disk), which is sufficient for the present analysis, as it is unlikely that aircraft will pass much closer to the wind turbine. The resulting flow field is shown Figure 3. 11

14 May 2016 NLR-TR Figure 3: Calculated flow field behind wind turbine 2. Calculation of severity. The only obstacle wake criterion in the Netherlands is the 7 kt (~3.5 m/s) criterion, as discussed in paragraph 2. However, this criterion applies to large commercial airliners. At Teuge airport the majority of air traffic concerns small general aviation aircraft. Clearly such aircraft are more susceptible to wake effects then large commercial aircraft. Based on consideration of the flight characteristics of general aviation aircraft, the 7 kt criterion was adapted as follows: a. wind speed reduction in wake > 7 kt: hazardous effect b. wind speed reduction in wake between 3.5 and 7 kt: major effect c. wind speed reduction in wake between 3.5 and 2 kt: minor effect d. wind speed reduction in wake < 2 kt: negligible 3. Calculation of frequency. For this part two issues have to be considered. The probability that an aircraft comes close to the wind turbine, and the probability of sufficient wind strength to cause an appreciable wake behind the turbine. Concerning the first issue, given the location of point SIERRA, it is considered likely (high probability) that during normal operation aircraft will come as close as 600 m from the wind turbine with a vertical margin of 200 ft. However, it cannot be excluded that aircraft will come closer to the wind turbine. Therefore a non-nominal scenario is taken into account that an aircraft comes as close as 300 m without vertical margin (the aircraft is then flying at 500 ft, i.e. the lowest allowed altitude. The likelihood of this non-nominal scenario is assumed to be low. Analysis of the wind turbine characteristics has shown that the worst wind deficit in the wake occurs for wind velocities between 7 and 10 m/s. Based on local wind conditions it is known that such condition occur less than 30% of the time. 4. Determination of tolerability. For the risk assessment of wind hindrance due to wind turbines the following risk tolerability matrix has been applied. 12

15 NLR-TR May 2016 Severity (maximum wind deficit in wake) <2 kt kt 3.5 7kt > 7kt Negligible Probability of occurrence Low Medium High Nominal track Non-nominal track From this risk tolerability matrix it can be seen that the risks concerning wind hindrance behind the wind turbines are qualified as acceptable (all risks are in the green area). No further risk mitigation is therefore required 4.2 Collision risk As mentioned in paragraph 2, a new Dutch regulation has defined an Outer Horizontal Surface, which limits obstacles in the proximity of airports like Teuge to 100 m height up to a distance of 5100 m from the airport. Four out of five of the planned wind turbines are located within this range and have a height of 150 m, and therefore violate the new regulation. The violation of this new regulation is only allowed, when it can be shown by a dedicated safety assessment that the operation nevertheless is sufficiently safe. For this again the mentioned safety assessment approach has been applied. 1. Identification of hazards. The following hazard scenarios have been identified: I. Traffic in conflict with wind turbines or other traffic, approaching SIERRA II. Traffic in conflict with wind turbines or other traffic, after passing SIERRA a. Traffic in conflict with wind turbines when departing from runway 21 b. Traffic in conflict with wind turbines after engine failure In addition for scenarios a) and b) two visibility conditions are taken into account: normal visibility (cloud base > 700ft and visibility > 1500 m) and reduced visibility (cloud base < 700 ft and visibility at minimum for visual flight, 1500 m). 2. Determination of severity. The identified hazard scenarios may lead to the following consequences a) Wind turbines may lead to distraction or disturbance of a flight, leading to reduced capability to see and avoid other traffic; b) Wind turbines reduce the area for maneuvering, leading to a concentration of traffic; c) Aircraft collides with wind turbine. 3. Determination of frequency. For each of the hazard scenarios the probability of occurrence has been determined. Because no objective, quantitative, methods exist to calculate this probability, expert judgment has been used to provide a best estimate. This is done by brainstorm sessions and 13

16 May 2016 NLR-TR interviews involving all necessary experts, such as representatives of the airport, local pilots, experienced general aviation pilots, test pilot and safety experts. Based on their inputs a consolidated assessment is made of each of the scenarios, resulting into a best estimate of the expected frequency of occurrence. 4. Determination of tolerability. The results of the frequency and severity assessment for each scenario (I IV) have been combined in the following risk tolerability matrix. Severity Distraction Reduced maneuvering area Collision with turbine Negligible III I (low visibility) II (low visibility) IV II (low visibility) III IV Low I (low visibility) III I (all visibility) II (all visibility) IV II (low visibility) Probability of occurrence Medium High I (normal visibility) I (normal visibility) II (normal visibility) As shown most of the identified risks are acceptable (in the green area), and none of the risks are clearly unacceptable (in the red area). However, a number of risk are assessed to be within the intermediate area (yellow area) which means that risks are not a priori acceptable and that further mitigating measures should be investigated, as far as practicable, to reduce the risks further. 5. Risk mitigating measures. A number of risk mitigating measures have been identified, i.e. a) Relocation of point SIERRA b) Use of the Northern circuit instead of the current Southern circuit c) Designate wind turbines as an ``area to be avoided`` on the applicable visual flight charts d) Improved visual markings of the wind turbines Based on expert judgment it was established that option a) is not feasible, options c) and d) would reduce risks slightly, but not enough to qualify the remaining risk as acceptable. Only option b) would mitigate all risks to the acceptable level. However this option is considered to be highly complex (due to noise considerations) and would require long lead times to be implemented. Therefore, this option was considered to be impracticable. This means that none of the identified measures are judged to be sufficient or practicable, leading to the conclusion that some remaining risk has to be accepted in case the wind turbines are realized. Clearly, such decision can only be taken with involvement of all relevant stakeholders, i.e. airport, pilots, safety regulator, surrounding residents and the applicant for the wind turbines. 14

17 NLR-TR May Conclusions The paper concludes that the growing claims of wind energy developers on the airspace around airports cannot be neglected by the aviation community. The traditional attitude of the aviation community that we own the sky is no longer tenable, as modern societies put ever increasing weight on the importance of green energy. The future adagio therefore will have to change to we share the sky. Clearly, if there is a shared use of airspace, the boundaries of what is acceptable from a safety and operational viewpoint have to be clearly established. To some extent new regulations are required to ensure that safety is guaranteed at a minimum acceptable level. However, due to the unique characteristics of specific projects, the fine line between what is acceptable from an aviation viewpoint with a minimal claim on the available airspace can in many cases only be determined with a dedicated safety assessment. The methods to conduct such assessments are well developed within the aviation community. The two examples in this paper illustrate that applying these methods allow to focus on the real problems and to find an optimal balance between aviation safety and wind energy requirements. The main conclusion of the paper is that a growing number of wind turbines is not a growing hazard to aviation safety, as long as the potential hazard is properly recognised, assessed and mitigated, if necessary. 15

18 May 2016 NLR-TR References 1. ICAO Annex 2, Rules of the air, ninth edition, July ICAO Annex 10, Aeronautical Telecommunications, Volume ICAO Annex 14, Aerodromes, Volume 1 Aerodrome Design and Operations, Fourth Edition, July ICAO Doc 8168, OPS/611, Procedures for Air Navigation Services, Aircraft Operations, Nieuwpoort, A.M.H. ; Gooden, J.H.M. ; Prins, J.L., Wind criteria due to obstacles at and around airports, NLR-CR Vermeer, L.J. et al., Wind turbine wake aerodynamics, Progress in Aerospace Sciences 39 (2003), pp

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20 NLR Anthony Fokkerweg CM Amsterdam p ) f ) e ) info@nlr.nl i )