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Wake Turbulence—An Obstacle to Increased Air Traffic Capacity 1 Introduction NEEDS OF NEXTGEN Demand for air transportation continues to increase, and it is estimated that demand will double or even triple by 2025. The present air transportation system cannot accommodate such a large increase in demand. Concerned that special efforts were needed to address this issue, Congress passed the Vision 100, Century of Aviation Reauthorization Act (P.L. 108-176) in 2003, establishing a framework for the Joint Planning and Development Office (JPDO) to facilitate activities in a wide-ranging initiative to transform the air transportation system. The Next Generation Air Transportation System (NextGen) aims to transform the U.S. air transportation system to accommodate a tripling of demand. To do this, the implementation of new capabilities of technologies such as satellite-based navigation, surveillance, and networking needs to be accelerated, and new air traffic control (ATC) procedures need to be designed and employed. Significant advances in surveillance and navigation technologies have high potential to increase system capacity by substantially reducing aircraft separation distances during flight. It is the goal of NextGen to develop and oversee the implementation of these technologies in order to increase airspace system capacity and address increased demand. There are a number of challenges that NextGen faces. These challenges may be operational, such as determining flight paths and reducing runway occupancy time. Taxes, fees, and ticket prices are economic
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Wake Turbulence—An Obstacle to Increased Air Traffic Capacity challenges that feed into the way the system is funded. Other challenges are societal: airport expansion with its increases in noise and pollution, and the transportation infrastructure that allows people to get from their doorsteps to the airport. Finally, challenges in weather and wind conditions, including turbulence, gust, shear, and visibility, are physical in nature, and not the topic of this study. This report concerns just one of these many obstacles, a challenge that has both operational and physical aspects: wake turbulence. All aircraft trail wake vortices as a consequence of lift developed in flight. The wake vortex can present a real danger to aircraft following each other, particularly when the leader is larger than the follower. When the Boeing 747 entered the airspace system in 1970, it was substantially bigger than existing commercial aircraft. As a result, wake vortex separation criteria were developed based on then-available technology. Though there have been a few revisions to the criteria over the intervening years, the state of the art has not provided a basis for substantial revisions. In many cases, these wake vortex separation requirements (discussed in the next section) prevent the use of reduced separation standards enabled by satellite and other technologies. This report reviews the nation’s wake turbulence research and development program and assesses its ability to provide wake vortex avoidance and/or mitigation technologies that will permit achievement of the NextGen goals. WAKE HAZARDS AND OVERVIEW OF CURRENT STANDARDS There are no Federal Aviation Regulations specific to wake vortex hazards, either for certification or operations. Current aircraft separation standards related to wake vortex hazards have evolved over the past 40 years. Guidance material for pilots is contained in FAA Advisory Circular 90-23F: Aircraft Wake Turbulence (FAA, 2002), and in the FAA Aeronautical Information Manual (FAA, 2006a), updated semiannually. These publications contain educational and operational advisory information pertaining to aircraft operations in potential wake vortex situations, including pilot responsibilities. Pilot groups and industry associations have also issued advisory material to their constituents to raise awareness and provide guidance for avoiding and dealing with potential encounters. Pilots are generally well aware of this information and adhere to the recommended operating procedures for wake vortex avoidance. However, these procedures generally require pilots to observe the flight path of the wake-generating aircraft; therefore they are currently effective only in visual meteorological conditions (VMC), i.e., clear weather. ATC aircraft separation standards and procedures for their use are contained in FAA Order 7110.65R: Air Traffic Control (FAA, 2006b). As
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Wake Turbulence—An Obstacle to Increased Air Traffic Capacity larger and more diverse aircraft types have been introduced into the national airspace system, there have been re-categorizations of aircraft types and separation standard adjustments to ensure safety in all operating environments. Research has focused primarily on characteristics and behavior of wake vortex phenomena. Findings from this research have been used in the development, validation, or redefinition of the operational aircraft separation standards and procedures in use today. Current separation standards are shown in Table 1-1. Agreement has been reached in the international aviation community regarding separation standards behind the Airbus A380. While the separation standards of FAA Order 7110.65R have generally proved to be safe and effective, they are much more conservative than visual separations, which can be achieved during VMC operations in accordance with Advisory Circular 90-23F and the Aeronautical Information Manual. Collective wisdom holds that air traffic capacity is greatly reduced in instrument meteorological conditions (IMC), compared to VMC, suggesting that spacing standards may be conservative. A 1978 study of observed VMC separation distances supports this interpretation (Haines, 1978; Table 1-2). In the NextGen system, improved navigation and surveillance systems will have substantially increased accuracy and update rates. The pressure to increase capacity with these new systems will accentuate the importance of determining safe separation minima for wake vortex avoidance in all phases of flight: en route, terminal, and approaches to single runways or closely spaced parallel runways (CSPRs for the runways TABLE 1-1 IFR Separation Requirements for Arrival on the Same Runway (NM) Leading Aircraft Trailing Aircraft Heavy Large Small (max. takeoff weight of 255,000 lb or more) (max. takeoff weight of 41,000-255,000 lb) (max. takeoff weight of 41,000 lb or less) Heavy 4 5 6 B757 4 4 5 Large 3(2.5) 3(2.5) 4 Small 3(2.5) 3(2.5) 3(2.5) NOTE: Numbers in bold are based on wake vortex separations. Numbers in parentheses are valid only at certain airports. IFR, Instrument Flight Rule; NM, nautical mile. SOURCE: FAA, 2006b.
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Wake Turbulence—An Obstacle to Increased Air Traffic Capacity TABLE 1-2 Separation Distances Observed During Self-separation in VMC (NM) Leading Aircraft Trailing Aircraft Heavy Large Small (max. takeoff weight of 300,000 lb or more) (max. takeoff weight of 12,500-300,000 lb) (max. takeoff weight of 12,500 lb or less) Heavy 2.7 3.6 4.5 Large 1.9 1.9 2.7 Small 1.9 1.9 1.9 NOTE: There was a recategorization in 1996, so “small,” “large,” and “heavy” are not the same between Table 1-1 and 1-2. However, given that before the recategorization, “smallers” were smaller, and “heavies” were heavier, the point is made more strongly that IMC standards have been conservative. VMC, visual meteorological conditions; NM, nautical miles. SOURCE: Haines, 1978. themselves or CSPAs for closely space parallel approaches). Automatic Dependent Surveillance-Broadcast (ADS-B) is a new surveillance system based on aircraft reporting of current position and velocity vector. ADS-B relies primarily on satellite navigation, which can be augmented by the Local Area Augmentation System (LAAS) and the Wide Area Augmentation System (WAAS). The improved navigation and surveillance accuracy of these systems have the potential to substantially improve surveillance accuracy over that possible with radar, thus reducing the effect of surveillance accuracy on spacing. In addition, the use of satellite position fixing can be used to substantially improve the accuracy of onboard aircraft navigation systems over that obtained using present ground-based navigation aids. In fact, the aircraft-to-aircraft separations that can be safely flown using modern navigation and surveillance systems are often considerably less than that permitted by wake vortex interference. The FAA is in the process of defining a target level of safety for wake turbulence operations, which will be invaluable for developing effective safety cases and assessments (as defined in the FAA Safety Management System Manual and in FAA Order 8040.4 (FAA, 1998)) to support proposed operational changes. There is currently a lack of aviation community consensus on how to define an acceptable level of risk associated with aircraft operations in all wake vortex scenarios. Furthermore, there is little information available that would help quantify the number and severity of wake turbulence encounters before and after an operational change. The Aviation Safety Reporting System (ASRS) historically has not included wake turbulence encounters as a reportable event, except
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Wake Turbulence—An Obstacle to Increased Air Traffic Capacity in reduced vertical separation minimums (RVSM) airspace at cruise altitudes. The FAA supported and funded a change to ASRS that provides for the limited collection of wake turbulence encounter information. The collection of data is limited to those airports where current procedural mitigation work is underway (Lambert-St. Louis International Airport [STL] and George Bush Intercontinental Airport [IAH]) and two of the airports where the A380 will initially operate (John F. Kennedy International Airport [JFK] and San Francisco International Airport [SFO]). Any further enhancement of the collection system is dependent on funding by the FAA. Development of a safety case to support a procedural change is an arduous and costly process. The approval process has historically taken 2 to 5 years and requires extensive data collection and analyses, multiple simulations, and monitoring (Lang, 2007). Even when an effective, successful safety case is made, approval often comes in the form of a waiver from standard procedures. Such waivers are generally not applicable system-wide—they may be specific to an airport or even to a particular runway configuration. The approval processes for re-categorization of aircraft weight classes or the construction of new runways can be even more onerous. There has never been an accident directly attributed to wake turbulence while aircraft were following IFR spacing criteria or the self-spacing criteria of AC 90-23F and the Aeronautical Information Manual in VMC. Current wake vortex separation criteria are conservative and sufficient for ensuring safe operations. The key question is: Can a reduction in wake vortex separation criteria be obtained while maintaining safety? The VMC spacing shown in Table 1-2 indicates that pilots will comfortably space more closely than the IMC standards allow, provided that they can see surrounding aircraft. In IMC the spacing could be reduced up to 1 nautical mile (NM), if new technologies or procedures can confirm the same level of safety and provide pilots and controllers with that same level of comfort. WAKE TURBULENCE: A LONG POLE FOR CAPACITY? Wake vortex limitations primarily restrict arrival capacity, and to a lesser extent, departure capacity. The degree of constraint will vary from airport to airport, based on diverse factors such as traffic mix, current capacity, runway configuration, and weather patterns. For the simplest case—reducing the spacing between two aircraft arriving on the same runway—various systems studies have estimated that the most viable concepts can yield roughly 5-15 percent extra capacity (Lunsford et al., 2005; Galpin et al., 2005; Rutishauser et al., 2003; Lebron, 1987). Although
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Wake Turbulence—An Obstacle to Increased Air Traffic Capacity the reduced spacing increases capacity when it is available, it is available only when crosswinds are strong enough to sweep the wake vortices off the runway. For CSPA, however, the needed wind conditions are less restrictive, requiring only that the leading aircraft’s wake will not be blown onto the follower’s runway. Therefore, reduced spacing is permissible a larger percentage of the time. Accordingly, various studies have estimated the benefit of new CSPA procedures to range from 20 percent to over 50 percent (Lunsford et al., 2005; Lang et al., 2003, 2004, 2005; Lebron, 1987). At best, it would allow the two dependent runways to be decoupled, allowing them the capacity of two separate runways. It should be noted that increases in arrival will be more significant during IMC conditions, since VMC spacing is already more efficient. However, if these new procedures could prove the safety case that would allow a reduction of the minimum lateral runway spacing, airports could build new runways where there is sufficient space between present runways, thereby increasing capacity in all weather conditions. For departures, it is estimated that 1 to 8 percent capacity could be gained on single runways (Lunsford et al., 2005) and about 10 to 15 percent on CSPRs, providing benefits of up to 23 percent for airports with a high proportion of large aircraft (Lang et al., 2003). Some airports may not be able to take advantage of the extra capacity afforded by reduced wake turbulence spacing. Today’s aviation system is a complex web of intertwined systems that constrain each other in ways that are not obvious. Runways, taxiways, gates, terminal traffic, emissions, and many other factors may limit capacity at a degree comparable to wake turbulence. Studies of tradeoffs at individual airports and at the systems level are useful in identifying the most fruitful areas for improvement. A JPDO systems study showed that a 3 to 7 percent decrease in wake vortex separation (depending on weather) reduced delays at SFO more significantly than did a 20 percent reduction in runway occupancy time (Borener, 2007). Furthermore, many existing capacity constraints will be eased or lifted as NextGen is implemented (JPDO, 2007). If wake vortex is not also addressed, other system improvements may not reach their full potentials. Finally, capacity in and of itself is not always the best benchmark. If that were the case, NextGen goals could be achieved by constructing large airports far from population centers. Instead, capacity increases must occur where there is excess demand. In particular, urban airports with high demand and a limited amount of real estate could benefit from increased use of CSPRs. For an airport operating at 90 percent capacity, an increase in capacity of only 6 percent yields a 40 percent reduction in delays (O’Connor and Rutishauser, 2001). Furthermore, delays tend to
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Wake Turbulence—An Obstacle to Increased Air Traffic Capacity propagate throughout the air transportation system; reducing delays at the busiest airports will increase the efficiency of the entire system. Finding 1-1. Air transportation system capacity could be significantly enhanced by applying the results of robust and focused wake vortex research and development. These results will be required in order to use the system at its maximum efficiency. Recommendation 1-1. Aircraft wake vortex characteristics of transport airplanes operating in the national airspace system should be assessed using the best standardized techniques prior to their introduction into service, so that appropriate separation criteria may be established with regard to each new aircraft model. The details of this assessment should vary based on the impact any new aircraft is expected to have on the system, with large and heavy aircraft receiving more emphasis than small ones in terms of data requirements. STUDY PROCESS As directed in the 2005 NASA Authorization Act (P.L. 109-155), NASA’s Aeronautics Research Mission Directorate contracted with the National Research Council for an independent analysis of what should be the appropriate elements for a national approach to overcoming wake turbulence challenges. The committee’s full statement of task is found in Appendix A. The Committee to Conduct an Independent Assessment of the Nation’s Wake Turbulence Research and Development Program (see Appendix B) first met for 3 days in April 2007. At that meeting, the committee received briefings from those who would be interested in the results of the study: NASA Headquarters, staff of the U.S. House of Representatives Committee on Science and Technology, and the JPDO. It also heard from many of the major stakeholders of the WakeNet USA forum: the FAA, NASA’s Langley Research Center, the FAA’s Volpe Transportation Center, MITRE, MIT Lincoln Laboratory, the Air Line Pilots Association, and the National Air Traffic Controllers Association. At the second meeting, which took place in May 2007, the committee received briefings on European research from Eurocontrol and the Deutsches Zentrum für Luft- und Raumfahrt (DLR), the current view of NextGen research from the U.S. Office of Management and Budget (OMB) and the Office of Science and Technology Policy (OSTP), and the state of the art in various wake turbulence technologies from a variety of experts in industry and academia.
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Wake Turbulence—An Obstacle to Increased Air Traffic Capacity The third and final meeting was held in July 2007. At that meeting the committee finalized the findings and recommendations contained in this report. All three meetings took place in Washington, D.C. A full list of speakers is presented in Appendix C. CHALLENGES The committee began its work by listing the important challenges to address in wake turbulence. These challenges fell into two major categories: organizational challenges and technical challenges. A successful research plan would yield no gains in capacity or safety if the resulting solutions could not be implemented. Therefore, the two organizational challenges presented in Chapter 2 are equally important and are critical to the success of the nation’s wake turbulence R&D enterprise. It was decided that the organizational challenges had to be overcome if the benefits of the technical challenges were to be realized. The technical challenges are presented in Chapter 3. Prioritization of the technical challenges and a complete program plan are presented in Chapter 4. This report refers to short-, medium-, and long-term time frames. “Short term” is defined as activities carried out prior to 2012, “medium term,” between 2012 and 2017, and “long term,” 2017 to 2025. While the committee does not believe that the time frame carries any inherent priority, it does believe that the federal government should plan its research to sustain a continuous level of effort over time, and include efforts to realize the NextGen system. Some of these efforts may not reach their peaks before 2025, but large gains in capacity may potentially result. REFERENCES Borener, S. 2007. Reduced wake vortex and runway occupancy time analysis, presentation to the Committee to Conduct an Independent Assessment of the Nation’s Wake Turbulence Research and Development Program, April 2, 2007, Washington, D.C. FAA (Federal Aviation Administration). 1998. Order 8040.4. Available at www.faa.gov/library/manuals/aviation/risk_management/ss_handbook/media/app_g_1200.PDF. FAA. 2002. Advisory Circular 90-23F: Aircraft Wake Turbulence. Available at www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/1472662A19F6603B86256C1600733DA7?OpenDocument. FAA. 2006a. Aeronautical Information Manual: Official Guide to Basic Flight Information and ATC Procedures. Available at www.faa.gov/airports_airtraffic/air_traffic/publications/atpubs/aim/. FAA. 2006b. Order 7110.65R: Air Traffic Control. Available at www.faa.gov/airports_airtraffic/air_traffic/publications/atpubs/ATC/index.htm . Galpin, D., C. Pugh, D. Cobo, and L. Vinagre. 2005. European Wake Vortex Mitigation Benefits Study. Available at www.eurocontrol.int/eec/gallery/content/public/documents/newsletter/2006/issue_1/ EuroBen_WP3_Report.pdf.
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Wake Turbulence—An Obstacle to Increased Air Traffic Capacity Haines, R.F. 1978. Parameters of Future ATC Systems Relating to Airport Capacity Delay. DOT/FAA/EM-78-8A. ICAO (International Civil Aviation Organization). 2006. Wake Turbulence Aspects of Airbus A380-800 Aircraft. ICAO letter ES-AN 4/44-0750, October 9. JPDO (Joint Planning and Development Office). 2007. Concept of operations for the Next Generation Air Transportation System, version 2.0. June 13. Available at jpdo.gov/library/NextGen_v2.0.pdf. Last accessed August 2, 2007. Lang, S. 2007. Wake turbulence research management plan, presentation to the Committee to Conduct an Independent Assessment of the Nation’s Wake Turbulence Research and Development Program, April 2, 2007, Washington, D.C. Lang, S., A. Mundra, W. Cooper, B. Levy, C. Lunsford, A. Smith, and J. Tittsworth. 2003. A phased approach to increase airport capacity through safe reduction of existing wake turbulence constraints. Air Traffic Control Quarterly 11(4):331-356. Lang, S., D. Domino, A. Mundra, and C. Bodoh. 2004. ATC feasibility of potential near term wake turbulence procedures. 23rd Digital Avionics Systems Conference, October 24-28. Lang, S., J. Tittsworth, C. Lunsford, W. Cooper, L. Audenaerd, J. Sherry, and R. Cole. 2005. An analysis of potential capacity enhancements through wind dependent wake turbulence procedures. 6th USA/Europe Seminar on Air Traffic Management Research and Development, June 27-30, Baltimore, Md. Lebron, G. 1987. Estimates of Potential Increases in Airport Capacity Through ATC System Improvements in the Airport and Terminal Areas. FAA-DL5-87-1. Lunsford, C., A. Mundra, L. Audenaerd, J. Cheng, C. Devlin, A. Gross, R. Mayer, J. Sherry, W. Bryant, E. Johnson, and B. McKissick. 2005. Wind dependent concepts for wake avoidance: A comparative analysis of capacity benefits and implementation risks. 24th Digital Avionics Systems Conference, October 30. O’Connor, C.J., and D.K. Rutishauser. 2001. Enhanced Airport Capacity Through Safe, Dynamic Reductions in Aircraft Separation: NASA’s Aircraft Vortex Spacing System (AVOSS). NASA TM-2001-211052. Available at ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20010089337_2001145219.pdf. Last accessed August 2, 2007. Rutishauser, D., G. Donohue, and R. Haynie. 2003. Measurements of aircraft wake vortex separation at high arrival rates and a proposed new wake vortex separation philosophy. 5th Eurocontrol/FAA ATM R&D Conference, June 23-27.