Wake Turbulence: An Obstacle to Increased Air Traffic Capacity (2008)

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Summary Wingtip vortices were first described by British aerodynamicist F.W. Lanchester in 1907. A product of lift on a finite-span wing, these counter- rotating masses of air trail behind an aircraft, gradually diffusing while convecting downward and moving about under mutual induction and the influence of wind and stratification. Should a smaller aircraft happen to be following the first aircraft, it could be buffeted and even flipped if it flew into the vortex, with dangerous consequences. Given the amount of air traffic in 1907, the wake vortex hazard was not initially much of a concern. Times have changed. The demand for air transportation continues to increase, and it is estimated that demand could double or even triple by 2025. One factor in the capacity of the air transportation system is wake turbulence and the consequent separation distances that must be main- tained between aircraft to ensure safety. In 2005, Congress passed the 2005 National Aeronautics and Space Administration (NASA) Authorization Act (P.L. 109-155), which, inter alia, directed the NASA administrator to enter into an arrangement with the National Research Council (NRC) to assess federal wake turbulence research and development (R&D) programs to address whether the fed- eral R&D goals and objectives were well defined, whether there were any deficiencies in them, and what roles should be played by each of the relevant federal agencies: NASA, the Federal Aviation Administra- tion (FAA), and the National Oceanic and Atmospheric Administration (NOAA, part of the Department of Commerce). This report is the result of  

 WAKE TURBULENCE—AN OBSTACLE TO INCREASED AIR TRAFFIC CAPACITY that assessment, based on the statement of task (see Appendix A) devel- oped by NASA’s Aeronautics Research Mission Directorate and the NRC in accordance with the congressional direction. Chapter 5 lists all findings and recommendations; this summary highlights some of them. Wake turbulence is an obstacle to increased capacity The frequency of air traffic delays reached an all-time peak in June 2007, and that frequency is only expected to grow. The current air trans- portation system has reached a limit in certain airspaces, particularly near hub airports, where increasing traffic density and current routing practices necessitate a new approach to air traffic spacing and control. The interagency Joint Planning and Development Office (JPDO) was established to usher in the Next Generation Air Transportation System (NextGen). The seven entities represented in the JPDO are the FAA, the Department of Transportation (DOT), NASA, the Department of Home- land Security (DHS), the Department of Commerce (principally NOAA), the Office of Science and Technology Policy (OSTP), and the Department of Defense (DOD). NextGen is expected to bring revolutionary changes in navigation, communications, and air traffic control, all designed to increase the capac- ity of the air transportation system. At most airports, this will mean more aircraft arriving and departing. Depending on their relative sizes, a cer- tain minimum separation distance between aircraft must be maintained during approach and landing to avoid wake vortex encounters. Unless the separation distance can be reduced, other NextGen technologies will have much less impact on arrival and departure capacity than they otherwise could be expected to have. When the Boeing 747 entered the airspace system in 1970, it was substantially bigger than the 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 substan- tial changes. In many cases, these wake vortex separation requirements do not allow taking advantage of reduced separation standards enabled by satellite and other new technologies. In the past, the focus of wake turbulence research was aimed at improving safety. Current wake vortex separation criteria are conserva- tive and sufficient for ensuring safe operations. The key question now is whether a reduction in wake vortex separation criteria can be obtained while maintaining safety. Unfortunately, there is still no way to judge how much this spacing can be reduced—that is, there is no clearly defined 

SUMMARY  “hazard boundary.” With no meaningful metric, it is impossible to tell whether a proposed alternative is acceptably safe. The Committee to Conduct an Independent Assessment of the Nation’s Wake Turbulence Research and Development Program concluded that there are both organizational and technical challenges involved in increas- ing air transportation system capacity through reducing aircraft separa- tion standards. It also recognized that reducing separation standards is only one factor in increasing air system capacity. Some airports may not be able to take advantage of the extra capacity afforded by reduced separation standards. Today’s aviation system is a complex web of inter- twined systems that constrain each other in nonobvious ways. Runways, taxiways, gates, terminal traffic, emissions, and many other factors may limit capacity to much the same extent as wake turbulence does. Studies of tradeoffs at individual airports and at the systems level are useful in identifying the most fruitful ground for improvement. Thus, while wake turbulence is an obstacle to increased capacity, it is not the only obstacle. The others are outside the scope of this committee’s charge, however. 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. Organizational challenges: Federal wake turbulence research needs leadership To best support a national approach to overcoming wake turbulence challenges, there needs to be a simple and clearly defined goal, agreed to and understood by all participants. Based on current needs, an appropri- ate goal would be to develop the technical and procedural capability to increase capacitywithout loss of safetyby reducing the required air- craft separation distances associated with wake turbulence avoidance.  Historically, NASA and the FAA shared leadership of wake turbu- lence research. This arrangement was successful when budgets were 

 WAKE TURBULENCE—AN OBSTACLE TO INCREASED AIR TRAFFIC CAPACITY not so tight, but it is no longer feasible. While NASA has the technical expertise to support this leadership, wake turbulence research now lies outside its priority research funding focus. The FAA is responsible for the establishment of civil aviation safety standards and implementation of air transportation system changes, and therefore has an interest in all phenomena that affect safety, including wake turbulence. The JPDO has the ability to coordinate research, but as a planning agency it does not have the necessary executive power or budget authority. NOAA and DOD occasionally contribute to wake turbulence research, but their efforts are motivated by their own needs, not by the goal of increasing the capacity of the air transportation system. Without a leader, the alignment of these efforts depends on the relationships between individual researchers and on temporary partnerships between agencies. This is sufficient leadership to ensure success in projects and programs that take place over a few years, but not enough to tie those successes together into solutions. Finding 2-1. There is no champion, spokesperson, or leader held account- able for goal achievement across the nation’s wake turbulence research and development efforts. Finding 2-2. Wake turbulence is a long-term problem. Although a total solution cannot be achieved within a decade, improvements will become available gradually, depending on funding, and it can be envi- sioned that these incremental improvements will provide incrementally increased capacity at airports where implemented. Recommendation 2-1. Federal wake turbulence research should have the following characteristics: • The FAA should be the lead agency for defining requirements for wake turbulence research. • The FAA should manage and fund capacity-focused wake turbu- lence research using academic, industry, and other government partners. • The FAA should appoint a strong and motivated leader to inte- grate and coordinate research across agencies, define priori- ties, and represent wake vortex research to the JPDO and other agencies. • Research should be sustained over the short, medium, and long term. • Resource allocations across functional lines of involved ­agencies should be coordinated among all agencies involved in this work. 

SUMMARY  Better coordination of the many independent entities that are cur- rently studying wake turbulence characteristics, dynamic predictive capa- bilities, sensor and display development, and adaptive procedures will be important.  Until recently, NASA provided essential fundamental wake turbulence research in partnership with the FAA. But budget constraints have severely limited NASA’s ability to support the wake vortex research required for NextGen, creating a technology gap. While NASA is still well-aligned to do this research, in that it possesses the proper expertise, facilities, and institutional experience, it does not have the necessary resources. The FAA does not have this expertise, and there appears to be no other government agency with this capability or capacity. Other orga- nizations with this technical capability will have to be identified so that the FAA can work with them. Elements of a successful study of wake turbulence include (1) being identified as a major program within NextGen, the wide ranging transfor- mation of the entire national air transportation system; (2) being included within the scope of one or more of the FAA Air Transportation Centers of Excellence; (3) being consolidated in a single location, perhaps at the FAA Field Office located at NASA’s Langley Research Center, funded by the FAA; (4) being identified by the Administration as a high priority; (5) being closely linked to all similar international studies; and (6) being identified as a high priority in the Aeronautics Research and Develop- ment Plan and the related Aeronautics Research, Development, Test, and Evaluation Infrastructure Plan, as it was in the NRC’s 2006 Decadal Survey of Civil Aeronautics. The committee found as follows (Findings 2-3, 2-4, and 2-5): The change in aeronautics research priorities at NASA has led to a gap in the wake turbulence program as previously envisioned; present federal investment does not place sufficient priority on wake turbulence research to achieve the results called for by NextGen goals; and NASA expertise is well-aligned to conducting medium- to long-term fundamental research, including wake vortex modeling and wake vortex alleviation work, while the FAA does not currently have such expertise. Recommendation 2-2. Because of its expertise, NASA should continue to conduct medium- to long-term fundamental research, including wake vortex modeling and wake vortex alleviation work at a level of effort sufficient to achieve NextGen goals. Operators (including airline pilots, airline management, and general aviation pilots) and controllers are important elements of this process. If these parties are not confident that new operations and technologies are safe and effective, they will not use them. By engaging users, researchers 

 WAKE TURBULENCE—AN OBSTACLE TO INCREASED AIR TRAFFIC CAPACITY will have a better understanding of what is and is not acceptable, and users will have a better understanding of new systems and be able to contribute to their development and implementation. Recommendation 2-3. Operators and controllers should be included in the process of designing, implementing, and evaluating wake turbu- lence-related changes to the air transportation system. The JPDO has an important role in assisting in the definition of fed- eral wake turbulence research. Its Integrated Work Plan and its Research and Development Plan currently in development are expected to provide some guidance, but more detailed plans will be necessary to ensure that wake turbulence solutions will both support and be supported by the NextGen architecture. Recommendation 2-4. JPDO should recommend to the FAA detailed wake vortex research efforts needed to support NextGen. TECHNICAL CHALLENGES The committee identified nine technical challenges that need to be addressed. Each is discussed in detail in Chapter 3, and associated mile- stones are identified for short-term (by 2012), medium-term (by 2017), and long-term (by 2025) research. The committee divided the technical challenges into three groups: capacity enhancers, enabling research, and supporting studies. Capacity Enhancers Improving Spacing System Design The committee considered three approaches (see Findings 3-2, 3-3, and 3-4 and Recommendations 3-2, 3-3, and 3-4): • Closely spaced parallel approach (CSPA) procedures. Wind monitoring will allow better use of parallel runways, because spacing can be reduced when the leading aircraft is downwind of its follower. When wind moni- toring is coupled with improved technology on the vortex location and revised air traffic control (ATC) procedures, even greater capacities can be achieved. • Recategorization. The current system of categorizing an aircraft as small, large, or heavy may not be the most efficient approach, particularly since it does not account for detailed characteristics of the wakes, or the encountering aircraft. 

SUMMARY  • Dynamic spacing. A combination of modeling and measurement that locates the wake vortices would allow aircraft to continually adjust their spacing in Visual Flight Rule (VFR) flight for optimization depend- ing on the mix of aircraft approaching, the speed and direction of wind, and the rate of dissipation of vortices on approach to a given runway. In Instrument Flight Rule (IFR) flight, the system would provide a safety backup to assist in ensuring avoidance of a hazardous wake encounter. Vortex Visualization: Cockpit and Controller Dynamic spacing of aircraft based on wake vortex motion will require prediction of wind behavior over roughly the next hour. It is also necessary for the pilot and/or controller to have information on the wake position in real time as a safety net to verify the predicted separation provided. The information could be presented to the pilot numerically, visually, or by a simple red/green light system. One option for presenting the real-time wake position that has been researched is visualization. Onboard wake vortex visualization has been demonstrated in a proof-of-concept trial and can provide a safety net for dynamic self-spacing procedures in both IFR and VFR flight. These concepts should be further explored and pursued (see Finding 3-5 and Recommendation 3-5). Vortex Alleviation There have been many attempts over the past several decades to evaluate systems that provide some alleviation of the wake vortices via laboratory simulations. The level of vortex alleviation activity in the United States is very small, with no discernible effort at any of the fed- eral agencies. Activity over the past decade has been concentrated in industry, in academia, and in Europe. Vortex alleviation has the potential to significantly impact aircraft spacing requirements in the long term. Vortex alleviation ideas, including configuration changes and active and passive forcing, should be explored. (See Findings 3-6 and 3-7 and Rec- ommendation 3-6.) Enabling Research Weather Forecasting European forecast models have already been demonstrated to forecast terminal area weather and wake vortex persistence in the short term. In the United States, existing weather forecast models have been used with some success at Lambert-St. Louis International Airport (STL) to forecast wake transport persistence. A higher-resolution model will be required 

 WAKE TURBULENCE—AN OBSTACLE TO INCREASED AIR TRAFFIC CAPACITY to improve on this performance and to better forecast wake vortex per- sistence. The Weather Research and Forecasting (WRF) model currently under development by the National Center for Atmospheric Research, NOAA, DOD, the FAA, the University of Oklahoma, and others has the potential to meet this requirement in the necessary time frame to support the needs of NextGen. However, as presently conceived these models lack the ability to incorporate eddy dissipation rates and thus will not provide the optimal set of parameters needed for predicting wake turbulence. More research is needed to ensure that weather modeling is adequate to predict wake vortex movement and decay. (See Findings 3-8 and 3-9, and Recommendations 3-7 and 3-8.) Wake Vortex Modeling Wake vortex modeling attempts to predict the basic characteristics of the vortices from the near field into the far field as a function of the generating aircraft and ambient atmospheric conditions. Modeling of the wake-initialization phase has not received much attention at any of the federal agencies. Recent efforts are focused in industry, in ­universities, and in European agencies. Modeling of the wake-evolution phase has been ongoing at NASA Langley Research Center, with research assistance from the Naval Postgraduate School and Northwest Research Associates. Wake vortex modeling plays a critical role in many concepts aimed at reducing IFR spacing requirements, and NASA’s aeronautics program is well-aligned to conduct medium- to long-term foundational wake vortex modeling. (See Findings 3-10 and 3-11.) Wake Vortex Measurement Research and development of high-resolution sensors to support wake vortex modeling efforts has stalled since the late 1990s. Sensors are needed to measure aircraft wake vortices as well as meteorological con- ditions for inputs into wake vortex and weather prediction models. No high-resolution wake vortex measurement system capable of operating in inclement weather currently exists. An all-weather, aircraft-based wake vortex measurement system that provides information on the location of the wake should be explored, and an all-weather wake vortex measure- ment system that provides high-resolution measurements of wake vortex characteristics sufficient to validate wake vortex modeling should be developed. (See Findings 3-12 and 3-13 and Recommendations 3-9 and 3-10.) 

SUMMARY  Supporting Studies Safety Analysis and Hazard Boundaries It is difficult to quantify acceptable reductions in wake turbulence spacing because there is no agreed metric for or definition of hazard boundaries for wake encounters. A “hazard boundary” provides a demar- cation between acceptable and unacceptable vortex encounters based on criteria developed in conjunction with the pilot community. Because defining a hazard standard is ultimately the responsibility of regula- tory bodies, this effort should be led by the FAA. However, substantial research at the aircraft vehicle level is required to define the wake hazard boundaries. This aspect of the work could be led by NASA, using contract support from aircraft manufacturers and airlines/pilots. A hazard bound- ary needs to be defined and used as a metric in forming spacing criteria. (See Finding 3-15 and Recommendation 3-11.) Systems to Gather Data About Wake Events The challenge is to develop a means of collecting information and data from wake events that have actually occurred as observed and reported by pilots. Currently only a very limited amount of information on wake events is collected in any form by any agency in the United States, so base- line data are not readily available. Without an event-driven database as a control, it will be very difficult to measure whether any future increase or decrease in wake events is the result of reduced spacing or is simply in line with current event levels. Pilots and controllers today do not have a simple system to report the impact of a wake event in all phases of flight. Implementing a system to gather data on wake events in the short term could establish a baseline that could be used to quantitatively evaluate potential solutions. It could also help gain support from the operator and ATC community. (See Finding 3-16.) System-Level Study of Benefits System-level studies are required to assess the relative benefits of wake-turbulence mitigation strategies and to help with setting research priorities and using resources effectively. It is important that system-level studies cover a range of operational scenarios, weather scenarios, fleet mixes, and airport layouts. They are essential for ensuring that (1) research priorities are set in a rational manner, (2) the actual realizable benefits of wake turbulence solutions will be known, and (3) key constraints can be identified and NextGen capacity goals can be achieved. The current JPDO research in system-level modeling of the air transportation system should 

10 WAKE TURBULENCE—AN OBSTACLE TO INCREASED AIR TRAFFIC CAPACITY be continued and resources should be directed to extending simulation capabilities to cover a range of operational scenarios, weather scenarios, fleet mixes, and airport layouts. (See Finding 3-17 and Recommendation 3-12.) Agency Roles in Addressing the Challenges Certain agencies are best able to address certain of these techni- cal challenges. Over the short and medium terms, the FAA, assisted by NASA, should continue its current improved spacing programs, and it should pursue work to determine minimum runway spacing for future airport expansions (see Recommendations 3-3 and 3-4). The JPDO should do three things: • Investigate and define specific requirements for research on the impact of cruise-altitude-generated wakes on capacity (including climb and descent) to avoid future problems as fleet diversity increases (Recom- mendation 3-1). • Conduct a detailed analysis of what wake turbulence research and development is needed to achieve its separation management capability goals, and provide a detailed plan with milestones that will lead to suc- cessful development in the required time frame (Recommendation 3-2). • Continue its research in system-level modeling of the air transpor- tation system and direct resources to extending simulation capabilities to cover a range of operational scenarios, weather scenarios, fleet mixes, and airport layouts in support of FAA and NASA research, as requested and agreed to in support of NextGen goals (Recommendation 3-12). For the technical challenges that do not suggest a specific agency, the FAA should manage the program, utilizing resources such as WakeNet USA, a support network including academia, industry, and various fed- eral research centers that coordinates federal wake turbulence research. These resources should be tapped whenever possible. Partnerships with this support system, as well as international partnershipsincluding con- tinuation of work with WakeNet Europeshould be encouraged, striving for a balanced mix of participation. CREATING A WAKE TURBULENCE PROGRAM PLAN In addition to identifying technical challenges, the committee was charged with prioritizing those challenges and generating a draft pro- gram plan. The committee found that the challenges were highly syn- ergistic, which made it very difficult to prioritize them as a single list; a 

SUMMARY 11 particular challenge’s ability to provide capacity was generally linked to the accomplishment of others, and thus contingent on the contents of the total research portfolio. The committee accordingly decided to roll priori- tization and program planning into a single step. Concurrently investigating capacity enhancers at varying stages of maturity reduces the technological risk of the program as a whole. Figure S-1 shows a notional program plan. Each challenge is accompanied by a bar stretching from the short term to the long term. The height of the bar represents the level of effort recommended by the committee; the shading of the bar represents its priority. The priorities and levels of effort were selected with the expectation that the program as a whole would have the greatest potential to create capacity; the expected deliverables of this program are listed in Table S-1. Because of the interrelationships among the challenges, budget changes should be absorbed by the program as a whole. That is to say, instead of cutting or eliminating one challenge in times of scarce resources, levels of effort should be lowered across the board, with medium-priority challenges taking a slightly larger cut than those with high priority. The committee did not judge any of the chal- lenges to be of low priority. In this way, the time horizon of the program may grow longer, but the quality of the results will not be jeopardized. Similarly, if more money becomes available, it should be used to bolster efforts for all of the challenges. That said, research is a dynamic enterprise. This report includes mile- stones (Table S-1) and metrics (Table S-2) for evaluating the progress in overcoming these challenges. In time, some may prove to be dead ends, or new ideas may surface, necessitating periodic reexamination of research priorities to ensure that wake turbulence research maintains relevance throughout the evolution of the NextGen system. Recommendation 4-1. Wake turbulence research should pursue mul- tiple tracks, with the goal of a robust, stable program that will provide continuing reductions in aircraft spacing as new ideas and technologies are developed and proven. Recommendation 4-2. Wake vortex research priorities should be peri- odically reexamined. Recommendation 4-3. The federal wake turbulence R&D enterprise should continue its relationships with a balanced mix of government laboratories, industry, and academia. 

12 WAKE TURBULENCE—AN OBSTACLE TO INCREASED AIR TRAFFIC CAPACITY (2012-2017) (2017-2025) Short Term Long Term High priority (through Medium 2012) Term Medium priority Level of effort Safety analysis and hazard Supporting Studies boundaries Gather event data System-level study of benefit Enabling Research Weather forecasting Modeling Measurement CSPA procedures Capacity Enhancers Spacing Recategorization Systems Dynamic spacing Visualization Alleviation FIGURE S-1  Recommended priority and level of effort for wake turbulence chal- lenges. Height of each bar indicates level of effort. Shade of each bar represents priority (darker corresponds to higher priority). CSPA, closely spaced parallel approach. fig S-1 and 4-1 

SUMMARY 13 TABLE S-1  Deliverables Short Term Medium Term Long Term Conservative hazard Fleetwide simulation Refined and tested hazard boundary capabilities for gate- boundary Outreach efforts to to-gate operations Pilot training for wake aviation community All-weather wake vortex vortex Wake turbulence measurement system System-level simulations encounter reporting Airborne wake vortex with effects of system sensor uncertainty System-level simulations Wake vortex High-resolution, all- of arrival/departure measurement weather wake vortex with operational network measurement system scenarios, weather Conditional spacing High-resolution, all- scenarios, fleet mixes, reduction to CSPA at weather wake vortex and airport layouts selected airports measurement network Weather data needs Conditional spacing coordinated with reduction to CSPA at all WRF effort airports Probabilistic wake vortex Conditional spacing model reduction for single runway approaches at selected airports Visualization systemsa Alleviation methods and devicesa Dynamic spacinga NOTE: WRF, Weather Research and Forecasting; CSPA, closely spaced parallel approach. aResearch would be completed in the long term, but deliverable would not yet be a ­ vailable. 

14 WAKE TURBULENCE—AN OBSTACLE TO INCREASED AIR TRAFFIC CAPACITY TABLE S-2  Evaluation Metrics Metrics Supporting studies Number of parameters included in analysis Amount of data collected Variety of data collected Computational efficiency of models Applicability to flight simulators for pilot training Enabling research Temporal and spatial resolution Low uncertainty Weather tolerance Accuracy in the characterization of Lateral wake location Vertical wake location Wake strength Meteorological conditions Speed Computational efficiency Time response of measurement systems Capacity enhancers Capacity provided Delays reduced Cost Precision Accuracy Predictability (Will you know how much capacity you have?) Robust to Airports Aircraft Weather conditions Traffic conditions