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Wake Turbulence—An Obstacle to Increased Air Traffic Capacity
4
Wake Turbulence Program Plan
PRIORITIZATION OF CHALLENGES
In addition to identifying the challenges described in Chapter 3, the committee was also charged with prioritizing these challenges and generating a draft program plan. The committee found that the challenges were highly synergistic, which made it very difficult to prioritize them as a single list; a particular challenge’s ability to provide capacity was generally linked to the accomplishment of other challenges and thus contingent on the contents of the total research portfolio.
The committee therefore decided to roll prioritization and program planning into a single step. In Figure 4-1, 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. Levels of effort are approximate and meant to show the relative changes of activity within that challenge and which challenges are emphasized during each time frame.
The utility of a prioritized list is that, in the absence of a sufficient budget, a program manager may decide to cut the lowest-priority items. Given additional resources, he or she may decide to allot them to the highest-priority challenge. The system shown in Figure 4-1 suggests a different approach. Because of the interrelationships among the challenges, instead of cutting or eliminating one challenge, levels of effort should be lowered across the board, with medium-priority challenges taking a slightly larger cut than high-priority challenges. The committee did not judge any of the challenges to be of low priority. In this way, the time
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FIGURE 4-1 Recommended priority and level of effort for wake turbulence challenges. Height of each bar indicates level of effort. Shade of each bar represents priority (darker corresponds to higher priority). CSPA, closely spaced parallel approach.
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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 all of the efforts. The priorities and levels of effort were selected to give the program as a whole the greatest potential to create capacity.
Supporting studies are high priority and merit high levels of effort in the short term. All of these studies will be of great value in characterizing and evaluating potential solutions, so creating them and putting them in place quickly will maximize their value. Once models have been created, hazard boundaries have been set, and reporting systems are in place, much less work will be needed to maintain and make use of these capabilities.
Capacity enhancers are focused projects that address specific changes to the air transportation system. Each follows the same general pattern: At the start, there is a low level of effort associated with performing studies of tradeoffs and evaluating potential concepts. As concepts are narrowed down, effort is ramped up, culminating in a testing effort. If the technology is accepted, a smaller level of research will continue to assist in certification and handoff to the operational side of the FAA. Federal research should be addressing multiple capacity enhancers at various levels of effort at any given time. This allows sufficient sharing of resources such as lidar systems and roughly constant funding. It also means that increased capacity does not hinge on the success of a single concept. In fact, these projects should be monitored closely. If, during concept evaluation, their value becomes questionable, they can be canceled or refocused with a minimal waste of resources. As the program goes on, new concepts for reducing wake vortex constraints may arise and can be rolled into the project schedule at the appropriate time.
Modeling and sensor development will generate important knowledge and the capabilities needed to implement the various capacity enhancers and provide inputs for the systems studies. A constant level of effort should be maintained, addressing needs relevant to each of the time frames. This enabling research provides the seed corn for future concepts and is a basis of the nation’s wake turbulence core competence.
The results of this program will be continuous reductions in required wake turbulence spacing. No single answer will totally eliminate the threat of wake vortices, certainly not by 2025. Most solutions will be appropriate for a certain kind of airport or certain weather conditions. However, as more and more capabilities are added, they will build on one another. Table 4-1 shows some of the deliverables distilled from the milestones of this program plan and when they are expected.
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TABLE 4-1 Deliverables
Short Term
Medium Term
Long Term
Conservative hazard boundary
Outreach efforts to aviation community
Wake turbulence encounter reporting system
System-level simulations of arrival/departure with operational scenarios, weather scenarios, fleet mixes, and airport layouts
Weather data needs coordinated with WRF effort
Probabilistic wake vortex model
Fleetwide simulation capabilities for gate-to-gate operations
All-weather wake vortex measurement system
Airborne wake vortex sensor
Wake vortex measurement network
Conditional spacing reduction to CSPA at selected airports
Refined and tested hazard boundary
Pilot training for wake vortex
System-level simulations with effects of uncertainty
High-resolution, all-weather wake vortex measurement system
High-resolution, all-weather wake vortex measurement network
Conditional spacing reduction to CSPA at all airports
Conditional spacing 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 available.
Recommendation 4-1. Wake turbulence research should pursue multiple 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.
PERIODIC ASSESSMENT
The program recommended in this chapter is a living one. Based on current knowledge, the challenges outlined in this report are believed to afford the best potential for achieving the goals of NextGen. However, in the next few years, the air transportation system will undergo many changes, some of which could invalidate some of these concepts and inspire new ones. Wake turbulence research should be constantly kept relevant throughout the evolution of the NextGen system. In fact, as the
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TABLE 4-2 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
program plan shows, research need not grind to a halt when NextGen is implemented. More capacity may be needed in the future; in addition, these technologies may provide other benefits such as safety, efficiency, and situational awareness.
There are two ways to measure progress in addressing a challenge. The first is to view its progression along a planned path. This can be done using the milestones listed for each challenge in Chapter 3 and the deliverables shown in Table 4-1. The second involves evaluation against a metric, which allows comparing alternative projects. The metrics for the three categories of challenge are listed in Table 4-2.
Recommendation 4-2: Wake vortex research priorities should be periodically reexamined.
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ROLES
In Chapter 2, the committee discusses the roles of various federal agencies and recommends that the FAA take leadership and oversight of wake turbulence research as a whole; that NASA assist in aeronautics and airspace systems research; that the JPDO assist in identifying research needs and providing system-level studies, and that the WRF modeling group expand its weather prediction system to include parameters needed for wake vortex characterization and implementation of dynamic spacing.
In the past, federal wake turbulence research has taken advantage of outside resources. The Volpe Transportation Research Center has collected a great deal of the existing empirical data. MITRE Corporation’s Center for Advanced Aviation System Development has performed systems modeling and human-in-the-loop testing. MIT’s Lincoln Laboratory has researched localized weather prediction and built operational prototypes of FAA systems. Companies like Boeing, Lockheed Martin, and Northwest Research Associates and universities like George Mason University, the University of California at Berkeley, and the Naval Postgraduate School have also been involved.
Recommendation 4-3: The federal wake turbulence R&D enterprise should continue its relationships with a balanced mix of government laboratories, industry, and academia.