NASA WAKE VORTEX RESEARCH FOR AIRCRAFT SPACING R. Brad Perry*, David A. Hinton**, and Robert A. Stuever*** NASA Langley Research Center Hampton, Virginia ABSTRACT MOA Memorandum of Agreement NASA National Aeronautics and Space The National Aeronautics and Space Administration Administration (NASA) is addressing airport capacity NOAA National Oceanic and Atmospheric enhancements during instrument meteorological Administration conditions through the Terminal Area Productivity RSO Reduced Spacing Operations (TAP) program. Within TAP, the Reduced Spacing TAP Terminal Area Productivity Operations (RSO) subelement at the NASA Langley TASS Terminal Area Simulation System Research Center is developing an Aircraft Vortex TRACONTerminal Radar Approach Control Spacing System (AVOSS). AVOSS will integrate the output of several inter-related areas to produce weather INTRODUCTION dependent, dynamic wake vortex spacing criteria. These areas include current and predicted weather Air travel delay and traffic congestion at major airports, conditions, models of wake vortex transport and decay projected increases in air travel, and environmental in these weather conditions, real-time feedback of wake restrictions on new airport construction, together with vortex behavior from sensors, and operationally associated costs to the traveling public and to the air acceptable aircraft/wake interaction criteria. In today’s carriers, have led to an increased interest in maximizing ATC system, the AVOSS could inform ATC the efficiency of the national airspace system. The controllers when a fixed reduced separation becomes National Aeronautics and Space Administration safe to apply to “large” and “heavy” aircraft categories. (NASA) is responding to this interest through its With appropriate integration into the Center/TRACON Terminal Area Productivity (TAP) program. The major Automation System (CTAS), AVOSS dynamic spacing goal of the TAP program is to develop the technology, could be tailored to actual generator/follower aircraft during instrument meteorological conditions, which pairs rather than a few broad aircraft categories. allows air traffic levels to approach or equal levels presently achievable only during visual operations. ACRONYMS Presently, a degradation in weather conditions which causes a loss of visual approach capability degrades ATC Air Traffic Control capacity due to numerous factors. These factors AVOSS Aircraft Vortex Spacing System include reducing the number of available runways and CTAS Center/TRACON Automation System the longitudinal wake vortex separation constraints DOT Department of Transportation used by air traffic control (ATC) in the spacing of FAA Federal Aviation Administration aircraft to a runway. Two major initiatives under TAP ILS Instrument Landing System are the enhancements of basic ATC automation tools ITWS Integrated Terminal Weather System and the development of a wake vortex spacing system MIT Massachusetts Institute of Technology to improve terminal area efficiency and capacity. The ____________________________________________ NASA Ames Research Center is developing * Manager, Reduced Spacing Operations. Associate enhancements to the base Center/TRACON Fellow AIAA. Automation System (CTAS). This automation will ** Research Engineer. provide aids and interfaces to the controller to *** Research Engineer, Senior Member AIAA. effectively schedule and sequence arrivals and Copyright ª 1996 by the American Institute of Aeronautics minimize variations in desired interarrival spacing. and Astronautics, Inc. No copyright is asserted in the United Enhanced CTAS automation will provide an States under Title 17, U.S. Code. The Government has a opportunity to dynamically alter the longitudinal wake royalty-free license to exercise all rights under the copyright vortex separation constraint as a function of both the claimed herein for Government purposes. All other rights are weather effects on wakes and aircraft leader/follower reserved by the copyright owner. pair types. The Reduced Spacing Operations (RSO) this matrix to an automated ATC system such as CTAS subelement of TAP, at the Langley Research Center, is described by Erzberger3 et al. Capacity gains can be developing the Aircraft Vortex Spacing System expected due to considering both wake transport and (AVOSS) which is described by Hinton1. The purpose decay, and also by providing a large matrix of aircraft of the AVOSS is to integrate current and predicted pair separations to ATC automation rather than just the weather conditions, wake vortex transport and decay three category system utilized in today’s system. knowledge, wake vortex sensor data, and operational The wake separation constraints will be delivered definitions of acceptable strengths for vortex to the automated ATC system with adequate lead time encounters to produce dynamic wake vortex separation and stability to be used in the process of metering and criteria. By considering ambient weather effects on spacing. Final approach aircraft spacing may be wake transport and decay, the wake separation established 5 to 10 minutes prior to landing, while the distances can be decreased during appropriate periods metering rate at which aircraft are accepted into the of airport operation. With the appropriate interface to TRACON is established earlier. Since a current CTAS, spacing can be tailored to specific weather observation will frequently not reflect the wake leader/follower aircraft types rather than just a few situation 5 to 50 minutes in the future, an effective broad weight categories of aircraft. In a manual ATC AVOSS must utilize short-term weather predictions system, a simplified form of the AVOSS concept may (nowcasting) to provide the lead time required for be used to inform ATC when a fixed alternate, reduced increasing terminal area capacity. Although wake separation standard becomes safe for the “large” AVOSS/CTAS interface simulations have not yet been and ”heavy” aircraft categories. performed, the concept currently envisions a zero to 15 The AVOSS development program has as its minute weather prediction being used to establish target a field demonstration of a prototype AVOSS individual aircraft pair spacing for final approach and a system in the year 2000. To support this goal, current 30 to 60 minute prediction being used to regulate the plans include three increasingly complex AVOSS field rate at which aircraft are accepted by the TRACON deployments to be conducted at the Dallas-Fort Worth facility from enroute airspace. This nowcasting International Airport. This paper describes the AVOSS capability, coupled with CTAS and the AVOSS concept and development program, the related wake predictor capability, will ensure that adequate aircraft vortex modeling effort, the wake vortex hazard are available for approach when minimal spacing is definition studies, and the wake vortex sensors possible and the arrival rate is reduced when larger development. spacing is required to avoid inefficient low altitude path stretching and holding. This weather predictive AVOSS CONCEPT requirement will drive all efforts in the development areas of meteorological sensors and system AVOSS is envisioned as an automated process architecture. The automated nowcasting element is an which combines meteorological data, rules describing important difference between the AVOSS concept and the atmospheric modification of wakes, and aircraft and earlier concepts that proposed to utilize only real-time airspace operational procedures to provide dynamic surface weather observations to regulate final approach wake vortex separation constraints to ATC. The spacing. AVOSS system concept by Hinton borrows from A number of ground rules will be followed during previous efforts conducted in the 1970's by Eberle2 et the AVOSS development. The development effort will al. The philosophy behind the AVOSS system is to be focused on a practical system that can be approved avoid aircraft encounters with vortices above an for operational use. This will require a large degree of "operationally acceptable strength." This avoidance is robustness, reliance on readily available meteorological obtained through consideration of two factors, wake and wake sensors, graceful system degradation when vortex motion away from the flight path of a following sensors or subsystems fail, and cost realism. The safety aircraft and wake vortex decay. Since these factors are provided must be equal to or greater than the current highly dependent on ambient meteorological system. The AVOSS will not require an increment in conditions, as well as the generating aircraft position pilot skill levels or training requirements, nor any and type, the wake vortex constraints on aircraft aircraft structural or on-board systems modifications. separation are expected to vary significantly with the The AVOSS will not alter current pilot functions nor weather. The AVOSS will quantify the wake change airborne/ground responsibilities. ATC separation required for generator/follower aircraft pairs controllers will not be required to monitor or predict during final approach, or the initial climb, and provide weather conditions. During peak traffic demand 22 American Institute of Aeronautics and Astronautics periods, however, efficient “vortex-limited” spacing airplanes from large aircraft wakes. The AVOSS will operations may require special ATC or flight require improvements in the current state of knowledge procedures compatible with current skill levels. of wake behavior in ground effect, meteorological Examples may include executing only straight-in predictions, and wake/aircraft interaction. Instrument Landing System (ILS) approaches and no The derivation of the approach corridor intentional operations above the glide slope by “large” dimensions was described by Hinton in 1995, refined in or “heavy” category aircraft. Finally, the AVOSS Hinton4, and depicted in Figure 1. These dimensions system must provide a meaningful increment in airport will likely be further refined based on industry and capacity during most instrument meteorological FAA inputs as the system is developed and conditions and not reduce capacity during visual demonstrated. The departure corridor shape has not yet meteorological conditions. been defined and will likely only include lateral AVOSS will provide a time-based matrix that separation due to the wide variation in climb angle provides only the wake vortex constraint for a between different aircraft. leader/follower aircraft pair. The automation/ATC interface will combine this constraint with other factors including radar resolution of aircraft position and runway occupancy time to determine the actual Glide Slope approach spacing. For maximum efficiency with an 70 ft 200 feet automated ATC system, the vortex spacing constraint Approach Side View will be dependent on individual aircraft leader/follower Approach Corridor Floor pairing, although the matrix could be reduced in real time to the current “small,” “large,” and “heavy” Runway Middle Marker categories for use by a manual ATC system. Approach Corridor Lateral Limits Approach Plan View Automation interface issues to be investigated during Localizer the development include required controller interface 1000 feet and displays, controller acceptance, maximum spacing 300 feet update rates, and overall system stability with dynamic spacing. Figure 1. AVOSS Approach Corridor Dimensions An important aspect of AVOSS is that it is not intended to be a fix to any perceived safety problems, nor is it intended for routine use at all airport facilities. The AVOSS purpose is to improve airport capacity at The AVOSS subsystem architecture is shown in major facilities that are capacity limited and that will be Figure 2. Each of the AVOSS subsystem areas will be equipped with ATC automation tools such as CTAS, described in turn. and state-of-the-art meteorological systems such as the Integrated Terminal Weather System (ITWS). This focus on capacity has strong implications for the development of AVOSS. For example, the wake sensor Prediction will not be required to detect the weak wakes from Subsystem “small” category aircraft, since current separation - Transport & Decay standards behind those aircraft are driven by runway - Predict Hazard occupancy time rather than wake vortex separations. Weather TRACON Likewise, the interaction between small aircraft and Subsystem AVOSS Automation wakes may not be modeled in the initial AVOSS since - ITWS Detection - Adaptive Separation small aircraft typically contribute a small percentage of - other Subsystem - Tactical Safety the traffic during capacity limited periods at major - Locate - Track airports. Under those traffic mix conditions, retaining - Quantify current separation standards for small aircraft in the subset of weather conditions that stall vortices in the approach corridor should have little impact on overall Figure 2. AVOSS Subsystem Architecture airport capacity levels. These considerations have led to an AVOSS development activity that is somewhat different from traditional efforts to protect small 33 American Institute of Aeronautics and Astronautics Prediction Subsystem variable winds, such as would be expected near air- The core of the AVOSS system is the prediction mass type thunderstorms, the uncertainty values may be subsystem. The predictor will utilize weather data, an quite large. In this environment, AVOSS will prescribe aircraft data base to predict the initial wake and the conservative spacing with a maximum value equal to threshold of wake vortex strength for an acceptable current separation rules. The weather subsystem will encounter (acceptable strength definition), airport utilize several available resources, including existing configuration data, and wake sensor feedback. Using and planned products from the FAA's ITWS program, weather data for current and projected times, at a off-the-shelf acoustic sodars or radar profilers, and number of "windows" along the path, the predictor will advanced numeric modeling techniques under compute both the time required for vortices from each development. aircraft to clear the AVOSS corridor (transport time) and the expected time to decay to an acceptable Sensor Subsystem strength (decay time) for each follower aircraft. At The wake vortex sensor subsystem is included for each window, the lesser of either the transport or decay several purposes. Operational test and evaluation of time will be taken as the wake spacing constraint at that any particular AVOSS installation will require a wake point. For each aircraft pair, the recommended final sensor to validate the performance of the weather and approach wake constraint will be the largest of the predictor subsystems at that airport. Once operational, wake constraint times from the various windows. the wake sensor would be used to continuously monitor Appropriate uncertainty buffers will be applied to the actual wake behavior and compare to the prior predictions to accommodate weather uncertainties, as predictions. In normal operations, the actual wake well as expected aircraft flight total system error on the behavior should fall close to the prediction and well approach path. A number of efforts are underway to inside the uncertainty buffer times provided to ATC. develop the prediction subsystem, including When actual behavior begins to deviate from development and validation of two-dimensional and predictions, the wake sensor feedback to AVOSS will three-dimensional wake and planetary boundary layer be used to refine aircraft spacing prior to any aircraft simulation codes, field studies to validate the numeric encountering an unsafe wake remaining in the corridor. codes and implement the infrastructure required for In the event of a wake unexpectedly persisting much AVOSS testing and demonstration, and simulation and longer than expected, the wake sensor input to ATC flight test to quantify the interaction between aircraft would be used to provide a go-around to the next wakes and following aircraft. aircraft and increase subsequent spacing. Obviously this last situation must be extremely rare. A secondary Weather Subsystem use of a wake vortex sensor is that some of the potential The weather subsystem is crucial to the AVOSS. technologies can also provide high quality approach This subsystem must provide detailed wind, vertical corridor wind information to the AVOSS predictor wind shear, turbulence, and temperature gradient system. information to the prediction subsystem for the current The basic requirements for a wake vortex sensor time and for times up to an hour in the future. The are to detect, locate, and quantify the strength of weather subsystem should anticipate boundary layer aircraft wake vortices. The sensor should perform this changes associated with sunrise and sunset, and provide function in clear weather as well as in low ceiling and discrete information to AVOSS when the atmosphere is visibility conditions compatible with the instrument about to be modified by frontal passages or other approach minima for the airport facility. The sensor phenomena. The AVOSS is not attempting to predict should protect the critical region of the approach, which how a particular vortex will behave 30 minutes in the begins near the aircraft touchdown zone and extends to future. Instead, the predictor is setting the bounds of a distance of approximately two miles from the runway. expected vortex behavior given that the supporting This definition of the critical region will be refined weather predictions will have some uncertainty. Wind during development, but must include that region values and their confidence boundaries will be where terrain and changing boundary layer specified. The predictor will use these uncertainty characteristics make vortex prediction most difficult, values to estimate a range of wake behaviors. In a and where the potential danger of an error is weather situation with moderately steady wind, for unacceptable. A sensor that does not meet all of the example, the uncertainty values may be small compared above requirements may be useful in a subset of to the wind, and accurate wake predictions should weather conditions, and the effectiveness of that sensor result. In a weather situation with moderate but highly would be evaluated with cost/benefit studies. 44 American Institute of Aeronautics and Astronautics NASA RESEARCH SUPPORTING AVOSS The NASA Langley wake vortex research required DEVELOPMENT the acquisition of planetary boundary layer and wake vortex data for the validation of numerical models. NASA is conducting a broad range of research, This element has been addressed through an agreement both in-house and out-of-house, to support the with MIT-Lincoln Laboratory which was initially development of AVOSS. In broad terms, AVOSS funded in early 1994. Within that year Lincoln requires: 1) a knowledge of the characteristics of designed, constructed, and deployed a van-mounted vortices which are generated by different aircraft, 2) a 10.6 micron, continuous wave coherent laser (lidar), predictive capability for the effects of weather on the equipped the Memphis International Airport with a 45 motion and decay characteristics of vortices from the meter meteorological tower, soil temperature and solar time at which they are generated until the arrival of a radiation sensors; and established agreements with the following aircraft, 3) feedback from a sensor system to prime aircraft operators at Memphis to supply aircraft support the vortex motion and decay predictive landing weight data. An existing FAA/Lincoln facility capability, and 4) criteria for determining when a wake was used by Lincoln to collect aircraft beacon and is not a concern on the approach path. Although flight plan data to identify each aircraft passing the research in each of these areas, or subsystems, will be lidar. described separately, the results of the research will be The National Oceanic and Atmospheric integrated in the AVOSS system. Administration (NOAA) contributed by acquiring and deploying a radar profiler and radio acoustic sounding Numerical Fluid Dynamics/Atmospheric Modeling system as well as an acoustic sodar at Memphis in The NASA Langley numerical modeling effort is 1994. Volpe National Transportation System Center expected to provide key support in the development of provided an acoustic sodar and the deployment of a line the AVOSS system. The Terminal Area Simulation of anemometers (wind line) in 1995. Lincoln System (TASS), a multi-dimensional large-eddy conducted dedicated rawinsonde balloon launches weather model by Proctor5,6, and by Proctor and during the observations and NASA Langley deployed Bowles7, has been adapted to accomplish this goal. The an OV-10A aircraft to measure atmospheric conditions TASS model was highly successful in the completed at spatially remote locations from the meteorological NASA-FAA wind shear program as described by site at the airport. The resulting system provided the Proctor8; and by Proctor9 et al., and more recently has most complete facility to date to simultaneously collect been modified to include initialization routines for post wake vortex data, meteorological data, and aircraft data roll-up simulations of aircraft wakes. The model in an operational setting. The deployments conducted includes the effects of turbulence and ground stresses, are described in more detail by Campbell12 et al., Dasey and is capable of simulating wake vortices within a and Heinricks13, and Campbell14 et al. The data and wide range of atmospheric environments that may operational experience gained at Memphis in 1994 and include vertical wind shear, stratification, fog, and 1995 will be utilized to complete validation of numeric precipitation. Other recent modifications to the initial wake simulation models, to develop improved wake and boundary conditions now permit simulations of the prediction algorithms, and to begin the engineering and Earth's planetary boundary layer, and including software build of an engineering model AVOSS in the thermals and turbulence eddies as described by 1997 through 2000 year time frame. Schowalter10 et al., and by Lin11 et al. This is a first step As in past studies conducted with the TASS model, for investigating the interaction between three- it is very important that all aspects of the model dimensional wakes and atmospheric turbulence. The simulations be compared and validated with observed ultimate goal is to inject three-dimensional wakes into data. For the planetary boundary layer simulations, realistic boundary layers accompanied by a spectrum of mean and turbulence flux profiles are being compared scales for ambient turbulence. This will allow a full against those measured in field studies. Results to date understanding of how eddy scales effect wake transport have been very encouraging. Validation of the wake and decay, for example, the determination of which vortex simulations are well underway as well. scales of turbulence act to decay wake vortices versus Recently, promising results have been obtained from those that are most responsible for transport. Output comparing the two-dimensional version of TASS with from the TASS modeling effort will be used for wake measured field data from the 1990 FAA Idaho Falls and vortex characterization and for development of AVOSS the 1994 and 1995 Memphis field experiments, as predictive algorithms. described by Proctor15. 55 American Institute of Aeronautics and Astronautics Wake Vortex Hazard Definition vortex encounter simulator which can be used to assist A wake vortex that moves out of the traffic in the model selection process. The encounter corridor by the time the next aircraft passes through is simulation will employ results from NASA wind tunnel not a hazard to that aircraft. However, if the wake and flight experiments designed to examine the effects stalls in the traffic corridor or moves very slowly, the of wakes on representative aircraft configurations. AVOSS must determine if the next aircraft can safely The second major issue in defining an acceptable and satisfactorily complete its intended operation, even wake encounter is the determination of the metric for if it encounters the wake. This determination follows satisfactory terminal area operations. In the 1970’s and the notion that a perfectly acceptable terminal area 1980’s, several key piloted simulation studies were operation may be completed with a wake in the traffic conducted by Sammonds16,17,18 et al., Jenkins and corridor, e.g., for a “heavy” aircraft following a Hackett19, and by Hastings and Keyser20 to obtain an business jet or when conditions or time increments estimate of the magnitude of vortex-induced motions allow a wake to decay to satisfactory levels. For these that would be acceptable near the ground. These cases, the AVOSS will determine the correct simulations provided data on the wake vortex hazard longitudinal spacing for the specific leader/follower perceived by pilots with repeatable encounter aircraft pair based upon the required characteristics of conditions, and have provided preliminary guidance the wake for satisfactory passage through it. into suitable metrics for satisfactory operation that can The adequate definition of an acceptable wake be used for a wake vortex hazard definition. In these encounter requires that three key issues be addressed as studies, the separation of occurrences into hazardous depicted in Figure 3. First, the effect of the wake on and non-hazardous categories was found to correlate the encountering aircraft’s trajectory must be with maximum roll or bank angle, proximity to the determined by developing and validating aircraft/vortex ground, vortex strength, and the encounter geometry. interaction simulator models. Second, the metrics However, there is no similar correlation on the metrics characterizing a satisfactory terminal area operation, which would define a satisfactory encounter, which regardless of whether a wake is present in the traffic may not be quite the same as those for a non-hazardous corridor, must be defined in order to determine if the encounter and may also include consideration of, for resultant wake encounter trajectory is acceptable. example, passenger comfort, chance of missed Finally, for the AVOSS system to realize its full approach, and runway occupancy times as additional potential, a method must be devised for applying these factors. One could conclude that if the metrics for a operational metrics and validated models to the entire satisfactory terminal area operation could be defined, commercial aircraft fleet. regardless of the cause of an upset or deviation, then the definition of whether a wake encounter is satisfactory will directly follow, given an assumed APPLICATION METRIC VALIDATION model of the encounter dynamics, pilot, and autopilot. Candidate metrics for satisfactory terminal area Aircraft/Wake operations could include limits on airplane attitude, Application Interaction to Fleet Landing angular rates, flight path deviations and the amount of Criteria control required to correct them, touchdown requirements, and ride quality. Some of these limits can be derived from the FAA certification regulations, from manufacturer’s aircraft limitations, and other sources to augment the first-order estimates from the Resulting previous piloted simulations. However, many of these Trajectories limits must come from subjective opinion, which necessitates a strong consensus among a team Figure 3. Major Hazard Definition Issues comprised, in part, of the operators, the regulators, the manufacturers, and government research agencies. Presently, efforts are underway to gather the To address the aircraft/wake vortex encounter information needed to define an initial candidate issue, several experimental and analytical efforts are operational metric using the results of the previous being conducted to provide a basis for the selection of a simulation studies, the certification requirements, and valid aircraft/wake interaction model. Key to these opinions of experienced airline pilots. efforts is the development and validation of a wake Government/industry interaction will be used to address 66 American Institute of Aeronautics and Astronautics the long-term consensus of a suitable metric, quite proven to be valuable through many years of FAA possibly involving additional simulation efforts to wake vortex research. However, these sensors at refine and augment the first-order estimates. present do not have the capability to operate in The final hazard definition issue addresses how precipitation and have limited range and tracking these wake encounter and terminal area operational capabilities. Lidar has the potential for increased range metrics are applied to the whole fleet. An initial and tracking capabilities, and has demonstrated the assessment was provided by Stuever and Greene21. The capability to detect and measure vortex velocity fields solution to this problem will likely depend on several in clear air. However, lidar capability in rain and fog analytical techniques which consider the "correctness" has not been demonstrated. Finally, radar has the of a given aircraft model for the size/weight of aircraft potential for all weather capability, but has not yet it represents, the accuracy of the predicted encounter demonstrated the capability to detect and track vortices dynamics, and the likelihood that the severity of a using radar systems that have reasonable power outputs representative encounter will approximate the worst and antenna sizes. To address all these wake vortex case for a given initial condition. sensor technologies, NASA is planning to conduct a The constitution of the fleet will need to be series of field experiments in conjunction with the defined and then a representative sample of that fleet Volpe National Transportation System Center at the will need to be agreed upon through John F. Kennedy International Airport beginning in late government/industry interaction. From that sample, a 1996. data base of suitable fidelity for those representative Based on the state of readiness and potential of the aircraft will be developed and used. Presently, a data sensor technologies mentioned above, the NASA base of some 67 aircraft has been prepared at NASA Langley wake vortex sensor research is presently Langley which will be used to establish the fleet concentrating on both lidar and radar technologies. representation for follow-on work. Next, the issue of NASA is working with Coherent Technologies, errors in the data base parameters, differences in data Inc. through a small business innovative research base parameters among representative aircraft of about contract to develop a 2 micron diode-pumped Doppler the same size, and even sensitivity to metrics must be lidar. Coherent Technologies, Inc. has previous addressed to determine how robust the hazard experience in developing a 5 pulse per second, 2 definition must be. A simulation capability is being micron flash lamp-pumped lidar system as reported by implemented at NASA Langley to evaluate these Hannon and Thomson23. This flash lamp-pumped effects as described by Reimer and Vicroy22. Given the system has successfully detected and measured vortex metrics of allowable path deviation and bank angle for velocity fields in a number of different locations. The 2 a satisfactory encounter, very small differences in micron diode-pumped system will have a greatly allowable vortex-induced bank angle can give increased pulse repetition frequency and should provide significant differences in the strength of a vortex that increased detection area coverage, better capability to can be encountered. detect in low signal to noise conditions, and increased definition of velocity fields in the vortices. Wake Vortex Sensors In order to determine the expected performance of The requirements for a vortex sensor subsystem are the experimental system and to assist in the system not completely defined at present because they are development, extensive computer simulations have driven by current technology limitations. There are been conducted. Simulation runs to date have predicted four broad ranges of technologies under consideration: that the system will successfully detect, track and mechanical sensors (such as propeller anemometers), quantify vortices in clear air at sensor offset distances acoustic sensors, lidar, and radar. At the present time, of 1 to 3 kilometers. Simulation of detection sensitivity none of these sensors have demonstrated all of the during inclement weather and fog are presently characteristics desired for the vortex sensor subsystem. underway. Preliminary results indicate that detection Mechanical anemometers are used routinely in all range during rainy conditions should be approximately weather conditions for a variety of applications. They equal to the ambient visibility. Detection range in fog are currently being used as wake vortex position is more difficult to predict and will be measured during sensors in field tests being conducted by the FAA. Due actual field tests. Field test results will be used to to their technical maturity, low cost, and reliability, modify the lidar simulation as necessary to obtain these sensors are candidates for use as part of a sensor correlation with actual lidar performance in the field. package but cannot meet all of the current AVOSS Initial efforts toward the development of a radar sensor requirements. Acoustic sensors have also been sensor for wake vortices were motivated by the desire 77 American Institute of Aeronautics and Astronautics for all-weather capability. It was determined that vortex behavior, quantification of aircraft/wake before attempting an all-weather design, models for encounter effects (hazard definition) wake vortex estimating the clear air reflectivity of vortices would be sensor assessment and development, and field data developed and preliminary field experiments would be acquisition and systems deployment. The effort is performed to attempt clear air detection and to validate scheduled to culminate in a AVOSS system and calibrate the models. In the event that all-weather demonstration in the year 2000. capability is determined to be unreasonable or unobtainable, a foul-weather capability could be REFERENCES developed which could complement a lidar sensor that might perform well in clear air, but have limitations in 1. Hinton, D. A., Aircraft Vortex Spacing System adverse weather such as fog and rain. (AVOSS) Conceptual Design. NASA TM-110184, Preliminary reflectivity models concentrated on August 1995. Bragg scattering due to turbulent eddies with a diameter 2. Eberle, W. R., Brashears, M. R., Zalay, A. D., of one half the radar signal wavelength. These initial Shrider, K. R., and Love, D. A., Aircraft Wake Vortex reflectivity models indicated the potential for useful Characteristics from Data Measured at John F. reflectivity at microwave frequencies up to and Kennedy International Airport: Lockheed Missiles and including X-band. Previous field experiments by Space Co. FAA-RD-78-47, March 1978. Chadwick24, Nespor25, and Gilson26, which detected 3. Erzberger, H., Davis, T. J., and Green, S., Design of vortices in clear air at wavelengths from 3 m to 5 cm, Center-TRACON Automation System. Presented at the appeared to support these models. Based on these AGARD Guidance and Control Symposium on results, target of opportunity field experiments were Machine Intelligence in Air Traffic Management, conducted at the NASA Wallops Flight Facility and Berlin, Germany, 11-14 May, 1993. White Sands Missile Range during 1995 using a C-130 4. Hinton, D. A., An Aircraft Vortex Spacing System airplane, and the data are currently being analyzed. (AVOSS) For Dynamical Wake Vortex Spacing Presently, planning is underway for the Criteria. Presented at the AGARD 78th Fluid development of a millimeter wave radar system to be Dynamics Panel Meeting and Symposium, Trondheim, available for initial field testing in 1998. This system is Norway, 20-23 May 1996. targeting operational capability in rain and fog 5. Proctor, F. H., The Terminal Area Simulation System conditions where many wake vortex sensors tested to Vol. I: Theoretical Formulation. NASA CR 4046: date have not been able to provide adequate wake DOT/FAA/PM-86/50, I., April 1987. detection. 6. Proctor, F. H., The Terminal Area Simulation System SUMMARY Vol. II: Verification Cases. NASA CR 4047: DOT/FAA/PM-86/50, II., April 1987. A concept and research program has been 7. Proctor, F. H., and Bowles, R. L., “Three- presented for the development and implementation of a Dimensional Simulation of the Denver 11 July 1988 prototype AVOSS. The AVOSS development is being Microburst-Producing Storm,” Meteorol. and Atmos. conducted by the RSO subelement of the NASA Phys., 49, 107-124, 1992. Terminal Area Productivity Program. The purpose of 8. Proctor, F. H., “Numerical Simulations of an the AVOSS is to use current and nowcasting Isolated Microburst, Part II: Sensitivity Experiments,” predictions of the atmospheric state in approach and J. Atmo. Sci., 46, 2143-2165, 1989. departure corridors to provide, to ATC facilities, 9. Proctor, F. H., Bracalente, E. M., Harrah, S. D., dynamical weather dependent separation criteria with Switzer, G. F., and Britt, C. L., Simulation of the 1994 adequate stability and lead time for use in establishing Charlotte Microburst with Look-Ahead Windshear arrival scheduling by CTAS. The AVOSS will Radar. Preprints, 27th Conference on Radar accomplish this task through a combination of wake Meteorology, Vail, CO, Amer. Meteor. Soc. Paper: vortex transport and decay predictions, weather state 10B.7, 530-532, 1995. knowledge, defined aircraft operational procedures and 10. Schowalter, D. G., DeCroix, D. S., Lin, Y. L., corridors, and wake vortex safety sensors. Work is Proctor, F. H., Arya, S. P., and Kaplan, M. L., presently underway to address the critical disciplines Turbulent Statistics in the Atmospheric Boundary and knowledge requirements necessary to implement Layer: a Comparison of Large Eddy Simulation with AVOSS. These disciplines include short term Observations. Preprints, 11th Symposium on Boundary atmospheric state prediction (nowcasting), validated Layers and Turbulence, Charlotte, NC, Amer. Meteor. computational fluid dynamic simulations of wake Soc., 552-555, 1995. 88 American Institute of Aeronautics and Astronautics 11. Lin, Y. L., Arya, S. P., Kaplan, M. L., and Hazard to Aircraft Response,” Journal of Aircraft, Vol. Schowalter, D. G., Numerical Modeling Studies of 14, No. 10, pp. 981-987, October 1977. Wake Vortex Transport and Evolution within the 19. Jenkins, M. W. M., and Hackett, J. E, A Pilot-in- Planetary Boundary Layer. NASA Grant NCC-1-188, the-Loop, Visual Simulation of Trailing Vortex FY95 Annual Report, NASA Langley Research Center, Encounters at Low Speed. AIAA 75-104, January Hampton, VA, 1995. 1975. 12. Campbell, S. D., Dasey, T., Heinricks, R., and 20. Hastings, E. C., and Keyser, G. L., Jr., Simulator Matthews, M., Overview of 1994 Memphis Wake Vortex Study of Vortex Encounters by a Twin-Engine, Testing Program. Presented at the Sixth Conference on Commercial Jet Transport Airplane. NASA TP 1966, Aviation Weather Systems, Dallas, TX, January 15-20, February 1982. 1995. 21. Stuever, R. A. and Greene, G. C., An Analysis of 13. Dasey, T., and Heinricks, R., An Algorithm for the Relative Wake Vortex Hazards for Typical Transport Recognition and Tracking of Aircraft Wake Vortices Aircraft. AIAA 94-0810, January 1994. with a Continuous Wave Coherent Laser Radar. 22. Reimer, H. M., and Vicroy, D. D., A Preliminary Presented at the Optical Society of America Conference Study of a Wake Encounter Hazard Boundary for a on Coherent Laser Radar, Keystone, CO, July 23-27, B737-100 Airplane. NASA TM 110223, December 1995. 1995. 14. Campbell, S., Dasey, T., Freehart, R., Heinricks, 23. Hannon, S. M., and Thomson, J. A., “Aircraft R., Matthews, M., Perras, G., Wake Vortex Field Wake Vortex Detection and Measurement with Pulse Measurement Program at Memphis, TN. Presented at Solid-state Coherent Laser Radar,” Journal of Modern the 34th Aerospace Sciences Meeting and Exhibit, Optics, November 1, 1994. Reno, NV, AIAA 96-0399, January 15-18, 1996. 24. Chadwick, R. B., Jordan, J., and Detman, T., Radar 15. Proctor, F. H., Numerical Simulation of Wake Detection of Wingtip Vortices. Presented at the Ninth Vortices Measured During the Idaho Falls and Conference on Aerospace and Aeronautical Memphis Field Programs. Presented at the 14th AIAA Meteorology, AMS, June 1983. Applied Aerodynamics Conference, New Orleans, LA, 25. Nespor, J. D., Hudson, B., Stegal, R. L., and AIAA 96-2496, June 17-20, 1996. Freedman, J. E., Doppler Radar Detection of Vortex 16. Sammonds, R. I., Stinnett, G. W. Jr., and Larson, Hazard Indicators. Proceedings of the Aircraft Wake W. E., Hazard Criteria for Wake Vortex Encounters. Vortices Conference, DOT, Washington, D.C., October NASA TM-X-62473, August 1975. 1991. 17. Sammonds, R. I., Stinnett, G. W. Jr., and Larson, 26. Gilson, W. H., Radar Measurements of Aircraft W. E., Wake Vortex Encounter Hazard Criteria for Two Wakes. Project Report AAW-11, Lincoln Laboratory, Aircraft Classes. NASA TM-X-75113, June 1976. September 1992. 18. Sammonds, R. I., Stinnett, G. W. Jr., and Larson, W. E., “Criteria Relating Wake Vortex Encounter 99 American Institute of Aeronautics and Astronautics