When an aircraft wing generates lift, it also produces horizontal, tornado-like vortices that create a potential wake-vortex hazard problem for other aircraft trailing. The powerful, high-velocity airflows contained in the wake behind the generating aircraft are long-lived, invisible, and a serious threat to aircraft encountering the system, especially small general aviation aircraft. Immediately behind the wake-generating aircraft is a region of wake turbulence known as the roll-up region, where the character of the wake that is shed from individual components (wingtips, flaps, landing gear, etc.) is changing rapidly with distance because of self-induced distortions. Farther away from the generating aircraft is an area of the wake known as the plateau region, where the vortices have merged and/or attained a nearly constant structure. Even farther downstream from the generating aircraft is a wake area known as the decay region, where substantial diffusion and decay of the vortices occur due to viscous and turbulence effects. Depending on the relative flight path of a trailing aircraft in the wake-vortex system, extreme excursions in rolling motion, rate of climb, or even structural load factors may be experienced during an encounter with the wake. If the encounter occurs at low altitudes, especially during the landing approach, loss of control and ground impact may occur. The severity of this wake-vortex hazard is mainly dependent on the size, geometry, and operating conditions of the generating and trailing aircraft; the distance between the two aircraft; the angle and altitude of the encounter; and local atmospheric conditions that influence the position, strength, merging, and decay of the vortices. In general, a pair of vortices drift downward with time behind the generating aircraft, and the strategy recommended to the pilot for avoiding vortex encounters is for the trailing aircraft to fly at altitudes equal to or above that of the flight path of the preceding aircraft. However, on many occasions (particularly near the ground), the vortices may persist at the generated altitude or even rise to a slightly higher altitude because of atmospheric conditions. If the vortices reach the ground, they typically move outward from the aircraft at a speed of about 2 to 3 knots in calm-wind conditions. However, if there is an ambient wind, then the net movement of the vortices is the sum of the ambient wind velocity and the no-wind motion of each vortex. A light crosswind can cause one vortex to remain nearly stationary over the runway, which will continue to pose a threat to the landing aircraft. Finally, operations from parallel runways with less than 2,500 ft separation require alertness for crosswinds that may push vortices onto the active runway. Because of these complex factors, the fundamental behavior of wake vortices and their avoidance have been especially challenging problems to the aviation community since the earliest days of flight.
Under visual flight rules (VFR) operations, the responsibility for aircraft separation distances may be given to the pilot during the approach phase. In this situation, the primary constraint on following distance is usually the time interval for the leading aircraft to clear the runway prior to the landing of the following aircraft. However, under instrument flight rules (IFR) conditions, air traffic control has direct responsibility for separation according to FAA-mandated standards that are a function of the weight classifications of the leading and trailing aircraft. A more complete discussion of the separation standards is included in a later section. An analysis of aviation accidents indicates that probable vortex-related accidents constitute a relatively small percentage of all single aircraft accidents and that the vortex safety problem has been largely confined to general aviation aircraft (including business jets) operating under VFR conditions. In addition, the most frequent cause of vortex-related accidents involves an aircraft landing behind another aircraft on the same runway; the takeoff condition has been virtually free of vortex accidents. Perhaps the most important observation is that no accidents under IFR conditions have happened when full FAA separations were provided between aircraft. Prime reasons for the extremely small accident rate due to wake-vortex encounters are the IFR separation standards and the increasing awareness of the wake vortex problem on the part of operational personnel for both VFR and IFR conditions.
Sketch of wake characteristics behind generating aircraft.
Different vortex hazards
created by relative flight
Sequence of photographs
of Boeing 747 on landing approach as industrial
Wake-vortex separation requirements
under instrument flight
Although separations standards have proven to be effective from a safety point of view, they are frequently well in excess of the spacing required (due to weather conditions that rapidly decay or drift the wakes) and, as a result, significantly reduce airport operating capacities and impose costly delays affecting the airlines and the general public. With the projected accelerated growth in air traffic operating from essentially the same number of airports in the future, these penalties will become extremely large. Some quantitative measure of the penalties imposed by wake-vortex separation distances can be obtained by considering an example case of a runway operating with a 3-nmi IFR radar spacing and a full capacity of 30 operations/hr. If the separation between aircraft using the runway is increased to 5 nmi, then capacity would be cut by a third. If the separation is increased to 7 nmi, capacity would be cut in half. Obviously, the operating costs to airlines and passengers are severely impacted.
In 1970 NASA Marshall Space
Flight Center conducted flyby studies of vortex-wake
behavior with smoke
Thus, strong incentives exist to remove wake-vortex turbulence as an impediment to air traffic operations at and around airports while retaining current levels of safety. Historically, two approaches have been used by researchers in attempts to mitigate the vortex problem. One approach has been to attempt to alter the aerodynamic vortex pattern shed by the generating aircraft so that its effects on other aircraft would be minimal. The second approach has been to develop and install wake-vortex detection and avoidance systems that would increase runway capacity by varying the separation distance to conform to the aircraft and the meteorological conditions present. A complete vortex separation system of this type must consider many factors, such as the detection or prediction of the presence and strength of the vortices at a given time; an evaluation of the threat on the basis of vortex strength, location, and trailing aircraft characteristics; and the determination of the proper hazard avoidance action.
Langley Research and Development Activities
The NASA Langley Research Center has actively pursued research to mitigate the wake-vortex hazard for over 45 years, beginning with flight tests in 1955 led by Christopher C. Kraft, Jr., to measure the wake-velocity characteristics of a P-51 aircraft. In 1968, Langley participated in a brief exploratory program to probe the vortices of an FAA Convair 880 transport using a T-33 aircraft. After the emergence of the first jumbo-sized transports, the FAA requested NASA’s assistance in 1969 to determine the wake characteristics of large aircraft; this resulted in flight tests of a B-52 and C-5 aircraft by the Dryden Flight Research Center. Impressed with the potential wake hazard of these large aircraft, the FAA issued (January 1970) interim IFR separation standards with a minimum trailing distance of 10 miles for aircraft behind the C-5 or Boeing 747. After additional NASA and FAA flights of the C-5, Boeing flights of the 747 and 720, and flyby studies by the FAA, the FAA revised the separation standards to 5 mi behind aircraft with gross takeoff weights of over 300,000 lb. At the same time, Langley researchers Harry A. Verstynen, Jr., and R. Earl Dunham, Jr., participated in flight tests using a T-33 to measure the velocity profile of the wake of a C-5 aircraft. Perhaps the most interesting result of the study was the fact that, under the atmospheric conditions of the test site (Wright Patterson Air Force Base), the vortices often could be found above the flight path of the C-5. This early preview of the powerful influence of meteorological conditions on vortex behavior highlighted one of the most complex factors associated with the vortex hazard.
The focus of Langley’s research in the early 1970s was to alter and minimize the aerodynamic wake-vortex characteristics of generating aircraft. In the 1980s, the studies were redirected to emphasize the fundamental character of wake-vortex phenomena and studies of the physics involved in the formulation of separation standards. The most recent focus in the 1990s has been the integration of advanced meteorological and vortex hazard technologies and sensors to permit the development of an integrated system for reduced separation for increased capacity. One of the most significant contributions by Langley researchers and their partners to the national air transportation system of the 1990s was joint activities with the FAA, which led to the quantification and development of separation standards and an emerging automated approach to spacing requirements.
NASA Wake-Vortex-Alleviation Program
In the summer of 1972, faced with what was a growing concern over the potential impact of large aircraft on the safety of small aircraft operations and the explosive growth of air traffic, the FAA requested NASA’s help to develop technology and design information that might be used to alter the aerodynamic characteristics of the vortex pattern of generating aircraft so that the intensity of wake-vortex encounters might be minimized. At the same time, the FAA (with some help from NASA) focused on the development and installation of wake-vortex detection and avoidance sensors and systems at airports. NASA responded to the FAA’s request, and research activities began immediately at the Langley Research Center, the Ames Research Center, and the Dryden Flight Research Center. The coordinated research program performed at the centers investigated the effectiveness of a myriad of aerodynamic schemes such as spoilers, vortex generators, wingtip vortex-attenuating devices, steady and pulsed mass injection, oscillating control inputs, and span load variations. The effectiveness of the schemes was assessed by measurements on trailing aircraft configurations and visualized with various flow visualization techniques. Tests were completed in several unique NASA facilities and several contractor water channels, as well as actual aircraft flight tests. A complete discussion of these extensive studies far exceeds the intended scope of this publication. Thus, the included information is restricted to only highlights of the program, and the reader is referred to the bibliography for sources of more detailed information.
At Langley, Joseph W. Stickle led the Center’s Wake-Vortex-Alleviation Program, with several teams conducting the effort by using various ground-based facilities and aircraft flight tests. In addition to a broad experimental research program, analytical studies of vortex viscous effects and interactions were conducted by industry and academia under Langley contracts. Noteworthy contributions in the analytical effort were contributed by Alan J. Bilanin and Coleman duP. Donaldson of Aeronautical Research Associates of Princeton, Inc.
An important initial task of Langley’s research program on wake-vortex alleviation was to assess the many devices and concepts that had been proposed to aerodynamically alter vortex formation and decay. In accomplishing this objective, Langley had to develop facility test capabilities that could permit studies of the characteristics of the wake-vortex system from the point of generation to locations far downstream, representing in scale dimensions the downstream distances of interest for trailing aircraft. The testing problems, measurement techniques, and scaling of Reynolds number and viscosity effects were predominant issues in the program. Langley’s ground-based facilities included the Langley 14- by 22-Foot Tunnel (formerly the Langley V/STOL Tunnel and the Langley 4- by 7-Meter Tunnel) and the Langley Vortex Research Facility (VRF), which was a new facility derived from an inactive NACA towing basin. The 1,800-ft long, water-filled towing basin had been modified with a new overhead carriage system to propel models, whereas observations and measurements of the wake characteristics of the passing model were made at a fixed observation position. In conjunction with complementary facility tests at Ames and the Tracor Hydronautics Ship Model Basin at Laurel, Maryland, these Langley facilities carried the load of Langley’s ground tests. Researchers at Ames and Langley agreed to use similar 0.03-scale models of the Boeing 747 for common representation of a generating aircraft, and trailing aircraft models used in the studies ranged from simple wing models to representative business jet aircraft. Other configurations of interest including the Lockheed L-1011 and the Douglas DC-10 were also tested.
Sketch of Langley Vortex Research Facility.
Model of Boeing 747 during
wake-vortex testing in Langley 14- by 22-Foot Tunnel
In the 14- by 22-Foot Tunnel, the test setup consisted of a generating transport model mounted to a static force test sting-strut apparatus in the tunnel test section, and the trailing model was mounted on a special strut-traverse mechanism that could be mounted at various distances downstream in the tunnel test section or farther downstream in the tunnel diffuser section. In this approach, the model could be remotely moved about 6 ft laterally and vertically to permitting researchers to probe the strength of the trailing vortices shed by the generating model. With the use of flow visualization techniques, such as smoke and neutrally buoyant hydrogen soap bubbles, positioning the model into specific areas of the wake-vortex pattern behind the generating model was possible. The magnitude of rolling moment imposed on the trailing model by the generating model was measured with a strain-gauge balance and analyzed for various generator configurations.
The Langley Vortex Research Facility was a unique approach to wake-vortex research in which the impact of the wake shed by a moving aircraft model was measured on a moving trailing model and observed and measured (by laser velocimetry) at a fixed observation point in the ambient air of the facility as time progressed following the passage of the model. James C. Patterson, Jr., conceived and led the development and operational research for the facility. A gasoline-powered automobile carriage was mounted on an overhead track with the vortex-generating model blade-mounted beneath the carriage. The trailing model was also attached to the carriage through a series of trailers, which resulted in the trailing model being located at a scale distance of about 1 mile downstream of the generating model. After the carriage was launched, the automotive drive system accelerated to a velocity of about 100 fps, which was held constant by cruise control throughout the length of the covered test area. At the test position, inside the covered area, smoke produced by vaporized kerosene was deployed and entrained by the wake for flow visualization. High-speed motion-picture cameras were used to film the motion of vortices produced by the generating model, and the aerodynamic forces experienced by the model were recorded.
Highlights of Wake-Vortex-Alleviation Research
The scope of Langley’s Wake-Vortex-Alleviation Research Program included ground-based subscale model tests, theoretical studies, and full-scale aircraft flight tests to identify concepts and techniques that would reduce the rolling motion imparted to a smaller trailing aircraft. The investigation of a particular concept usually began with a preliminary evaluation through flow visualization of the wake-vortex pattern, with and without the vortex-alleviation concept. If the initial flow visualization indicated a change in the vortex structure, a quantitative assessment of the effectiveness was undertaken, either through detailed velocity measurements or through measurements of the vortex-induced rolling moment imposed on a trailing-wing model. Many in the international research community believed it would be impossible to alter the wake, whereas others believed that the task could be easily accomplished.
Flow visualization of wake
of Boeing 747 model with inboard wing flaps deflected
in Langley Vortex
During the course of the program, several concepts were identified that altered the wake and reduced the upset on a trailing aircraft. For example, the injection of turbulence into the vortex field was found to alter the vortex structure and cause premature aging and dissipation of the trailing vortices. Turbulence from jet engines also changed the vortex structure, as did an alteration of the span-load distribution. The combined effects of turbulence injection and span-load alteration through the use of wing spoilers were also found to be effective in altering the wake. Even oscillating inputs to the wing control surfaces proved effective.
Langley researchers were constantly challenged by the complexity of the wake flow field for representative transports. Many concepts that appeared to affect the wake properties in the immediate roll-up area behind the generating aircraft were found to have little impact on the magnitude of roll upset at downstream distances representative of the location of trailing aircraft. Furthermore, it was found that numerous interacting vortices were shed by the typical transport in the landing configuration. For example, in addition to the vortices expected at the wingtips, strong vortices were also shed at the edges of wing trailing-edge flaps, and aft fuselage. As a result of these types of interactive vortex effects, some wingtip vortex control concepts that were known to provide beneficial effects for cruise drag (such as winglets) had little or no effect on the wake vortex hazard when the aircraft was in the flaps-down, landing approach configuration.
As previously mentioned, the program was closely coordinated such that common concepts were cross tested in different facilities for correlation and verification of effectiveness; however, certain researchers focused on particular concepts in their studies. An excellent summary of the results of both successful and unsuccessful vortex alleviation concepts by the Langley staff is given in NASA SP-409, Wake Vortex Minimization (see bibliography).
Research in the VRF was led by James C. Patterson, Jr., with assistance from by Frank L. Jordan, Jr. Both researchers had worked under the supervision of Richard T. Whitcomb in the Langley 8-Foot Transonic Pressure Tunnel, and were familiar with Whitcomb’s interest in controlling the wingtip vortex with drag-reduction wingtip concepts, such as winglets. Patterson and Jordan investigated a wide range of alleviation concepts in the VRF, which included the potential of utilizing the high-energy wake produced by the large jet engines incorporated on wide-body transports for vortex alleviation. Interest in this concept was stimulated by earlier work that indicated forcing a mass of air forward into a vortex would interrupt the vortex axial flow (which provides the energy that normally sustains the vortex long after its generation). Because the jet exhaust wake of large engines is a source of high energy, it was hypothesized that when this energy was directed into the vortex, the wake hazard might be mitigated. After considerable research on engine location effects, thrust reversers, and differential engine thrust, it was determined that operating the outboard engines at maximum thrust while operating the inboard engine thrust reversers at idle thrust resulted in a significant reduction in the roll upset of the trailing aircraft. Although considered not practical from an operational viewpoint, these positive results provided additional incentive for further research.
Laser beams used to measure air velocities at specific points in wake for Boeing 747 model in VRF.
Langley’s early flight test activity in the aircraft wake-vortex minimization program was led by Joseph W. Stickle, Earl C. Hastings, Jr., and James C. Patterson, Jr. In one Langley flight project led by Hastings, Robert E. Shanks, and test pilot Robert A. Champine, a vortex-attenuating device referred to as a “spline” was developed from ground-based testing and assessed during flight tests of a C-54 propeller-driven transport. Early research leading to the spline concept had been conducted by Patterson in the VRF with a wing panel. It was proposed that an unfavorable or positive pressure gradient applied just downstream of the wingtip might force the vortex to dissipate. In the initial testing, this mechanism was verified by the brute-force approach of utilizing a decelerating parachute at each wingtip. As a more practical application of the idea, the spline concept was tested and found to produce the same vortex-attenuating effect as the decelerating chute. The spline configuration was envisioned to be retracted during cruise flight and deployed only for landing, when the vortex hazard was greatest. After extensive parameter variations in the VRF and in-flight assessments with the C-54 as a generating aircraft and a Piper PA-28 general aviation aircraft as a probe aircraft, the researchers concluded that the spline device was effective in reducing the strength of the trailing vortex. When the PA-28 approached behind the basic C-54 (no splines), the PA-28 could not approach closer than about 3 nmi before full aileron deflection was required to prevent a severe roll off. For the C-54 with splines, however, the PA-28 could be easily flown to a separation distance of less than 1 nmi. The vortex attenuation achieved in flight was greater than that obtained in the VRF, and the researchers anticipated that the installation of stowable spline devices on commercial jet aircraft would avoid the obvious penalties in climb capability in the landing phase of flight and increased approach noise due to increased power settings.
Langley’s evaluation of spline device on C-54. Top photographs show VRF results for basic (left) and modified model, showing diffuse wake with spline. Lower photographs show installation of splines on full-scale aircraft.
R. Earl Dunham, Jr., conducted cooperative studies with Vernon J. Rossow at the Ames Research Center on the effect of trailing-edge-flap settings on wake alleviation. These far-ranging tests included wind-tunnel studies at Ames and Langley, water tow experiments, and full-scale flight tests of a Boeing 747 at the Dryden Flight Research Center. The motivation for this project was to distribute the lift on the wing so that the interactions of wake vortices would lead to a very diffused wake. This approach was attractive because of the potential ease of application and minimal retrofit costs. The results of these extensive investigations demonstrated that variations in span-load distribution using flap deflections could produce significant reductions in the wake-vortex hazard. For example, an approximately 50-percent reduction in both the wake rolling moment imposed on a trailing aircraft and aircraft separation requirement was achieved in ground-based and flight experiments by deflecting the inboard trailing-edge flaps more than the outboard flaps.
NASA conducting flight
assessments of wake-alleviation concepts
Langley’s Delwin R. Croom led research activities in the Langley 14- by 22-Foot Tunnel. Yet another concept that significantly modified the wake of representative jet transport configurations was developed by Croom and the intercenter NASA team. The focus of Croom’s studies was the use of wing spoilers, commonly used by jet transports (for speed brakes and to decrease aerodynamic lift after landing), as a possible method of vortex attenuation, because the deflection of spoilers will inject turbulence into the wake as well as alter the span-load distribution. Croom had been inspired by earlier investigations conducted at Ames in 1970 that combined wind-tunnel and flight investigations of the effects of wingtip-mounted spoilers on the wake characteristics of a Convair 990 transport. The Ames experiment, however, ended with reports from pilots of a Learjet probe aircraft citing no differences in wake behavior between the modified and unmodified transport. In 1971, Langley initiated more detailed semispan wing studies in the VRF to determine the proper location for a spoiler to cause the largest alteration to the trailing vortex. Testing then shifted to the 14- by 22-Foot Tunnel and the Tracor Hydronautics Ship Model Basin, with emphasis on the impact of deflecting various combinations of the wing spoilers available on the Boeing 747 configuration. Exploratory testing by Croom in the 14- by 22-Foot Tunnel, which began in March 1975, defined an effective spoiler configuration, which had a spoiler deflection of about 30∞. The effectiveness of the spoiler concept was verified in the VRF and the Hydronautics facility. With these promising results, a flight program using a NASA Boeing 747 aircraft was initiated at Dryden. In the flight program, NASA T-37 and Learjet aircraft were used to penetrate the trailing vortex.
Tests without the wing spoilers of the Boeing 747 deployed produced violent roll upset problems for the T-37 aircraft at a distance of approximately 3 miles. In one instance, the wake of the 747 caused the T-37 to perform two unplanned snap rolls and develop a roll rate of 200 deg/sec, despite trailing the jetliner by more than 3 miles. Tests showed the rotational velocity of the 747 wake vortex could exceed 240 km/hr and persist for a distance of 30 km. With two spoilers on the outer wing panels deflected, the T-37 could fly within a distance of 3 miles and not experience the upset problem. Although initial flight results tended to verify the effectiveness of the spoiler concept, additional flight tests indicated that the effectiveness was sometimes not repeatable (probably because of atmospheric conditions) at low altitudes. Later, flight tests of a Lockheed L-1011 at Dryden indicated considerably less effectiveness of the spoiler concept. Finally, additional concerns over unacceptable buffet characteristics produced by the spoilers shelved further research on the concept.
At the conclusion of the NASA Wake-Vortex-Alleviation Program, Langley and its intercenter partners had conducted extensive ground-based and flight research for a myriad of alleviation concepts. These concepts included altered span loading, turbulence ingestion, mass ingestion, oscillating controls, and combinations of these approaches. Actual flight evaluations indicated that several aerodynamic attenuation concepts were effective and these concepts would probably be operationally practical. However, in addition to issues such as effects on buffet and aircraft controllability and costs of retrofit, a significant obstacle to the implementation of this technology remained unconquered: the tremendous impact of variations in meteorological conditions on wake persistence and decay characteristics. Because of these appropriate concerns, none of the promising wake-alleviation concepts identified by the NASA research was incorporated into civil aircraft of the 1990s. Nonetheless, an immense increase in knowledge of the nature and sensitivity of aircraft aerodynamic wake characteristics was obtained and served as the fundamental building block for future advanced approaches to the operational prediction, detection, and avoidance of the wake hazard. For an excellent summary of experiences and the outlook for wake alleviation concepts, the reader is referred to the excellent paper by Vernon J. Rossow in the bibliography.
As the funding and momentum of the NASA Wake-Vortex-Alleviation Program were dramatically reduced in the 1980s, the focus of wake-vortex research turned away from aerodynamic alleviation concepts. Instead, Langley researchers directed their efforts toward more fundamental studies of the impact of atmospheric conditions on wake-vortex formation and decay.
During the 1980s, George C. Greene, Dale R. Satran, G. Thomas Holbrook, and others from Langley began to reassess the impact of meteorological conditions on wake characteristics. Critical changes in vortex position, strength, and decay were analytically and experimentally determined as a result of atmospheric density stratification caused by temperature gradient effects and temperature inversions. Dramatic demonstrations of the impact of stratification in the VRF, together with earlier FAA-sponsored observations of vortex-wake descent and decay characteristics, and the experiences of the Wake-Vortex-Alleviation Program during flight tests at Dryden, provided new insight and sensitivity to the powerful influences of real atmospheric effects on the wake-vortex hazard problem.
Typical meandering of trailing vortices due to local atmospheric effects.
Aircraft Separation Standards
Before 1970, radar operating limits and, to a lesser extent, runway occupancy restrictions dictated aircraft separation standards—no regulatory aircraft separations were imposed because of wake vortices. Separation requirements for IFR conditions were established in 1970 after NASA, the FAA, industry, and others conducted flight tests to determine the wake-vortex characteristics of existing jet aircraft. Until March 1976, separation distances of 5 nmi were required for “nonheavy” aircraft (less than 300,000 lb) trailing heavy aircraft (greater than or equal to 300,000 lb), and separations of 3 nmi for all other conditions. In 1976, the distances were increased, with the maximum being 6 nmi for a “small” aircraft (less than 12,500 lb) trailing a heavy aircraft.
As the 1980s closed, new concerns arose regarding the wake-vortex characteristics of certain new transports and the spacing requirements for them. In particular, a series of wake-related accidents and incidents involving the Boeing 757 during landing approaches resulted in a concern over the wake characteristics of this particular transport. In one accident on December 18, 1992, a Cessna Citation crashed while on a VFR approach at the Billings Logan International Airport, Billings, Montana. The two crew members and six passengers were killed. Witnesses reported that the airplane suddenly and rapidly rolled left and then contacted the ground while in a near-vertical dive. Recorded ATC radar data showed that, at the point of upset, the Citation was about 2.8 nmi behind a Boeing 757 and on a flight path that was about 300 ft below the flight path of the 757. Then, on December 15, 1993, an Israel Aircraft Industries Westwind, operating at night, crashed while on a VFR approach to the John Wayne Airport, Santa Ana, California. The two crew members and three passengers were killed. Once again, witnesses reported that the airplane rolled abruptly and that the onset of the event was sudden. The Westwind was about 2.1 nmi behind a Boeing 757 and on a flight path that was about 400 ft below the flight path of the 757. An additional accident, involving a Cessna 182 during VFR conditions, resulted in loss of the aircraft but no fatalities. Additionally, significant but recoverable losses of control occurred for a McDonnell Douglas MD-88 and a Boeing 737 (both required immediate and aggressive flight control deflections by their flight crews) trailing Boeing 757 aircraft.
Although all the wake accidents had occurred during visual conditions, when pilots are responsible for wake turbulence avoidance, the NTSB sent an urgent recommendation to the FAA to increase the controller-imposed IFR landing separation distances behind the Boeing 757 and similar weight aircraft to 4 nmi from 3 nmi for the 737, MD-80, and DC-9; to 5 nmi from 3 nmi for aircraft such as the Westwind or Citation; and to 6 nmi from 4 nmi for small airplanes. By June 1994, the FAA had accepted some of the NTSB recommendations, and separation standards were modified August 1994. Meanwhile, the aviation community initiated several exercises to determine if the wake of the Boeing 757 was more hazardous than other transports.
Tower flyby tests of the Boeing 757 and 767 had been conducted at the NOAA vortex facility in Idaho Falls, Idaho, during 1990. The NOAA results proved to be controversial because they showed, for a peculiar set of weather conditions that lasted about 0.5 hour, the vortex velocity of the 757 was approximately 50 percent higher than that of the 767 at similar vortex ages (younger than 60 sec) measured in less favorable weather conditions. However, the results also showed that overall the wake of the 757 decayed faster than that of the 767; in fact, the wake behaved as would be expected for an aircraft of the size and weight of the 757. However, the single unusual measurement was widely quoted as showing that the 757 should be treated as a heavy category aircraft like the Boeing 747 and 767. Another factor cited as relevant to the 757 accidents was the approach speed of the aircraft (125 knots) is relatively slow, in part because of its relatively low wing sweep and large wing area. As a result, business jets (such as those involved in the accidents) approach at higher speeds with inadvertently close separation.
Current FAA standards for
aircraft separation during IFR conditions. Note
At the request of the FAA, Langley’s Roland L. Bowles, George C. Greene, and others participated in the analysis and deliberations over the Boeing 757 wake characteristics. A wake turbulence government and industry team, composed of representatives from the FAA, NASA, air carriers, pilots, air traffic controllers, and manufacturers, provided the FAA with recommendations on how to best separate aircraft to prevent wake turbulence incidents and accidents. A key analysis providing support to the reclassifications of weight was performed by Bowles and George Washington University (GWU) graduate student Chris Tatnall. Following hotly debated analyses and discussion among various agencies, industry, and the airlines, the FAA implemented new aircraft separation standards on August 17, 1996, for all aircraft operating in the United States under IFR conditions. Separation standards for small aircraft traveling behind a Boeing 757 increased from 4 to 5 nmi, and 57 types of aircraft, including several business jets and some smaller commercial aircraft, were moved from the large to small aircraft category. Specifically, the small category was changed to less than 41,000 lb (previously less than 12,500 lb); the large category, to 41,000 lb to 255,000 lb (previously 12,500 lb to 300,000 lb); and the heavy category, to 255,000 lb or more (previously 300,000 lb or more).
Controversy over the new standards existed, however, with objections to the increased distances. Opponents of the new regulations pointed out that all reported wake turbulence incidents or accidents had occurred in VFR conditions. There had been no reports of wake turbulence upset accidents (757 related or otherwise) from aircraft operating in IFR conditions; therefore, controversy still exists as to whether the modifications to IFR separation standards will prevent VFR accidents such as the Billings and Santa Ana crashes.
carried by Langley OV-10 research
As the separation issues spawned by the 757 controversy became a high-level concern, Langley researchers were stimulated to examine the more general subject of the development of a more scientific approach to determining separation standards. Extensive studies were required to provide the tools and understanding necessary to provide confidence if the separation standards were to be reduced for improved airport capacity. Efforts in the NASA Terminal Area Productivity Project within the NASA Advanced Subsonic Transport Program provided the impetus and funds for these contributions. Key capabilities in achieving the goals set for the program were the definition of valid wake models, analytical tools to examine the severity of encounters, and the development of a high-fidelity simulation model that could be used to develop wake vortex encounter hazard criteria. Langley’s R. Earl Dunham, Jr., Eric C. Stewart, Dan D. Vicroy, Robert A. Stuever, and George C. Greene contributed significant leadership during ground- and flight-testing activities that provided the foundation for the development of simulations (both piloted and unpiloted) for wake-vortex encounter analysis. In addition to this analytical work, a successful first-ever feasibility study was conducted by Jay M. Brandon, Frank L. Jordan, Jr., Catherine W. Buttrill, and Robert A. Stuever to determine if free-flying models in the Langley 30- by 60-Foot (Full-Scale) Tunnel could be used in the analysis of the vortex hazard.
In 1995 and 1997, Langley conducted flight tests of a modified North American Rockwell OV-10 research aircraft behind a C-130 at the NASA Wallops Flight Facility to generate a quantitative, detailed set of data on wake characteristics for use in the validation of simulators and wake prediction methods. A unique feature of this research was that atmospheric data were obtained along with the wake measurements. The OV-10, which was equipped with special instrumentation for atmospheric measurements, first probed the atmosphere and completed a “weather profile” run before joining with the C-130 and probing the wake of the C-130. About 230 wake penetrations at different atmospheric conditions were accomplished; this provided valuable data for wake characterization research.
Analytical studies of separation effects on upset parameters were conducted by Stewart and Stuever, as well as further improvements in the state of the art for piloted simulator studies of hazard criteria. The ultimate objective of this effort was to permit the definition of hazardous and nonhazardous levels of vortex encounters for various aircraft types and atmospheric conditions and pave the way for a predictive element that might be used in a real-time ATC system. Unfortunately, funding for the study was eliminated before the final objective could be accomplished.
During the 1990s, the Langley staff continued to participate in supporting national issues and safety investigations involving wake-vortex phenomena. After USAir Flight 427 (a Boeing 737) plunged from the sky near Pittsburgh on September 8, 1994, killing 127 passengers and 5 crew members, the NTSB frantically accelerated its efforts to determine what might have triggered the 6,000-ft nose dive. A bump (a sudden airspeed increase detected by the plane’s flight-data recorder) indicated that the 737 had encountered wake turbulence created by a Delta 727 that preceded Flight 427 into the Pittsburgh International Airport. Flight 427 trailed the 727 by 4.1 nmi, well within the FAA regulation that requires two planes of such weights to maintain a separation of 3 nmi. As part of the investigation to determine the potential impact of such an encounter, the NTSB requested that Langley conduct flights of its specially instrumented OV-10 and 737 research aircraft trailing an FAA 727 generating aircraft. Following the longest aviation accident investigation in safety board history (4 years), the results of this cooperative activity helped investigators conclude that the vortex encounter might have been the initiating mechanism resulting in a hardover failure of the rudder actuator, which was determined to be the primary cause of the accident.
Aircraft Vortex Spacing System Program
In the 1990s, the NASA Terminal Area Productivity (TAP) Program directed its resources toward the extremely challenging objective of providing the same levels of airport capacity during instrument operations that are presently experienced during visual airport operations. Within the elements of the TAP Program, the Langley Research Center was tasked to perform the research and development required to devise an automated wake vortex spacing system, known as the Aircraft Vortex Spacing System (AVOSS). The AVOSS concept would use available and emerging knowledge of aircraft wake generation, atmospheric modification of the wakes, wake encounter dynamics, and operational factors to provide dynamic spacing criteria for use by Air Traffic Control (ATC). When considering ambient weather conditions, the wake separation distances between aircraft could possibly be relaxed during appropriate periods of airport operations. With an appropriate interface to planned ATC automation, spacing could be tailored to specific generating/trailing aircraft types rather than the existing broad weight categories of aircraft. The fundamental architecture for the AVOSS system was first proposed by Roland L. Bowles of Langley, who had played a key role in the initiation and highly successful completion of the Langley Wind-Shear Program. The lead researcher and project manager for AVOSS was David A. Hinton, assisted by Leonard Credeur and an extraordinary team that included Fred H. Proctor and others that had made many contributions.
The development of the AVOSS concept built on past wake-vortex research conducted by NASA, the FAA, the Volpe National Transportation Systems Center, and industry. Advances in computational fluid dynamics modeling, weather sensors, ATC automation, and aircraft vortex behavior predictions had advanced to the point that encouraged the implementation of a practical AVOSS concept. The most critical single element in the development of the system was the accurate representation of vortex behavior in the airport area, especially the effects of meteorological characteristics. The AVOSS differed substantially from previous efforts to characterize wake vortex systems in that the atmospheric conditions from the surface to the top of the instrument approach path were measured and used rather than only surface winds. This analysis is significant in situations where temperature inversions or wind gradients along the approach path may require greater spacing intervals than would be predicted by surface winds alone. Because metering of aircraft to meet airport acceptance rates occurs during the vectoring and descent process as aircraft enter the initial approach area, the AVOSS system was required to provide a predictive capability of 30 to 50 min in advance of the actual approach to take full advantage of reduced wake constraints.
The general AVOSS structure designed by Langley includes a meteorological subsystem that provides current and expected atmospheric states to a predictor subsystem. The predictor subsystem utilizes the atmospheric data, airport configuration, and aircraft specifications to predict the separation time required for a matrix of aircraft. A sensor subsystem monitors actual wake-vortex position and strength to provide feedback to the predictor subsystem and to provide a warning. The AVOSS as demonstrated did not interact with ATC but actually ran in a “shadow mode.”
David A. Hinton’s conceptual design for AVOSS included a broad perspective of the research required in the areas of analytical studies, wind-tunnel testing, field evaluations, and flight tests. In the area of numeric wake vortex modeling, Fred H. Proctor’s Terminal Area Simulation System (TASS), which had proven highly effective in the successfully completed NASA-FAA windshear program, was modified to model the effects of various atmospheric conditions of the behavior of aircraft vortices. Crucial to the validation of TASS, prediction algorithm development, and full system testing and demonstration was a field effort sponsored by Langley and conducted by the MIT Lincoln Laboratory. This effort provided comprehensive field capability to gather meteorological, aircraft, and wake data at major airports. The Lincoln effort established a facility at the Memphis International Airport and in 1994 provided the most comprehensive wake-vortex and weather data obtained to date with approximately 600 aircraft wakes studied. Data collection was also performed in 1995. During these field measurements, the Langley OV-10 aircraft participated by collecting atmospheric wind, temperature, and humidity data along the approach path to answer questions concerning the variability of critical atmospheric parameters.
MIT Lincoln Laboratory continuous wave lidar at Memphis for wake measurements in 1994.
The AVOSS concept was originally conceived to use two factors, singly or in combination, for reducing aircraft spacing. These factors would be wake-vortex motion out of a predefined approach corridor and wake decay below a strength that is operationally significant. Initial predictions indicated that AVOSS technology has the potential to reduce takeoff delays as well as increase single-runway throughput by 10 percent or more during conditions requiring instrument approaches.
An ambitious goal of demonstrating the AVOSS concept at the Dallas-Fort Worth International Airport (DFW) was set for 2000, and a proof-of-concept prototype of AVOSS was installed there in 1997. MIT Lincoln Laboratory and Langley set up an extensive suite of meteorological sensors using two sodar (sound detection and ranging) systems and a Doppler radar profiler (to measure winds aloft), an instrumented 150-ft tower, and shorter towers to estimate the required atmospheric profiles. In addition, algorithms were developed for using the two FAA Terminal Doppler Weather Radars (TDWR) in Dallas-Fort Worth as high-resolution wind profilers and to combine the wind data from the various sensors into a single wind profile.
One key instrumentation capability developed under the Langley leadership of Ben C. Barker, Jr., was a pulsed coherent lidar system. The pulsed coherent lidar system was designed by Coherent Technologies, Inc. (CTI), under a NASA Small Business Innovation Research (SBIR) contract to provide the necessary confirmation that actual wake-vortex behavior agreed with predictions. The transceiver uses a solid-state, eye-safe laser beam that is expanded through a telescope and directed to a hemispherical scanner that scans the beam across the approach path. Light reflected from microscopic particles in the air, and shifted in frequency due to the swirling particle motion in the vortex, is detected by the lidar transceiver. These return signals are then analyzed to detect, track, and measure strength of the wake vortices.
The milestone demonstration of AVOSS at Dallas-Fort Worth International Airport occurred July 18–20, 2000. As implemented for the demonstration, the system provided spacing values to separate aircraft from wake-vortex encounters by defining a corridor of protected airspace, predicting wake motion and decay at numerous locations along the approach path for all aircraft, providing a safe separation criteria for the entire approach, and monitoring safety with a wake-vortex sensor. AVOSS did not render any go- or go-around decisions nor did it actually alter real spacing. AVOSS produced recommended reductions in spacing, measured actual wakes to compare with the predictions, and then developed statistics to determine the effectiveness and safety of the reduced spacing. This system ran in real time with automated weather observations, data quality assessments, and automated comparison of predicted and measured wake behavior. Although the data were not provided to Air Traffic Control during the demonstration, a large audience of airport, airline, and government officials were able to watch AVOSS predictions and confirm lidar measurements in real time.
Locations of AVOSS components
Langley pilot Philip Brown
flies Thrush Commander over
Key members of the weather subteam included MIT Lincoln Laboratory, North Carolina State University, Langley, and the National Oceanic and Atmospheric Administration (NOAA); the prediction subsystem team was composed of Langley, NorthWest Research Associates, Inc., and the Naval Post-Graduate School; and the wake detection subsystem team was Langley, the Research Triangle Institute, MIT Lincoln Laboratory, and the Volpe National Transportation Systems Center.
Based on results of the DFW experiments, the increase in calculated daily throughput averaged 6 percent and ranged from 1 to 13 percent. At DFW, a capacity increase of 6 percent means 6 additional planes that would normally face delays would be allowed to land each hour. The average throughput gain translates to a 15- to 40-percent reduction in delay when applied to realistic capacity ratios at major airports.
In May 2001, the AVOSS project won the Administrator’s Award at NASA’s Turning Goals Into Reality Conference, and the Air Transport Association named AVOSS to its top 10 list of air traffic control improvements.
Other Wake-Vortex Activities
Another Langley aircraft wake research activity of the late 1970s and early 1980s involved the potential alteration of the wake of agricultural aircraft used in aerial applications for improved efficiency (more uniform distribution patterns) and reduced drift of potentially harmful insecticides and herbicides. As part of a larger NASA Aerial Applications Program that began in 1976, these aerodynamic studies included ground testing in the Langley 30- by 60-Foot Tunnel and the Vortex Research Facility (VRF), as well as cooperative flight tests of an Ayres Thrush Commander agricultural aircraft. In the 30- by 60-Foot Tunnel, Frank L. Jordan, Jr., and H. Clyde McLemore conducted extensive aerodynamic evaluations of the full-scale Thrush Commander aircraft with various dispersal systems installed. In addition to identifying performance-enhancing airframe modifications, such as wing-fuselage fillets, the characteristics of droplets spread by liquid spray rigs (using water spray) were also determined in the wind tunnel. In the VRF, the ability to simulate the aerial dispersal of materials from small-scale models and the development of numerical methods to predict particle trajectories were demonstrated. Exploratory tests of various wake control concepts, including wingtip winglets, were also conducted.
In 1984, pilot Philip W. Brown and researchers Dana J. Morris, Cynthia C. Croom, and Bruce J. Holmes conducted flight tests of the Thrush Commander aircraft at the NASA Wallops Flight Facility to collect experimental data on the wake characteristics of the aircraft and the impact of aircraft modifications on particle deposition patterns during representative aerial spraying operations. The researchers developed theoretical methods simultaneously with the experimental efforts to simulate the dispersal of particles, including the complex interaction of the dispersed particles with the aircraft wake. The results of the study indicated good agreement between the experimental results and the theoretical predictions, and the ability to change the wake characteristics to produce desirable effects on deposition and drift characteristics were demonstrated. The success of the study provided fundamental information for aerial application operators, and the theoretical method known as AGDISP has been provided to designers and other pertinent users such as the U.S. Forestry Service.
At the same time that researchers were attempting to modify the wake-vortex characteristics of aircraft to provide solutions to safety and airport capacity issues, others were attempting to control and harness the energy expended in the formation of wingtip vortices in an effort to improve aircraft performance. The pioneering efforts of Langley’s Richard T. Whitcomb and his conceptual development and maturation of wingtip-mounted winglets is the most outstanding example of an application of performance-enhancing control of wingtip vortices. Whitcomb’s approach, however, stimulated additional efforts to reduce aircraft-induced drag through the use of wingtip devices. James C. Patterson, Jr., worked for Whitcomb in both wake alleviation research as well as drag-reduction efforts. Patterson led efforts on research on two wingtip vortex control concepts for drag reduction. In the first concept, Patterson and Whitcomb explored the potential of using wingtip-mounted jet engines to modify the formation of the wingtip vortex in a manner beneficial to reducing induced drag. The scope of the studies included tests of powered semispan models in the Langley 8-Foot Transonic Pressure Tunnel and other facilities. Although significant reductions in drag were measured in these experimental studies, real-world concerns involving aircraft controllability and other issues have limited the application of this concept to date.
Another performance-enhancing concept explored in ground testing and limited flight testing by Patterson was the use of wingtip turbines for reduced cruise drag or power extraction. In this concept, multiblade turbines mounted at each wingtip are either fixed in the swirling wingtip vortex for induced drag reduction or allowed to freewheel and rotate (driven by the wingtip vortex) for the generation of electrical power for aircraft systems. Patterson’s research on the tip turbines included tests in several Langley wind tunnels and limited flight tests using Langley’s PA-28R research aircraft.
To complete this brief survey on Langley contributions to wake-vortex technology, it should be pointed out that Langley researchers have participated on numerous occasions with the DOD on many high-priority classified activities requiring expertise in the field.
As indicated by the foregoing discussion, the Langley Research Center has expended considerable effort and made valuable contributions to the Nation’s knowledge and approach to the wake-vortex hazard. The results of the early Wake-Vortex-Alleviation Program, although frustrating to the researchers who had hoped for an aerodynamic solution to the problem, nonetheless serve as a foundation of knowledge for potential airframe modifications to mitigate the problem. In addition to providing clarification and data for the civil applications, the work resulted in activities in support of the military, such as analysis and improvement of C-17 paratroop capabilities. The vortex characterization research focused the attention of the research community on the impact of atmospheric conditions on the prediction and control of vortices. Combined meteorological and wake-vortex data sets from Memphis and DFW deployments are in use internationally, including Canada, Germany, and France. Langley’s research on spacing requirements provided analysis to support reclassification of weight categories. Finally, the development of an integrated aircraft spacing concept such as AVOSS has developed weather profiling, wake lidars, and wake prediction to a point where a wake system implementation is feasible and demonstrated the potential of technology to provide solutions to the current and impending capacity issues at major U.S. airports.
As the new millennium begins, the Nation faces a rapidly growing issue of airport capacity, and the FAA must ultimately provide options for solutions. NASA’s research has provided fundamental technology and stimulated interest by airport management, the Air Transport Association (ATA), and the FAA in developing wake systems for delay reduction. Aircraft manufacturers like Boeing and Airbus are also beginning to design new aircraft with wake characteristics in mind.
Gail S. Langevin
Gail S. Langevin
October 17, 2003