Wind Shear


Since the beginning of manned flight, low-altitude encounters with atmospheric turbulence and gusts have been among the most challenging safety issues facing aircraft operators, air traffic controllers, and the aerospace engineering community. The prediction, detection, and avoidance of potentially hazardous wind conditions have been a high priority technical target internationally. Major wind-induced accidents caused by the inability of the pilots to maintain aircraft performance and control have historically plagued the entire spectrum of civil aircraft types, including large commercial transports, regional airliners, business jets, and small personal-owner general aviation vehicles.

In the 1970s and 1980s, an alarming number of fatal accidents in the United States and abroad were attributed to the phenomenon known as wind shear, defined as any rapid change in wind direction or velocity. Severe wind shear is a rapid change in wind direction or velocity and causes horizontal velocity changes of at least 15 m/sec over distances of 1 to 4 km, or vertical speed changes greater than 500 ft/min. About 540 fatalities and numerous injuries resulted from wind-shear crashes involving 27 civil aircraft between 1964 and 1994. Wind shear also caused numerous near accidents in which the aircraft recovered just before ground contact.

Research focused by industry, government agencies, academia, and airlines on this major threat to aviation safety in the 1980s resulted in a vastly improved fundamental knowledge of the atmospheric environment and the critical properties associated with wind shears. In particular, the experimental and analytical efforts of meteorologists, coupled with analyses of piloting strategies during wind-shear encounters, pilot training, and the development of ground-based and airborne sensing technology, paved the way for technical solutions to mitigate this serious problem. Prior to this concentrated research effort, a gust front, or a leading edge of rain-cooled air, was widely believed to be the main wind-shear threat presented by thunderstorms to aircraft in takeoff or landing. A gust front is formed along the leading edges of large domes of rain-cooled air that result from cold downdrafts from individual thunderstorm cells. At the leading edge of this gust front, a dynamic clash occurs between the cool outflowing air and the warmer thunderstorm inflowing air and produces the familiar wind shift, temperature drop, and gusty winds that precede a thunderstorm.

However, extensive studies of thunderstorm and related downburst phenomena in the vicinity of airports by the late meteorological researcher, T. Theodore Fujita of the University of Chicago, played a key role in reversing the incorrect implication of the gust front to aircraft accidents. Fujita, who developed the tornado severity scale that bears his name, had conducted extensive studies on airline crashes. A key causal factor in his analysis was the generation and effects of a rapidly descending vertical column of air formed when air at high altitudes quickly cools due to the evaporation of ice, rain, or snow. Fujita submitted that a concentrated, strong three-dimensional outflow associated with the ground impact of the downdraft was the real fatal hazard in aircraft encounters. Although not totally technically correct in details, a layman’s interpretation of this physical phenomenon is the flow from a water-hose nozzle directed straight at a driveway, producing a spray of water in all directions. In this simplified model, the impact pressure field causes the downflow component to decelerate as air approaches the surface, and the horizontal component of the wind to accelerate outward from the impact center. But Fujita’s theory of a critical vertical “downdraft” in the mid-1970s was highly controversial at the time. Subsequently, photographic evidence of the phenomenon was obtained, and Fujita coined the name “microburst” for it. Fujita defined a microburst as a relatively small downburst whose outward, damaging winds extend no more than 4 km (2.2 nmi) over the surface. Radar meteorologists have redefined a microburst as a divergent low-level wind field with a velocity change of at least 15 knots over a distance between 1 and 4 km. The microburst exhibits severe, low-altitude wind-shear gradients that are experienced by a landing aircraft as rapid changes in the relative wind vector, sometimes to an extent that the performance capabilities of the airplane are exceeded, which results in ground impact. Roughly half of microbursts, as defined by radar meteorologists, are truly hazardous to aircraft.


Cross-sectional view of microburst.

Aircraft landing in microburst first experiences
headwinds followed by downdraft and tailwinds.


Another characteristic feature of the microburst is air circulation in the form of a vortex ring surrounding the downdraft core. This vortex ring contains strong outflow winds that contribute to the larger hazards caused by horizontal shears and vertical winds over scales between 1 and 4 km. Most microbursts last for a few minutes, and generally less than 10 min. Microbursts can occur anywhere convective weather conditions (thunderstorms, rain showers, or virga) occur. Virga is rain that evaporates before it reaches the ground and is associated with a “dry” microburst. The terms “microburst” and “wind shear” are often used interchangeably because the vast majority of dangerous wind shears result from microbursts.

An aircraft flying through a microburst may experience extremely hazardous airspeed fluctuations. As the aircraft enters the edge of the downburst outdraft, it initially encounters an increased head wind. This head wind increases the lift of the aircraft and, therefore, the altitude of the aircraft. If the pilots are unaware that this speed increase is caused by wind shear, they are likely to react to correct the aircraft approach angle by reducing engine power. The aircraft then passes into the vertically descending microburst core, where it encounters an abrupt change from head winds to downflow winds, which results in a loss of lift and altitude. Immediately thereafter, the aircraft crosses into a region of tail winds. This wind change reduces the relative airspeed of the aircraft and further decreases lift, which causes the aircraft to lose more altitude. Because the aircraft is now flying on reduced power, it is vulnerable to sudden losses of airspeed and altitude. The pilots may be able to escape the microburst by adding power to the engines, but if the engine response time is not rapid or if the shear is strong enough, the aircraft may crash.


Photograph of vortex ring preceding downdraft core in microburst. Photograph ”1991 William Bunting.


Obviously, technology that permits the detection and avoidance of severe wind-shear conditions is a key element in the national air transportation system. Working with industry, academia, and the FAA, Langley researchers provided key concepts and the validation of advanced airborne detection systems that have been implemented by airlines in the 1990s. As a result of these breakthrough efforts, wind-shear accidents have been virtually eliminated for large commercial transports.

Several excellent technical summaries that form the basis for the following brief discussion of Langley’s contributions and technical leadership for this critical national program are available, especially those by Roland L. Bowles, P. Douglas Arbuckle, Michael S. Lewis, David A. Hinton, Fred H. Proctor, and Lane E. Wallace (see bibliography).

Langley Research and Development Activities

At Langley, initial interests in wind-shear studies were stimulated by a series of tragic accidents during the early 1980s. The national concern and technological challenges resulting from these accidents urgently demanded the identification of potential solutions that would eliminate future occurrences of such horrible events.

The first accident involved a Pan American Boeing 727 that attempted to depart the New Orleans International Airport on July 9, 1982, during a severe thunderstorm. Witnesses observed the aircraft climb after takeoff to an altitude of about 100 ft when it began to descend, striking some trees about 2,400 ft past the end of the runway, crashing into a residential area, and demolishing six houses. All 145 onboard the aircraft were killed, as well as 8 people in the residential area. The probable cause of the accident was identified as “The airplane’s encounter during the liftoff and initial climb phase of flight with a microburst-induced wind shear which imposed a downdraft and a decreasing head wind, the effects of which the pilot would have had difficulty recognizing and reacting to in time for the airplane’s descent to be arrested before its impact into trees. Contributing to the accident was the limited capability of current ground-based low-level wind-shear detection technology to provide definitive guides for controllers and pilots for use in avoiding low-level wind-shear encounters.” Unfortunately, the Low Level Windshear Alert System (LLWAS) developed by the FAA in 1976 did not provide adequate warning in this particular accident. The early LLWASs were more “gust front” detectors than microburst detectors and had a sparse array of sensors. These sensors were only installed around the airport, not 1 or 2 miles away where needed. These LLWAS sensors were also installed before the physical characteristics of microbursts were common knowledge.

Following the accident, the FAA contracted with the National Academy of Sciences (NAS) to review the technological state of the art in wind-shear alerting systems and to define technical options that might be used to mitigate and reduce wind-shear induced accidents. The NAS participants in the study met at the Langley Research Center in 1984, and participation in the ensuing discussions stimulated Langley researcher Roland L. Bowles and others to analyze and brainstorm the wind-shear issue. Bowles and his peers in the FAA began to plan a joint technical program to address one of the major deficiencies cited in the NAS report—the need for accelerated research on airborne wind-shear detection systems—especially forward-looking systems that could provide adequate warning to the pilot before a wind shear was encountered.

Unfortunately, before Langley could assemble enough momentum to aggressively address airborne wind-shear detection, a second horrifying accident was thrust into the national spotlight. On August 2, 1985, a Delta Airlines Lockheed L-1011 approaching the Dallas-Fort Worth International Airport in a thunderstorm that included heavy rain and lightning encountered a microburst. The aircraft touched down in a field about 6,000 ft short of the runway, bounced, struck a car on a highway, collided with two water tanks, broke up, and burst into flames. The human toll was 137 fatalities. Subsequent analysis indicated that the pilot was able to traverse the downdraft winds, but the aircraft crashed as it tried to fly into the outflow winds that contained high velocity tail winds. The probable cause was stated as “The flight crew’s decision to initiate and continue the approach into a cloud which they observed to contain visible lightning; the lack of specific guidelines, procedures, and training for avoiding and escaping from low-level wind shear; and the lack of definitive, real-time wind-shear hazard information. This resulted in the aircraft’s encounter at low altitude with a microburst-induced, severe wind shear from a rapidly developing thunderstorm located on the final approach course.” The ground-based LLWAS wind-shear warning system finally detected the microburst a full 2 min after the aircraft had crashed.

Immediately following the accident, national pressures on Congress to provide solutions to these traumatic accidents resulted in a fact-finding visit by Congressman George Brown of California to Langley for a briefing on wind-shear research. At the time, the most directly relevant NASA research was being conducted in the area of piloted simulation technology, involving cloud-scale modeling. Jeremiah F. Creedon, then Director for Flight Systems, briefed Brown on Langley’s potential plans for a technical attack on the problem. An enthusiastic endorsement by Brown of the concepts and capabilities offered by Langley resulted from this briefing. Brown carried his impressions of the briefing back to Congress where he played a key role in pursuing support for wind-shear research. Subsequently, in 1986, the FAA announced a National Integrated Windshear Plan, which included Langley as the lead organization for airborne wind-shear detection research under a joint NASA and FAA Airborne Windshear Program. At Langley, wind-shear research was organized under the leadership of Roland L. Bowles. Bowles and his team quickly formulated analyses, simulations, laboratory tests, and flight tests that would help the FAA reach the objective of certifying predictive wind-shear detection systems for installation on all commercial aircraft. The program consisted of three main elements: hazard characterization, sensor technology, and flight management systems. This joint program was later expanded in 1990 to include the integration of both airborne and ground-based detection technology.

Hazard Characterization

One critical and fundamental element of Langley’s Wind-Shear Program was a major effort aimed at understanding the detailed characteristics and relative hazard of microbursts. The use of Langley’s supercomputer capability had already been instrumental in the development of a mesoscale numerical weather model by Langley contractor, Michael L. Kaplan, and others in the early 1980s. This sophisticated weather model became a useful research tool in understanding large severe storms events, and it also became the concept for the National Weather Service to develop a numerical weather model that became operational in the period. A more detailed model of wind-shear phenomena was developed by Fred H. Proctor and his associates. Known as the Terminal Area Simulation System (TASS), this three-dimensional, time-dependent model included representation of liquid and ice microphysics. The effects of condensation, evaporation, freezing, and sublimation in the atmosphere and their impact on atmospheric winds could be numerically simulated by this impressive tool to promote an understanding of microburst formation and structure. Using TASS, Proctor could actually simulate the time-dependent life cycle of a convective storm, including microbursts that might develop at subsequent times. Data sets generated from this model were eventually used by the FAA in its certification process for onboard wind-shear sensors. In 1993, Proctor and Bowles were awarded NASA Langley’s prestigious award for the best technical paper, the H. J. E. Reid Award, for a case study of a Denver wind-shear incident using the TASS model.

Throughout the program, TASS was applied to numerous actual microburst cases and demonstrated the ability to produce simulation results that agreed closely with observations. The new capabilities provided by TASS were used in a multitude of analysis objectives: an understanding of microburst events, reconstructing missing information from actual observations, evaluating microburst sensor capabilities, and providing answers regarding flight management strategies during microburst encounters.

In addition to defining the detailed atmospheric characteristics produced by microbursts, the Langley researchers addressed the key issue of defining the relative hazard associated with microburst encounters. A brilliant approach to this problem was conceived and implemented by Bowles. He examined the overall performance capabilities of aircraft during a microburst encounter and subsequently derived a metric he named the “F-Factor,” which quantifies the loss in aircraft performance capability that would be experienced during a specific wind shear. The nondimensional F-Factor is based on a consideration of the weight, thrust, and drag of the aircraft as well as the effects of local velocities in the wind shear on the specific excess thrust (thrust minus drag divided by weight) required to maintain steady flight conditions due to wind variations in a microburst. For example, if a representative aircraft was capable of a specific excess thrust value of 0.15 at a flight condition of interest, a wind shear with intensity greater than an F-Factor of 0.15 would exceed the maximum performance capability of the airplane. When encountering such a wind shear, the airplane would lose airspeed, altitude, or both, regardless of pilot inputs. The typical transport aircraft traveling at 150 knots and encountering a wind shear with an F-Factor of 0.15 over 1 nmi (24 sec) would lose 911 ft of altitude if recovery action was not taken. Refinements and the ultimate development of the F-Factor principle by Bowles, Michael S. Lewis, and David A. Hinton included a consideration of the length of time over which the aircraft is exposed to the wind shear to produce a refined definition of the F-Factor averaged over 1 km (about 15 sec of exposure at typical jet transport low-altitude airspeeds).

The breakthrough analysis and derivation of the F-Factor by Bowles is regarded by many as the key contribution of NASA in the taming of the wind-shear threat. As discussed in a later section, the F-Factor provided enabling analyses and assessments for advanced airborne wind-shear sensors and is now used as a FAA-mandated tool in the development and commercial sales of wind-shear sensors. In recognition of his outstanding contributions in wind-shear research, Roland Bowles was awarded an R&D 100 Award (1993), the Langley H. J. E. Reid Award (1993), and the AIAA Engineer of the Year Award (1994).

The scope of the Langley program to characterize wind-shear hazards also involved other aspects of operations in thunderstorms. For example, stimulated by concerns over the unknown effects of the extremely heavy rainfall that is typically experienced during wind-shear conditions, researcher R. Earl Dunham, Jr., led work to determine experimentally the impact of heavy rain on aerodynamic characteristics (especially lift) of representative transport airfoils. In this unique study, wind-tunnel tests were initially conducted in the Langley 14- by 22-Foot Subsonic Tunnel using water spray bars in the tunnel to determine if simulated rain particles would degrade the aerodynamic performance of representative subscale wing models. Subsequently, a “car wash” test section was constructed along the track of the Langley Aircraft Landing Dynamics Facility (ALDF), and a large-scale instrumented wing-flap model was propelled through simulated rain to obtain results more representative of actual aircraft conditions. Results of the wind-tunnel and ALDF testing showed significant degradation in the maximum lift, and a marked decrease in the stall angle of attack under extremely heavy rain conditions (greater than about 1,000 mm/hr). Results of the tests were then used by Dan D. Vicroy in a theoretical analysis of wind-shear encounters where it was determined that climb performance reductions equivalent to an F-Factor of about 0.01 would be experienced. A major conclusion of these activities, however, was that for the vast majority of wind-shear encounters, heavy rain was an insignificant effect. The extremely large rain rates needed to impact aircraft lift are extremely rare in nature.

Sensor Technology

While Langley researched the airborne detection technology, the FAA undertook an aircrew training program that focused on wind-shear recognition and procedures for recovering from its effects. The FAA also led the development of advanced ground-based wind-shear detection instruments, including the Terminal Doppler Weather Radar (TDWR) now being installed at major U.S. airports. Developed by the Raytheon Corporation, the TDWR can accurately measure wind velocities in terminal areas and generate real-time aircraft hazard displays that are updated every minute.

Langley’s focus, however, was on the development of airborne systems capable of predicting the presence of wind shear in a forward-looking mode. In response to the terrible accidents of the early 1980s, the FAA in 1988 directed that all commercial aircraft have onboard wind-shear detection systems by the end of 1993. Three airlines—American, Northwest, and Continental—received exemptions until the end of 1995 in order to install and test emerging predictive wind-shear sensors rather than reactive systems that do not report the condition until an airplane already has encountered it. The reactive system processes data from standard aircraft instruments to determine the presence of wind shear. The reactive system, therefore, only advises a pilot of a wind-shear event, which allows an increase in engine power and possibly escape of the hazard; however, the airplane might not be capable of recovering from a severe wind shear at that point. Langley concentrated on a predictive system in the cockpit that would provide 10 to 40 sec of advance warning; thereby, the pilot would be able to determine the proper maneuver, add power for flight stability, or avoid the wind-shear area altogether. A Wind-Shear Program Office was established about the time that Langley realized that multiple sensors had to be flight-tested for assessments.


Langley TSRV research aircraft at Orlando during wind-shear flight research efforts.


Pushed by the 1995 implementation decision imposed by the FAA wind-shear implementation plan and augmented by technology application experts from the Research Triangle Institute, the Langley team initiated the development and assessments of three different types of microburst sensors. The one that ultimately became the first in airline service was the Doppler microwave radar, which sends a radio wave ahead of the aircraft to bounce off raindrops in the thunderstorm and return to the instrument. Computerized measurement of the Doppler shift (the difference in wavelength frequency between the outbound wave and the returning signal) provides an indication of wind-shear velocity. A second type of system known as Lidar, for Light Detection and Ranging, operates under the same Doppler shift principle but employs a laser beam instead of a radio wave. A third type, a passive infrared sensor, is based on the fact that a microburst, usually cooler than the surrounding air, can theoretically be detected by infrared measurement of the temperature differential ahead of the airplane.

Langley’s task was essentially to build a technology base that would enable manufacturers to develop their own commercially viable, proprietary systems. The enormous job began with characterizing the wind-shear hazard and determining the warning time required. Extensive computational simulations by Langley’s Fred H. Proctor, using his personal experience and the TASS computer models (which had been thoroughly validated by actual observations), documented the structure, strength, and evolution of microbursts. This work established the basic specifications for sensors and enabled development of algorithms for rejecting ground “clutter” that could confuse sensor signals. As would be expected, the Bowles F-Factor was a key factor in these analyses. All this knowledge gave manufacturers a broad knowledge base about how to extract wind-shear information from a sensor signal, how to process the data against hazard criteria, and how to alert flight crews to valid threats while rejecting “nuisance” indicators.

By 1991, 5 years into Langley’s wind-shear sensor development program, the technology had advanced to the point where validation of the sensors required actual flight tests in wind-shear conditions. For that challenging job, Langley outfitted its unique Boeing 737 flying laboratory. Formally known as the Transport Systems Research Vehicle (TSRV), this particular 737 was the first production aircraft of the Boeing 737-100 jetliner series. This unique aircraft was extensively modified by Langley and equipped with a rear research cockpit in what would have been the forward section of the passenger cabin for studies of advanced flight displays and technology. Although it had been used for over 20 years in very significant aeronautical research at Langley, the wind-shear program was arguably the most important technical project for the TSRV. The aft flight deck was used as a command post to monitor ground radar uplinks and airborne sensors and to fly the airplane during maneuvers to intercept a microburst. In the rear cockpit, a moving-map display, with radar-derived microburst icons, was used as an efficient tool for setting up the straight-in approaches needed to allow radar and lidar scanning before entry. Before microburst entry, the forward deck took control and manually flew the microburst penetration.

Three types of forward-looking wind-shear sensors were evaluated during the flight tests: a modified doppler radar transmitter from Rockwell International, Collins Air Transport Division (Langley developed the research signal-processing algorithms and hardware for the wind-shear application); a doppler lidar by Lockheed Corporation, Missiles and Space Division, United Technologies Optical Systems, Inc., and Lassen Research; and an infrared detector by Turbulence Prediction Systems. Emedio Bracalente, Langley, led the Airborne Radar Development Group.


Laser beams used to align lidar and infrared detection systems on TSRV.


Langley selected two field sites for the joint NASA and FAA flight test program: one at Orlando, Florida, and the other at Denver, Colorado. Both areas were noted for frequent microbursts in summertime, but it was anticipated that microbursts in the Orlando area would be predominantly “wet,” whereas a major portion of the microbursts at Denver would be “dry.” The flight test plan was challenging. The researchers had to anticipate when and where a microburst would form based on radar and other meteorological data because the life cycle of a microburst (and its parent thunderstorm) is shorter than the time it would take the aircraft to be boarded and airborne. When ground controllers predicted a potential wind shear, the Langley crew would scramble, take off, fly directly toward and into the microburst, observe and record the sensor findings, then validate them by cross-checking with ground radar data and with data from an airborne reactive system for measuring wind-shear velocities in situ. Orlando flights were supported by a TDWR operated by the Lincoln Laboratory at the Massachusetts Institute of Technology. At Denver, a research radar of similar capability was operated by the National Center for Atmospheric Research. For safety purposes, the TSRV was flown into wind shear at speeds higher than those of a normal jetliner approach and at altitudes greater than takeoff and landing levels (speeds of 240–260 mph and altitudes of 750–1,500 ft). Other safety factors employed during the deployment were to avoid flying into areas with radar reflectivity greater than 50 dBz (45 dBz in Denver), and F-Factors greater than 0.15. Over the summers of 1991 and 1992, the Langley team conducted 130 flights and experienced 75 wind-shear events. The airplane flew through heavy rains and dust clouds, near hail and frequent lightning, all in proximity to major airports without any safety incidents. The results of the test program demonstrated that Doppler radar systems offered the greatest promise for early introduction to airline service. The Langley forward-looking Doppler radar detected wind shear consistently and at longer ranges than other systems, and it was able to provide 20 to 40 sec warning of upcoming microbursts. Some of the predictive sensors showed good correlation with data from ground radars and the onboard reactive systems.


TSRV approaching thunderstorm for microburst studies.


The personal professionalism, dedication, and individual contributions of the TSRV wind-shear flight test team were shining examples of this outstanding Langley contribution to aircraft of the 1990s. Michael S. Lewis led these flight efforts in his role as Deputy Program Manager, augmented by an enthusiastic team that included electronic technicians led by Artie D. Jessup; flight operations led by TSRV crew chief Michael Basnett; and research pilots Lee H. Person, Jr., and Kenneth R. Yenni.

Flight Management Systems

Yet another key element in the NASA and FAA Airborne Wind-Shear Program was directed at defining operational concepts to minimize or mitigate the hazards associated with microbursts. Research efforts included defining aircraft wind-shear recovery strategies, using ground-based radar information on the flight deck as an alternative for airborne weather radar, and determining the most effective crew information and procedures. David A. Hinton led numerical and piloted simulations to determine the pilot control strategy that would result in a minimum aircraft energy loss when a microburst was encountered. Hinton examined several methods of controlling aircraft energy, and all findings indicated that the factor that most strongly affected a microburst wind-shear recovery was the time the recovery was initiated. In the studies, the average recovery altitude for all strategies only varied by about 20 ft; however, the average recovery altitude varied by almost 300 ft when the initiation time of the recovery was advanced by 5 sec.

Hinton also led Langley’s studies of the use of information provided by the FAA airport TDWR in the cockpit. Major challenges for this approach included the impact of time delays in view of the changing dynamics of microburst characteristics between updates from the TDWR (each minute) and correcting for the differences between the height of the ground-based radar beam and the altitude of the airplane. During flight tests of the Langley TSRV Boeing 737 aircraft in 1991 and 1992, uplinked TDWR data were used to locate microbursts and maneuver the TSRV to penetrate the event. However, the results of Hinton’s study indicated that the ground-based TDWR information was more appropriate for microburst awareness and advisories rather than as a flight deck wind-shear hazard alerting system.

Langley conducted or sponsored several important studies aimed at issues regarding the most effective crew-alerting information and responses during wind-shear encounters. These detailed studies addressed issues such as how much advanced warning was required, should lateral maneuvers be attempted for escape, and should existing wind-shear pilot training be modified for forward-looking sensor systems. The scope of studies included numerical and piloted simulations using microburst models and candidate cockpit display formats. Another important issue identified by Langley involved the interpretation of valid forward-looking wind-shear system alerts as nuisance alerts. This concern involved the fact that pilots, having been alerted by the system, might conclude that the system had issued a false alarm if the microburst penetration was uneventful.

Following these research studies, the technology development effort of NASA for airborne wind-shear detection systems was essentially complete, but the Langley group continued working in a consulting capacity on the matter of FAA certification. No certification standards existed—they had to be invented and the Langley researchers now represented the most knowledgeable body in the world of wind-shear expertise. Langley worked with the FAA and industry to develop a set of standards for certification of wind-shear sensors. Collectively, the standards define the hazard, the cockpit interface and alerts to be given to flight crews, a suggested methodology for certification, and the requisite sensor performance levels. Langley research was the basis for most of the specifications. In addition to providing guidance for development of specifications, TASS simulations provided the microburst data sets required for certification testing. This set consists of a range of possible events (dry, wet, large scale, small diameter, multicore) that an aircraft may encounter.

A final wind-shear related Langley contribution occurred immediately after the NASA and FAA program ended. On July 2, 1994, a USAir DC-9-31 crashed following a missed approach at the Charlotte-Douglas International Airport. The accident resulted in 37 fatalities of the 57 onboard. As an altitude of 350 ft was reached during the go-around, the aircraft rapidly began to descend. The aircraft then collided with trees and a private residence and broke up. The accident investigation stated that the probable causes were (1) the decision of the flight crew to continue an approach into severe convective activity that was conducive to a microburst, (2) the failure of the flight crew to recognize a wind-shear situation in a timely manner, (3) the failure of the flight crew to establish and maintain the proper airplane attitude and thrust setting necessary to escape the wind shear, and (4) the lack of real-time adverse weather and wind-shear hazard information dissemination from air traffic control, all of these factors led to an encounter with and failure to escape from a microburst-induced wind shear that was produced by a rapidly developing thunderstorm located at the approach end of the runway. Contributing to the accident were (1) the lack of air traffic control procedures that would have required the controller to display and issue radar weather information to the pilots, (2) the failure of the Charlotte tower supervisor to properly advise and ensure that all controllers were aware of and reporting the reduction in visibility and the low-level wind-shear alerts that had occurred in multiple quadrants, (3) the inadequate remedial actions by USAir to ensure adherence to standard operating procedures, and (4) the inadequate software logic in the wind-shear warning system of the airplane that did not provide an alert upon entry into the wind shear.

In response to an NTSB request for assistance, a Langley team of Fred H. Proctor, Emedio Bracalente, and Steve Harrah, together with George Switzer and Charles Britt of Research Triangle Institute, used the TASS model to simulate the microburst and then used radar simulation to show what a wind-shear radar would have seen if onboard the aircraft. The Charlotte microburst was one of the smallest and most intense that the Langley team had ever seen (just under 1 km but with very high F-Factor); this was a worst-case situation because it gave the pilots no reaction time. The primary conclusion of the study was that the accident may have been avoided if the aircraft had been equipped with a wind-shear radar.


In addition to the usual practice of disseminating technical papers to effect technology transfer, NASA and the FAA jointly sponsored five well-attended national wind-shear conferences beginning in 1987 and ending in 1993. The Airborne Wind-Shear Program was based on one of the most effective methods of technology transfer—the participation of potential manufacturers, industry, and regulatory agencies to track the development and assessments of wind-shear detectors. Three major avionics manufacturers (Allied Signal, Westinghouse Electronic Systems Group, and Rockwell Collins Commercial Avionics) sent engineering teams to Langley to meet directly with the radar engineering personnel and follow the developmental effort of Langley step by step. The three companies each requested and were provided Langley wind-shear simulations, which they used extensively in developing their own commercial systems. Langley personnel and contractors also participated in several government-industry efforts to develop standards for forward-looking wind-shear detection systems.

On September 1, 1994, Allied Signal Bendix RDR-4B became the first predictive wind-shear system to gain FAA certification for airline operations. The RDR-4B forward-looking radar was the product of the decade-long FAA, NASA, industry, and academia research program spearheaded by Langley that developed the technology base to enable commercial manufacture of the Allied Signal and other wind-shear detection-prediction systems.

Many of Langley’s technology concepts have been incorporated into industry’s implementation of advanced wind-shear detection systems. For example, the F-Factor became a regulatory parameter. According to FAA regulations, wind-shear warnings must be given for F-Factors of 0.13 and greater. FAA regulations determine the combinations of altitude, airspeed, distance from wind shear, and F-Factor that result in an advisory, caution, or warning to the flight crew.

Three major U.S. airlines (United, Northwest, and Continental) subsequently selected the RDR-4B; collectively, they ordered more than 1,000 units. The technology is also being extended to foreign airlines, and among those who have purchased the RDR-4B are Swissair, Alitalia, Iberia, Gulf Air, and Kuwait Airways.

On November 30, 1994, Continental Airlines Flight 1637, a Boeing 737-300 jetliner, took off from Washington (D.C.) National Airport bound for Cleveland. It was a routine, regularly scheduled flight, but to aviation safety officials all over the world it was something more—it was a historic moment that marked the introduction to commercial airline service of an onboard cockpit instrument for detecting and predicting wind shear in a forward-looking mode.

In the mid- and late-1990s, avionics companies rapidly provided advanced wind-shear detection systems. By June, 1996, vendors such as Allied Signal/Bendix (now Honeywell), Rockwell Collins, and Westinghouse Electric produced certified forward-looking wind-shear radar systems. Over 2,000 orders had been placed for the systems from foreign and domestic carriers as well as the U.S. Air Force. These wind-shear detection systems issue microbursts warnings within a specific distance (e.g., 0.25 nmi above or below the flight path of the aircraft) and within a specific angular sector (e.g., -30∞) of the aircraft heading. A warning icon is displayed on the radar display, and an aural warning is also issued by a voice synthesizer. Wind-shear alerts are typically inhibited during takeoff from the time the aircraft reaches 100 knots airspeed until it is 50 ft above ground level (AGL) to discourage avoidance maneuvers during this critical flight time. Similarly, alerts are inhibited during approach when the aircraft is below 50 ft AGL. Alerts are never given above a maximum altitude (e.g., 1,500 ft AGL).

It took almost a decade to bring the predictive wind-shear system from concept to commercial availability, but aviation experts say that was a remarkably brief period when the complexity of the phenomenon is considered. The program stands as a model of cooperative endeavor by a broad segment of the U.S. aviation community, including government agencies, aircraft manufacturers, sensor manufacturers, airlines, research organizations, and academia.

The contributions of the NASA Langley Research Center to this national success story stand out among the thousands of legendary technological accomplishments of the Center. Cited as “NASA at its best” by NASA’s Aeronautics Advisory Committee, the NASA and FAA Airborne Wind-Shear Research Program was nominated in 1994 for the Nation’s prestigious Robert J. Collier Trophy in recognition of the most significant aerospace accomplishment.



NASA Official
Gail S. Langevin
Page Curator
Peggy Overbey
Last Updated
October 17, 2003