Подпись: 13As clearly evidenced by U. S. military experiences, the technical area of high-angle-of-attack/departure/spin behavior will continue to challenge design teams of highly maneuverable aircraft. The Nation has been for­tunate in assembling and maintaining unique expertise and facilities for the timely identification and resolution of problems that might have had a profound impact on operational capability or program viability. In the author’s opinion, several situations are emerging that threaten the tra­ditional partnerships and mutual resources required for advancing the state of the art in high-angle-of-attack technology for military aircraft.

The end of the Cold War has naturally resulted in a significant decrease in new military aircraft programs and the need for continued research in a number of traditional research areas. As technical person­nel exit from specialty areas such as high-angle-of-attack and spin behav­ior, the corporate knowledge and experience base that was the jewel in NASA’s crown rapidly erodes, and lessons learned become forgotten.

Of even more concern is the change in traditional working-level relationships between the NASA and DOD communities. During the term of NASA Administrator Daniel S. Goldin in the 1990s, NASA turned its priorities away from its traditional links with military aircraft R&D to the extent that long-time working-level relationships between NASA, indus­try, and DOD peers were ended. At the same time, aeronautics funding within the Agency was significantly reduced, and remaining aeronautics activities were redirected to civil goals. As a result of those programmatic decisions and commitments, NASA does not even highlight military – related research as part of its current mission. It has become virtually impossible for researchers and their peers in the military, industry, or DOD research laboratories to consider the startup of highly productive, unclassified military-related programs such as the NASA F/A-18 High – Angle-of-Attack Technology program.

Meanwhile, leaders in military services and research organiza­tions have now been replaced with many who are unfamiliar with the traditional NASA-military ties and accomplishments. Without those
relationships, the military R&D organizations have turned to hiring their own aeronautical talent and conducting major research undertakings in areas that were previously exclusive to NASA Centers.

Finally, one of the more alarming trends underway has been the massive closures of NASA wind tunnels, which have been the backbone of NASA’s ability to explore concepts and ideas and to respond to high – priority military requests and problem-solving exercises in specialty areas such as high-angle-of-attack technology.

Подпись: 13In summary, this essay has discussed some of the advances made in high-angle-of-attack technology by NASA, which have contributed to a dramatic improvement in the capabilities of the Nation’s first-line mil­itary aircraft. Without these contributions, many of the aircraft would have been subject to severe operational restrictions, excessive develop­ment costs, significantly increased risk, and unacceptable accidents and safety-of-flight issues. In the current era of relative inactivity for devel­opment of new aircraft, it is critical that the resources required to pro­vide such technology be protected and nurtured for future applications.


Three important NASA research aircraft representing different approaches to V/STOL flight pass in review over NASA’s Ames Research Center. Left to right: the deflected lift QSRA, the tilt rotor XV-15, and the vectored-thrust Harrier. NASA.

National Plan for Civil Aviation Human Factors: 1995

In June 1995, the FAA announced its plans for a joint FAA-DOD-NASA initiative called the National Plan for Civil Aviation Human Factors. The plan detailed a national effort to reduce and eliminate human error as the cause of aviation accidents. The plan called for projects that would iden­tify needs and problems related to human performance, guide research programs that addressed the human element, involve the Nation’s top scientists and aviation professionals, and report the results of these efforts to the aviation community.[221]

NASA’s extensive involvement in human factors issues is detailed in another case study of this volume.

Aviation Safety Reporting System

NASA initiated and implemented this important human-based safety program in 1976 at the request of the FAA. Its importance can best be judged by the fact it is still in full operation—funded by the FAA and managed by NASA. The Aviation Safety Reporting System (ASRS) col­lects information voluntarily and confidentially submitted by pilots, controllers, and other aviation professionals. This information is used to identify deficiencies in the National Aviation System (NAS), some of which include those of the human participants themselves. The ASRS analyzes these data and refers them in the form of an "alerting message” to the appropriate agencies so that problems can be corrected. To date, nearly 5,000 alert messages have been issued.[377] The ASRS also educates through its operational issues bulletins, its newsletter CALLBACK and its journal ASRS Directline, as well as through the more than 60 research studies it has published.[378] The massive database that the ASRS main­tains benefits not only NASA and the FAA, but also other agencies world­wide involved in the study and promotion of flight safety. Perhaps most importantly, this system serves to foster further aviation human fac­tors safety research designed to prevent aviation accidents.[379] After more than 30 years in operation, the ASRS has been an unqualified success. During this period, pilots, air traffic controllers, and others have pro­vided more than 800,000 reports.[380] The many types of ASRS responses to the data it has collected have triggered a variety of safety-oriented actions, including modifications to the Federal Aviation Regulations.[381]

It is impossible to quantify the number of lives saved by this impor­tant long-running human-based program, but there is little dispute that its wide-ranging effect on the spectrum of flight safety has benefitted all areas of aviation.

Effect of External Stores

External stores have been found to have large effects on spin and recovery, especially for asymmetric loadings in which stores are located asymmetrically along the wing, resulting in a lateral displace­ment of the center of gravity of the configuration. For example, some aircraft may not spin in the direction of the "heavy” wing but will spin fast and flat into the "light” wing. In most cases, model tests in which the shapes of the external stores were replaced with equivalent weight ballast indicated that the effects of asymmetric loadings were primarily due to a mass effect, with little or no aerodynamic effect detected. However, very large stores such as fuel tanks were found, on occasion, to have unexpected effects because of aerodynamic char­acteristics of the component. During the aircraft development phase, spin characteristics of high-performance military aircraft must be assessed for all loadings proposed, including symmetric and asymmet­ric configurations. Spin tunnel tests can therefore be extensive for some aircraft, especially those with variable-sweep wing capabilities. Testing

of the General Dynamics F-111, for example, required several months of test time to determine spin and recovery characteristics for all poten­tial conditions of wing-sweep angles, center-of-gravity positions, and symmetric and asymmetric store loadings.[513]

The Early Evolution of General Aviation

The National Advisory Committee for Aeronautics (NACA) was formed on March 3, 1915, to provide advice and carry out much of cutting-edge research in aeronautics in the United States. This organization was modeled on the British Advisory Committee for Aeronautics. President Woodrow Wilson created the advisory committee in an effort to orga­nize American aeronautical research and raise it to the level of European aviation. Its charter and $5,000 initial appropriation (low even in 1915) were appended to a naval appropriations bill and passed with little fan­fare. The committee’s mission was "to supervise and direct the scientific study of the problems of flight, with a view to their practical solution,” and to "direct and conduct research and experiment in aeronautics.”[775] Thus, from its outset, it was far more than simply a bureaucratic panel distanced from design-shop, laboratory, and flight line.

The NACA soon involved itself across the field of American aero­nautics, advising the Government and industry on a wide range of issues including establishing the national air mail service, along with its night mail operations, and brokering a solution—the cross­licensing of aeronautics patents—to the enervating Wright-Curtiss patent feud that had hampered American aviation development in the pre-World War I era and that continued to do so even as American forces were fighting overseas. The NACA proposed establishing a Bureau of Aeronautics in the Commerce Department, granting funds to the Weather Bureau to promote safety in aerial navigation, licensing of pilots, air­craft inspections, and expanding airmail. It also made recommenda­tions in 1925 to President Calvin Coolidge’s Morrow Board that led to passage of the Air Commerce Act of 1926, the first Federal legislation regulating civil aeronautics. It continued to provide policy recommen­dations on the Nation’s aviation until its incorporation in the National Aeronautics and Space Administration (NASA) in 1958.[776]

The NACA started working in the field of GA almost as soon as it was established. Its first research airplane programs, undertaken pri­marily by F. H. Norton, involved studying the flight performance, sta­bility and control, and handling qualities of Curtiss JN-4H, America’s iconic "Jenny” of the "Great War” time period, and one that became first great American GA airplane as well.[777] The initial aerodynamic and performance studies of Dr. Max M. Munk, a towering figure in the his­tory of fluid mechanics, profoundly influenced the Agency’s subsequent approach to aerodynamic research. Munk, the inventor of the variable – density wind tunnel (which put NACA aerodynamics research at the forefront of the world standard) and architect of American aerodynamic research methodology, dramatically transformed the Agency’s approach to airfoil design by introducing the methods of the "Prandtl school” at Gottingen and by designing and supervising the construction of a rad­ical new form of wind tunnel, the so-called "variable density tunnel.” His GA influence began with a detailed study of the airflow around and through a biplane wing cellule (the upper and lower wings, connected with struts and wires, considered as a single design element). He pro­duced a report in which the variation of the section, chord, gap, stag­ger, and decalage (the angle of incidence of the respective chords of the upper and lower wings) and their influence upon the available wing cell space for engines, cockpits, passenger, and luggage, were investigated with a great number of calculated examples in which all of the numer­ical results were given in tables. Munk’s report was in some respects a prototypical example of subsequent NACA-NASA research reports that, over the years, would prove beneficial to the development of GA by investigating a number of areas of particular concern, such as air­craft aerodynamic design, flight safety, spin prevention and recoveries, and handling qualities.[778] Arguably these reports that conveyed Agency research results to a public audience were the most influential product of NACA-NASA research. They influenced not only the practice of engi­neering within the various aircraft manufacturers, but provided the latest information incorporated in many aeronautical engineering text­books used in engineering schools.

Though light aircraft are often seen as the by-product of the air trans­port revolution, in fact, they led, not followed, the expansion of com­mercial aviation, particularly in the United States. The interwar years saw an explosive growth in American aeronautics, particularly private flying and GA. It is fair to state that the roots of the American air trans­port revolution were nurtured by individual entrepreneurs manufactur­ing light aircraft and beginning air mail and air transport services, rather than (as in Europe) largely by "top-down” government direction. As early as 1923, American fixed-base operators "carried 80,888 passengers and 208,302 pounds of freight.”[779] In 1926, there were a total of 41 private airplanes registered with the Federal Government. Just three years later, there were 1,454. The Depression severely curtailed private ownership, but although the number of private airplanes plummeted to 241 in 1932, it rose steadily thereafter to 1,473 in 1938, with Wichita, KS, emerging as the Nation’s center of GA production, a distinction it still holds.[780]

Two of the many notable NACA-NASA engineers who were influ­enced by their exposure to Max Munk and had a special interest in GA, and who in turn greatly influenced subsequent aircraft design, were Fred E. Weick and Robert T. Jones. Weick arrived at NACA Langley Field, VA, in the 1920s after first working for the U. S. Navy’s Bureau of Aeronautics.[781] Weick subsequently conceived the NACA cowling that became a feature of radial-piston-engine civil and military aircraft design. The cowling both improved the cooling of such engines and streamlined the engine installation, reducing drag and enabling aircraft to fly higher and faster.

Подпись: This Curtiss AT-5A validated Weick's NACA Cowling. The cowling increased its speed by 19 miles per hour, equivalent to adding 83 horsepower. Afterwards it became a standard design feature on radial-engine airplanes worldwide. NASA. Подпись: 8

In late fall of 1934, Robert T. Jones, then 23 years old, started a tem­porary, 9-month job at Langley as a scientific aide. He would remain with the Agency and NASA afterwards for the next half-century, being particularly known for having independently discovered the benefits of wing sweep for transonic and supersonic flight. Despite his youth, Jones already had greater mathematical ability than any other of his coworkers, who soon sought his expertise for various theoretical anal­yses. Jones was a former Capitol Hill elevator operator and had previ­ously been a designer for the Nicholas Beazley Company in Marshall, MO. The Great Depression collapsed the company and forced him to seek other employment. His work as an elevator operator allowed him to hone his mathematical abilities gaining him the patronage of senior officials who arranged for his employment by the NACA.[782]

Jones and Weick formed a fruitful collaboration, exemplified by a joint report they prepared on the status of NACA lateral control research. Two things were considered of primary importance in judging the effec­tiveness of different control devices: the calculated banking and yaw­ing motion of a typical small airplane caused by control deflection, and the stick force required to produce this control deflection. The report included a table in which a number of different lateral control devices
were compared.[783] Unlike Jones, Weick eventually left the NACA to con­tinue his work in the GA field, producing a succession of designs empha­sizing inherent stability and stall resistance. His research mirrored Federal interest in developing cheap, yet safe, GA aircraft, an effort that resulted in a well-publicized design competition by the Department of Commerce that was won by the innovative Stearman-Hammond Model Y of 1936. Weick had designed a contender himself, the W-1, and though he did not win, his continued research led him to soon develop one of the most distinctive and iconic "safe” aircraft of all time, his twin-fin and single-engine Ercoupe. It is perhaps a telling comment that Jones, one of aeronautics’ most profound scientists, himself maintained and flew an Ercoupe into the 1980s.[784]

The Early Evolution of General Aviation

The Weick W-1 was an early example of attempting to build a cheap yet safe General Aviation airplane. NASA.

The NACA-NASA contributions to GA have come from research, development, test, and evaluation within the classic disciplines of aero­dynamics, structures, propulsion, and controls but have also involved functional areas such as aircraft handling qualities and aircrew

Подпись: Weick's Ercoupe is one of the most distinctive and classic General Aviation aircraft of all time. RPH. Подпись: 8

performance, aviation safety, aviation meteorology, air traffic control, and education and training. The following are selected examples of such work, and how it has influenced and been adapted, applied, and exploited by the GA community.

Gathering the Data for Supersonic Airplane Design

Подпись: 10NACA supersonic research after 1947 concentrated on the practical problems of designing supersonic airplanes. Basic transonic and low supersonic test data were collected in a series of experimental aircraft that did not suffer from the necessary compromises of operational mil­itary aircraft. The test programs were generally joint efforts with the Air Force and/or Navy, which needed the data in order to make reasonable decisions for future aircraft. The X-1 (USAF) and D-558-1 and D-558-2 (Navy) gathered research data on aerodynamics and stability and con­trol in the transonic regime as well as flight Mach numbers to slightly above 2. The D-558-1 was a turbojet vehicle with a straight wing; as a result, although it had longer mission duration, it could not achieve supersonic flight and instead concentrated on the transonic regime. For supersonic flights, the research vehicles generally used rocket engines, with their corresponding short-duration data test points. Other experi­mental vehicles used configurations that were thought to be candidates for practical supersonic flight. The D-558-2 used a swept wing and was able to achieve Mach 2 on rocket power. The XF-92A explored the pure delta wing high-speed shape, the X-4 explored a swept wing that dis­pensed with horizontal tail surfaces, the X-5 configuration had a swept wing that could vary its sweep in flight, and the X-3 explored a futuris­tic shape with a long fuselage with a high fineness ratio combined with very low aspect ratio wings and a double-diamond cross section that was intended to reduce shock wave drag at supersonic speeds. The Bell X-2 was a NACA-USAF-sponsored rocket research aircraft with a swept wing intended to achieve Mach 3 flight.[1056]

Valuable basic data were collected during these test programs appli­cable to development of practical supersonic aircraft, but sustained supersonic flight was not possible. The limited-thrust turbojets of the era limited the speeds of the aircraft to the transonic regime. The X-3 was intended to explore flight at Mach 2 and above, but its interim engines made that impossible; in a dive with afterburners, it could only reach Mach 1.2. The XF-92A delta wing showed promise for supersonic

Подпись: NACA stable of experimental aircraft. The X-3 is in the center; around it, clockwise, from lower left: X-1A, D-558-1, XF-92, X-5, D-558-2, and X-4. NASA. Подпись: 10

designs but could not go supersonic in level flight.[1057] This was unfortu­nate, as the delta winged F-102—built by Convair, which also manufac­tured the XF-92—was unable to achieve its supersonic design speeds and required an extensive redesign. This redesign included the "area rule” concept developed by the NACA’s Richard Whitcomb.[1058] The area rule principle, published in 1952, required a smooth variation in an aircraft’s cross-section profile from nose to tail to minimize high drag normal shock wave formation, at which the profile has discontinuities. Avoiding the discontinuities, notably where the wing joined the fuselage, resulted in the characteristic "Coke bottle” or "wasp waist” fuselage adja­cent to the wing. This was noticeable in supersonic fighter designs of the late 1950s, which still suffered from engines of limited thrust, after­burner being necessary even for low supersonic flight with the resultant

short range and limited duration. The rocket-powered swept wing X-2 Mach 3 test program was not productive, with only one flight to Mach 3, ending in loss of the aircraft and its pilot, Capt. Milburn "Mel” Apt.[1059]

NASA’s Cool Research Continues

Подпись: 12With additional research required on SLDs and engine core ice accre­tion, new updates always in demand for the LEWICE software, and the still-unknown always waiting to be discovered, NASA maintains its research capability concentrated within the Icing Branch at GRC. The branch performs research activities related to the development of meth­ods for evaluating and simulating the growth of ice on aircraft surfaces, the effects that ice may have on the behavior of aircraft in flight, and the behavior of ice protection and detection systems. The branch is part of the Research and Technology Directorate and works closely with the staff of the Icing Research Tunnel and the Twin Otter Icing Research Aircraft. Its mission is to develop validated simulation methods—for use in both computer programmed and real-world experiments—suit­able for use as both certification and design tools when evaluating air­craft systems for operation in icing conditions. The Icing Branch also fosters the development of ice protection and ice detection systems by actively supporting and maintaining resident technical expertise, exper­imental facilities, and computational resources. NASA’s Aircraft Icing Project at GRC is organized into three sections: Design and Analysis Tools, Aircraft Ice Protection, and Education and Training.[1262]


The XV-5B demonstrated that lift-fan aircraft are capable of performing steep simulated instrument approaches with up to 20° flight-path angles. Once more, lack of an integrated powered-lift flight control system was the primary cause of adverse handling qualities and operational limitations. The SSTOVLF’s integrated powered-lift system must provide decou­pled flight path control for glide slope tracking where a sin­gle controller, such as a throttle-type lever is used for direct flight-path modulation while airspeed and/or angle-of-attack are held constant. Simulator evaluations of such systems have indicated significant improvements in handling qualities and reductions in pilot workload, an integrated powered-lift sys­tem a must in a single-piloted SSTOVF.

Evaluations of the XV-5B’s ability to perform simulated instrument landing approaches along a 10° glide slope revealed that pilots preferred to approach with a deck-parallel attitude (near-zero angle-of-attack) instead of using deck-level attitude (near 10° angle-of-attack) instead of 15°. Fan-stall boundary and random aerodynamic lift disturbances were cited as the causes.

SSTOVLF designers should encourage the development of lift – fans with increased angle-of-attack capability which would enhance Instrument Meteorological Conditions (IMC) oper­ational capability and improve safety.

Подпись: 14All pilots that flew the XV-5 (the "XV-5 Fan Club”) were of the unanimous opinion that the conversion handling qualities of the Vertifan were completely unsatisfactory for IMC oper­ations. Trying to contend with the large power changes, atti­tude and altitude displacements, and abrupt airspeed changes while trying to fly instruments with the XV-5’s "manual” con­trol system was too much to handle. The enhanced operational flexibility requirement laid on the SSTOVLF requires that it have full IMC operational capability.

Eluding Aeolus: Turbulence, Gusts, and Wind Shear

Eluding Aeolus: Turbulence, Gusts, and Wind ShearKristen Starr

Since the earliest days of American aeronautical research, NASA has studied the atmosphere and its influence upon flight. Turbulence, gusts, and wind shears have posed serious dangers to air travelers, forc­ing imaginative research and creative solutions. The work of NASA’s researchers to understand atmospheric behavior and NASA’s deriva­tion of advanced detection and sensor systems that can be installed in aircraft have materially advanced the safety and utility of air transport.


EFORE WORLD WAR II, the National Advisory Committee for Aeronautics (NACA), founded in 1915, performed most of America’s institutionalized and systematic aviation research. The NACA’s mission was "to supervise and direct the scientific study of the prob­lems of flight with a view to their practical solution.” Among the most serious problem it studied was that of atmospheric turbulence, a field related to the Agency’s great interest in fluid mechanics and aerody­namics in general. From the 1930s to the present, the NACA and its suc­cessor—the National Aeronautics and Space Administration (NASA), formed in 1958—concentrated rigorously on the problems of turbulence, gusts, and wind shear. Midcentury programs focused primarily on gust load and boundary-layer turbulence research. By the 1980s and 1990s, NASA’s atmospheric turbulence and wind shear programs reached a level of sophistication that allowed them to make significant contribu­tions to flight performance and aircraft reliability. The aviation industry integrated this NASA technology into planes bought by airlines and the United States military. This research has resulted in an aviation transportation system exponentially safer than that envisioned by the pioneers of the early air age.

An Unsettled Sky

When laypeople think of the words "turbulence” and "aviation” together, they probably envision the "bumpy air” that passengers are often

subjected to on long-duration plane flights. But the term "turbulence” has a particular technical meaning. Turbulence describes the motion of a fluid (for, our purposes, air) that is characterized by chaotic, seem­ingly random property changes. Turbulence encompasses fluctua­tions in diffusion, convection, pressure, and velocity. When an aircraft travels through air that experiences these changes, its passengers feel the turbulence buffeting the aircraft. Engineers and scientists charac­terize the degree of turbulence with the Reynolds number, a scaling parameter identified in the 1880s by Osborne Reynolds at the University of Manchester. Lower numbers denote laminar (smooth) flows, inter­mediate values indicate transitional flows, and higher numbers are characteristic of turbulent flow.[1]

Eluding Aeolus: Turbulence, Gusts, and Wind ShearA kind of turbulent airflow causes drag on all objects, including cars, golf balls, and planes, which move through the air. A boundary layer is "the thin reaction zone between an airplane [or missile] and its exter­nal environment.” The boundary layer is separated from the contour of a plane’s airfoil, or wing section, by only a few thousandths of an inch. Air particles change from a smooth laminar flow near the leading edge to a turbulent flow toward the airfoil’s rear.[2] Turbulent flow increases friction on an aircraft’s skin and therefore increased surface heat while slowing the speed of the aircraft because of the drag it produces.

Most atmospheric circulation on Earth causes some kind of turbu­lence. One of the more common forms of atmospheric turbulence expe­rienced by aircraft passengers is clear air turbulence (CAT), which is caused by the mixing of warm and cold air in the atmosphere by wind, often via the process of wind shear. Wind shear is a difference in wind speed and direction over a relatively short distance in Earth’s atmosphere. One engineer describes it as "any situation where wind velocity varies sharply from point to point.”[3] Wind shears can have both horizontal and vertical components. Horizontal wind shear is usually encountered near coastlines and along fronts, while vertical wind shear appears closer to Earth’s surface and sometimes at higher levels in the atmosphere, near frontal zones and upper-level air jets.

Large-scale weather events, such as weather fronts, often cause wind shear. Weather fronts are boundaries between two masses of air that have different properties, such as density, temperature, or mois­ture. These fronts cause most significant weather changes. Substantial wind shear is observed when the temperature difference across the front is 9 degrees Fahrenheit (°F) or more and the front is moving at 30 knots or faster. Frontal shear is seen both vertically and horizontally and can occur at any altitude between surface and tropopause, which is the lowest portion of Earth’s atmosphere and contains 75 percent of the atmosphere’s mass. Those who study the effects of weather on aviation are concerned more with vertical wind shear above warm fronts than behind cold fronts because of the longer duration of warm fronts.[4]

Eluding Aeolus: Turbulence, Gusts, and Wind ShearThe occurrence of wind shear is a microscale meteorological phe­nomenon. This means that it usually develops over a distance of less than 1 kilometer, even though it can emerge in the presence of large weather patterns (such as cold fronts and squall lines). Wind shear affects the movement of soundwaves through the atmosphere by bend­ing the wave front, causing sounds to be heard where they normally would not. A much more violent variety of wind shear can appear near and within downbursts and microbursts, which may be caused by thun­derstorms or weather fronts, particularly when such phenomena occur near mountains. Vertical shear can form on the lee side of mountains when winds blow over them. If the wind flow is strong enough, turbu­lent eddies known as "rotors” may form. Such rotors pose dangers to both ascending and descending aircraft.[5]

The microburst phenomenon, discovered and identified in the late 1970s by T. Theodore Fujita of the University of Chicago, involves highly localized, short-lived vertical downdrafts of dense cool air that impact the ground and radiate outward toward all points of the compass at high speed, like a water stream from a kitchen faucet impacting a basin.[6]

Speed and directional wind shear result at the three-dimensional boundary’s leading edge. The strength of the vertical wind shear is directly proportional to the strength of the outflow boundary. Typically, microbursts are smaller than 3 miles across and last fewer than 15 min­utes, with rapidly fluctuating wind velocity.[7]

Eluding Aeolus: Turbulence, Gusts, and Wind ShearWind shear is also observed near radiation inversions (also called nocturnal inversions), which form during rapid cooling of Earth’s sur­face at night. Such inversions do not usually extend above the lower few hundred feet in the atmosphere. Favorable conditions for this type of inversion include long nights, clear skies, dry air, little or no wind, and cold or snow-covered surfaces. The difference between the inversion layer and the air above the inversion layer can be up to 90 degrees in direction and 40 knots. It can occur overnight or the following morn­ing. These differences tend to be strongest toward sunrise.[8]

The troposphere is the lowest layer of the atmosphere in which weather changes occur. Within it, intense vertical wind shear can slow or prevent tropical cyclone development. However, it can also coax thun­derstorms into longer life cycles, worsening severe weather.[9]

Wind shear particularly endangers aircraft during takeoff and land­ing, when the aircraft are at low speed and low altitude, and particularly susceptible to loss of control. Microburst wind shear typically occurs during thunderstorms but occasionally arises in the absence of rain
near the ground. There are both "wet” and "dry” microbursts. Before the developing of forward-looking detection and evasion strategies, it was a major cause of aircraft accidents, claiming 26 aircraft and 626 lives, with over 200 injured, between 1964 and 1985.[10]

Eluding Aeolus: Turbulence, Gusts, and Wind ShearAnother macro-level weather event associated with wind shear is an upper-level jetstream, which contains vertical and horizontal wind shear at its edges. Jetstreams are fast-flowing, narrow air currents found at cer­tain areas of the tropopause. The tropopause is the transition between the troposphere (the area in the atmosphere where most weather changes occur and temperature decreases with height) and the stratosphere (the area where temperature increases with height).[11] A combination of atmo­spheric heating (by solar radiation or internal planetary heat) and the planet’s rotation on its axis causes jetstreams to form. The strongest jet – streams on Earth are the polar jets (23,000-39,000 feet above sea level) and the higher and somewhat weaker subtropical jets (33,000-52,000 feet). Both the northern and southern hemispheres have a polar jet and a subtropical jet. Wind shear in the upper-level jetstream causes clear air turbulence. The cold-air side of the jet, next to the jet’s axis, is where CAT is usually strongest.[12]

Although most aircraft passengers experience clear air turbulence as a minor annoyance, this kind of turbulence can be quite hazard­ous to aircraft when it becomes severe. It has caused fatalities, as in the case of United Airlines Flight 826.[13] Flight 826 took off from Narita International Airport in Japan for Honolulu, HI, on December 28, 1997.

At 31,000 feet, 2 hours into the flight, the crew of the plane, a Boeing 747, received warning of severe clear air turbulence in the area. A few minutes later, the plane abruptly dropped 100 feet, injuring many pas­sengers and forcing an emergency return to Tokyo, where one passenger subsequently died of her injuries.[14] A low-level jetstream is yet another phenomenon causing wind shear. This kind of jetstream usually forms at night, directly above Earth’s surface, ahead of a cold front. Low-level vertical wind shear develops in the lower part of the low-level jet. This kind of wind shear is also known as nonconvective wind shear, because it is not caused by thunderstorms.

Eluding Aeolus: Turbulence, Gusts, and Wind ShearThe term "jetstream” is often used without further modification to describe Earth’s Northern Hemisphere polar jet. This is the jet most important for meteorology and aviation, because it covers much of North America, Europe, and Asia, particularly in winter. The Southern Hemisphere polar jet, on the other hand, circles Antarctica year-round.[15] Commercial use of the Northern Hemisphere polar jet began November 18, 1952, when a Boeing 377 Stratocruiser of Pan American Airlines first flew from Tokyo to Honolulu at an altitude of 25,000 feet. It cut the trip time by over one-third, from 18 to 11.5 hours.[16] The jetstream saves fuel by shortening flight duration, since an airplane flying at high altitude can attain higher speeds because it is passing through less – dense air. Over North America, the time needed to fly east across the continent can be decreased by about 30 minutes if an airplane can fly with the jetstream but can increase by more than 30 minutes it must fly against the jetstream.[17]

Strong gusts of wind are another natural phenomenon affecting avi­ation. The National Weather Service reports gusts when top wind speed reaches 16 knots and the variation between peaks and lulls reaches 9 knots.[18] A gust load is the wind load on a surface caused by gusts.

Eluding Aeolus: Turbulence, Gusts, and Wind Shear

Otto Lilienthal, the greatest of pre-Wright flight researchers, in flight. National Air and Space Museum.

The more physically fragile a surface, the more danger a gust load will pose. As well, gusts can have an upsetting effect upon the aircraft’s flightpath and attitude.

Surface Management System

Making the skyways safer for aircraft to fly by reducing delays and lowering the stress on the system begins and ends with the short jour­ney on the ground between the active runway and the terminal gate. To better coordinate events between the air and ground sides, NASA devel­oped, in cooperation with the FAA, a software tool called the Surface Management System (SMS), whose purpose is to manage the move­ments of aircraft on the surface of busy airports to improve capacity, efficiency, and flexibility.[261]

The SMS has three parts: a traffic management tool, a controller tool, and a National Airspace System information tool.[262]

The traffic management tool monitors aircraft positions in the sky and on the ground, along with the latest times when a departing air­liner is about to be pushed back from its gate, to predict demand for taxiway and runway usage, with an aim toward understanding where backups might take place. Sharing this information among the traffic control tools and systems allows for more efficient planning. Similarly, the controller tool helps personnel in the ATC and ramp towers to bet­ter coordinate the movement of arriving and departing flights and to

advise pilots on which taxiways to use as they navigate between the runway and the gate.[263] Finally, the NAS information tool allows data from the SMS to be passed into the FAA’s national Enhanced Traffic Management System, which in turn allows traffic controllers to have a more accurate picture of the airspace.[264]