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4 Conclusions LOW-ALTITUDE WIND-SHEAR-RELATE1) ACC I1)ENTS From analys es o f a ircra f t ac c ident s where 1 ow-a 1 ~ i tude wind shear was a factor, it appears that ache greatest hazards are caused by downdrafts and outflows produced by convective storms. Serious aircraft accidents have also been caused by terrain-induced and frontal wind shears. Since 1964 the NTSB has documented at least 27- accidents or incidents, 14 of which involved fatalities or serious . . . Ins urges. 2. WIND-SHEAR WARNINGS Pilots now receive inconsistent wind-shear warnings that are of questionable reliability. The effectiveness of warnings is reduced by the inconsistent terminology used by flight crews and control towers. The likelihood of aircraft accidents resulting from low-altitude wind shear can be reduced by improving the ground-based Low-Level Wind Shear Alert System (LLWSAS) and by improving interpretation, communication, and training related to the use of this system. 3 . INADEQUATE U SE OF AVAILABLE INFORMAT1011 The existing network of national weather radars can warn of precipitation, which is sometimes associated with wind shear, but the information is not made available to air traffic controllers in an appropriate or timely fashion. In addition, pilot reporting of wind shears in and around airports is seriously inadequate or not used effectively--a situation that may have led to accidents that could have been avoided. 4. INADEQUATE CLIMATIC INFORMATION To establish how often various wind shears occur, the hazards they pose, and the best ways to detect them, it is necessary to record wind-velocity data at LLWSAS-equipped airports and to analyze other relevant data obtained by suitable radar and airborne equipment. Data 79
on wind-shear frequency and intens ity and on aircraf t turbulence are needed to place detection equipment and to develop forecasting methods. Such data can also be used to construct three-dimensional wind models. 5. WIND SHEAR ASSOCIaTED WITH CONVECTIVE STORMS Mos t detailed observations of low-altitude wind shear have come from short-term research pro jects conducted near O'Hare International Airport (NIMROD Pro ject ~ in Chicago and Stapleton International Airport (JAWS Project) in Denver. A network of wind sensors detected 186 microburs ts in 49 days over the 86-day operational period of JAWS. The Doppler radar sets used in the JAWS Project observed 75 microburs ts on 33 days out of a total of 86 days during which the radars were operating. The mic~.robursts had short lifetimes (2-10 minutes ~ and were nearly randomly distributed over the roughly 600-square-mile area of observation. As a consequence, the JAWS data showed that the 1 ikel ihood o f a dangerous microburst occurring over a runway or approach to the Stapleton Airport presented a small but not insignificant hazard. 6 . LLWSAS AS A PARTIAL SOLUT ION LLWSAS can detect wind shears, such as occur in gust fronts, air-mass frontal passages, solitary waves, and sea-breeze fronts that typically spread across many miles, persist for 10 or more minutes, and travel across the ground, but it is inadequate for detecting microbursts. By improving the spatial and time resolutions of the surface-wind measurements, LLWSAS should be better capable of detecting the more dangerous wind-shear conditions in the vicinity of airports. In particular, the LLWSAS signal processing needs major improvement to increase the system's ability to discern the presence of small-scale wind shears, such as microbursts, with greater accuracy and reliability. An improved LLWSAS system is being developed for installation at New Orleans International Airport. This upgraded system, to be operationally tested in early 1984, should provide the teas is for modification of current LLWSAS installations and for improved system performance for future installations. 7. USE OF PRESSURE SENSORS Pressure sensors may be able to augment the ability of surface anemometer arrays, in some meteorological circumstances, to detect low-altitude wind shears. Full definition of their potential has not yet been established. Because of their simplicity and low cost, pressure sensors might supplement LLWSAS arrays at minimal expense. 8. GROUND-BASED PULSED DOPPLER RADAR A pulsed Doppler radar at a suitable microwave frequency can detect and make quantitative measurements of many, perhaps most, of the low-altitude wind shears that represent a hazard to aircraft. 80
This conclusion is based on the results of the NIMROD and JAWS projects as well as research at the NSSL and the NCAR. 9. THE NEXT GENERATION WEATHER RADAR (NEXRAD) NEXRAD, a pulsed Doppler radar designed to operate at a wavelength of 10 centimeters, is urgently needed as an all-purpose weather radar. It can be used to measure precipitation, to detect and track storms, and to identify the precursors of low-altitude wind shear. Although not intended to sense wind shears directly at the required 1- to 2-minute repetition frequency, NEXRAD's advanced technology will contribute substantially to the development of a radar for sensing wind shear at airport terminals. 10. FAA TERMINAL RADAR High data-collection rates and density of coverage are needed to detect low-altitude wind shear near airports. This requirement dictates a terminal-dedicated Doppler radar system to complement the information that NEXRAD will provide. This will require judgments to be made on such factors as the optimum wavelength, type of antenna, beamwidth, scanning mode, data analysis, transmission, and display and on the most appropriate place to site the antenna--on or off the airport. 11. AUTOMATION OF WIND-SHEAR SENSING AND COMMUNICATIONS Whereas a number of viable techniques appear suitable for sensing low-altitude wind shear, it is critical that the data be displayed for controllers and pilots in a reliable, unambiguous manner. Automation appears vital because of the need to update the data rapidly. In addition, the information needs to be presented in a graphical and/or digital format that can be easily understood by controllers and pilots. 12. CURRENT AIRBORNE WIND-SHEAR WARNING SENSORS An ultimate solution to the wind-shear problem would be a practical airborne system for detecting wind velocities ahead of an aircraft and for displaying wind-shear information in an easily comprehensible form. Current airborne sensors include forward-looking, continuous-wave laser radar (CW-lidar); pulsed lidar; passive infrared (JR) radiometric sensing; and microwave pulsed Doppler radar. The CW lidar system can sense the headwind/tailwind component of the wind but only up to about 1,000 feet ahead of an aircraft. Pulsed lidar should be explored to see if a practical system can be developed to sense shears up to, say, 2 miles. Although more investigation is warranted, the passive IR device seems unlikely to provide an unambiguous indication of the presence of wind shear, because the shear may exhibit local cooling, warming, or no temperature change at all. 81
Several manufacturers are building turbulence-measuring Doppler radars and some air carriers are using them. However, these radars cannot detect wind shear (i.e., wind-velocity differences over a known distance) in either precipitation or clear air. A program of develop- ment and testing could establish if it is technically feasible to develop a practical airborne pulsed Doppler radar to detect wind shear even at low altitudes where ground clutter presents special problems. 13. WIND-SHEAR PREDICTION Certain types of wind shear, such as those produced by air-mass fronts, sea breezes, and low-level jet streams, can be predicted some hours in advance with some degree of accuracy. There also appears to exist some ability to predict the occurrence of convective clouds and thunderstorms that may generate downdrafts and associated hazardous small-scale wind shears. However, the downbursts that pose the most severe hazards to aviation have extremely short lifetimes, and their initiation can be detected only a few minutes in advance. Additional research is essential to improve the ability to predict these events. 14. PILOT AWARENESS OF WIND-SHEAR HAZARD There is widespread lack of awareness among pilots as to the origins, nature, and potential hazards of downbursts and wind variability. The problem is particularly acute in the general aviation community. This is due to the diversity of skill levels and training of operators--ranging from the operators of highly sophisticated multiengine corporate jet transports to recreational flyers of small single-engine airplanes. 15. FAA EDUCATIONAL INFORMATION The FAA needs to augment and better distribute its manuals, circulars, and films on wind shear. The FAA's 1979 advisory circular on low-altitude wind shear (AC 00-50A) remains the basic source of information for pilots on this subject. It describes the meteorological phenomena, how to recognize wind shears, their effects on aircraft performance, and procedures for recovery from wind shear encounters. Since its publication, much new information has been generated on the nature and characteristics of wind shear and its detection. In addition, alerting procedures have been instituted. This advisory circular should be revised and updated. The FAA Airman's Information Manual (AIM) contains minimal information on wind shear. disseminated widely. 16. OPERATING PROCEDURES The AIM needs to be expanded, updated and The Federal Aviation Regulations (FAR) for air carriers and the implementing procedures appear to be adequate regarding aircraft operations and pilot training for wind-shear encounters. There are 82
inconsistencies, however, among operators of all categories of aircraft as to the preferred techniques to recognize, cope with, and tra in for wind-shear encounters . Some air carriers provide very extens ive instructions in their flight operations manuals, while others are far less complete. Both content and uniformity of terminology need improvement to ensure general unders Landing among pilots of aspects of flight at high angles of attack, near stick- shaker speed, and when emergency engine power is recommended. In many cases, insufficient emphasis is placed on the potential severity and hazard from s bong wind shears and the importance of their early recognition and immediate reaction to strong shear conditions. 17. SIMULATOR TRAINING There is no FAA requirement for specific flight training for wind-shear encounters. Because pilots cannot practice wind-shear encounters in an aircraft, simulator training is the only means for helping pilots cope with inadvertent wind-shear encounters and acquire an appreciation of the seriousness of such encounters. However, only a few pilots receive training on "advanced" simulators meeting F. M specifications. Simulators that come under FAR 121, Appendix a, are the only ones required by FAA to have wind-shear training capability. 18. WIND-SHEAR MODELING . . . Wind-shear models currently being used in the design, development, and certification of aircraft flight control and avionics systems and in pilot training do not accurately portray actual wind-shear situations identified in JAWS and other studies. Adequate meteorological data exist to build wind-shear models required for aircraft flight control systems; avionic system design, development, and certification; and for use in simulators used for flight training and pilot qualification. Development of new wind-shear models requires coordination among manufacturers, operators, and the government. Simplified models that can be easily manipulated to vary shear severity and encounter conditions are needed. Four-dimensional models, which represent as accurately as possible the time dependence of measured wind shears, are of interest and importance in research. However, the use of time dependence in these models greatly increases the compu- tation capacity required and are not required for most nonresearch applications . 19 . AI RCRAFT PERFORMANCE AND FL IGHT CHARACTE RI ST I C S Some low-altitude wind shears are so severe that they cannot be successfully penetrated by any aircraft. The risk involved, however, depends on the severity of the shear and the altitude of an aircraft's entry. It also depends on the aircraft's performance capability, pilot recognition and react ion, and the technique used to recover. The probability of successfully penetrating an inadvertently encountered wind shear can be increased with improved pilot awareness, warning, guidance systems, and piloting techniques. 83
Many pilots misunderstand or are confused about the proper techniques for recovery from wind-shear encounters. Such procedures may be contrary to normal experience and training. For example, arresting the descent of large commercial transports during recovery from shear encounters during either takeoff or landing may require extreme nose-high attitudes and sustained flight near stall angles of attack. Since both general aviation aircraft and swept-wing transport aircraft are vulnerable to a significant extent to wind shears, the hazards of encounters need to be publicized widely throughout the entire aviation community. There has been little examination of the effects of low-altitude wind shear on general aviation airplanes and helicopters. 20. HEAVY RAIN Hazardous wind shear is sometimes accompanied by heavy rain, which may adversely affect aircraft aerodynamic characteristics and, hence, flight performance. The magnitude of the effect has not been firmly establishede Rain may be a significant factor to consider in the analysis of wind-shear encounters and the establishment of recommended procedures for recovery. 21. GUIDANCE AND CONTROL AIDS To improve the probability of recovery from inadvertent wind-shear encounters, aircraft should be equipped with devices that warn pilots and instrument displays that augment pi lot control . The technology exists for an aircraft to "sense" that it is in a wind shear, although this technology has not been widely implemented. Optimum guidance in wind-shear encounters requires precise control of the angle of attack, a parameter that is seldom displayed in the cockpit . Many modern jet transports have air data sensors and inertial reference sys tems that could be used for both wind-shear warning and guidance displays. The newer transport aircraft have autopilots and flight direc tars that incorporate control laws for coping with wind shear. Further improvements in cockpit displays, guidance and control logic, and innovative use of primary and auxiliary controls could be made to alert pilots earlier to a wind-shear encounter and to augment pilot control for recovery. There are retrofit possibilities for older transport and general aviation aircraft that could improve pilot performance in wind-shear encounters. 84
