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E· ~ xecut~ve summary Congressional concern over the crash of Pan American World Airways Flight 759, a Boeing 727, minutes after takeoff from the New Orleans International Airport on July 9, 1982, resulted in legislation passed in December 1982 providing that the FAA enter into an agreement with NAS to study and assess the hazards of low-altitude wind shear on takeoff and landing aircraft operations. To accomplish this task the NRC established the Committee on Low-Altitude Wind Shear and Its Hazard to Aviation, consisting of two panels: the Panel on Low-Altitude Wind Variability and the Panel on Aircraft Perfo`~ance and Operations. The committee's principal finding confirmed that low-altitude wind variability (or wind shear) presents an infrequent but highly significant hazard to aircraft landing or taking off. Fortunately, most severe types of wind shear are relatively infrequent, generally short lived, and affect only local areas. Some wind shears have been understood by meteorologists for a number of years. These include those found in gust fronts, warm and cold air-mass fronts, mountain waves, low-level jet streams, gravity waves, terrain-induced turbulence, and sea-breeze fronts. Most are predictable, sometimes hours in advance. The more-skilled pilots recognize the potential presence of these shears and the dangers they pose. Scientists have recently begun to recognize the importance of storm downdrafts that are unusually small in horizontal cross sections and that are of short duration. Such downdrafts have been called microbursts. These often severe but localized events present the greatest danger to aircraft operations. Wind shear that resulted from the strongest microbursts actually measured in the summer of 1982 Joint Airport Weather Studies (JAWS) in Denver could not have been penetrated safely if encountered below 300-500 feet of altitude by an aircraft* during takeoff or landing. *In tints report the term aircraft includes commercial transports and general aviation aircraft, helicopters and airships. 1
Adding to the seriousness of the situation, microbursts are highly transient, existing for only a matter of minutes; thus, prediction and detection are difficult. On the positive side, because of their small size and short duration, an aircraf t encountering a microburs t near a runway i s a re latively rare event. Near-Term Actions Information and Education The committee found that the risks posed by all forms of wind shear can be reduced. This can be accomplished almost immediately by an urgent information and education campaign aimed at all aircraft pilots, including general aviation. Such a program should include warnings to avoid encountering wind shear if possible as well as the best advice available on piloting techniques to minimize risk when a shear is inadvertently encountered at low altitude. As part of this campaign, the FAA should revise and update its 1979 advisory circular on wind shear (AC 00-50A) to incorporate new information. Low-Leve 1 Wind Shear Alert Sys tem Beyond this first step, risks can be reduced by improving and automating the existing Low-Leve 1 Wind Shear Alert System (LLWSAS ~ . This system of ground-level wind sensors currently installed at 59 airports should be upgraded and the improved system should eventually be deployed at all high-traffic density airports with terminal automation systems (153 airports) where there is likelihood of the occurrence of dangerous wind shears. LLWSAS data need to be better interpreted and disseminated quickly to air traffic controllers and pilots. Also, radar observations currently unused as well as pilot reports (PIREPs) should similarly be made available to controllers and pilots in a format that can be easily understood, and pilots and ground personnel should be better trained in their use. Pilot Training Steps also should be taken in the near term to improve our under- standing of how an aircraft responds to wind shear under various piloting techniques and guidance and control systems. These studies should cons ider the effect of aircraft type. The information gained could lead to recommendations for improved training of pilots to cope with wind-shear encounters. Medium-Term Actions Airborne Alert of Wind-Shear Encounters In the medium term, it should be possible to design better guidance and control aids in the cockpit for business and transport aircraft based on existing (but not widely used) sensing and display systems. 2
These systems will not warn pilots of a wind-shear event ahead of their aircraft but will improve their ability to recover from a wind-shear encounter and to avoid an accident when a shear is inadvertently encountered. Long-Term Actions Terminal Doppler Radar There is no single solution to all hazards caused by wind shear. Utilization of a highly automated, ground-based terminal Doppler radar offers considerable hope. A radar system, currently being considered, could use subsystems from the NEXRAD Project, a joint effort of NOAA, DoD, and the FAA. The terminal radars would be located at or near major airports. They would do much to detect all forms of wind shear and would provide adequate warning to pilots. Airborne Remote Detectors Since it is not realistic to assume that all airports might be equipped with Doppler radar, there is a need for airborne detectors that could see ahead of an aircraft and give advance warning of the presence of wind shear. Some systems offer promise, but much further research and development is required to yield a small-sized, lightweight, economical airborne detector. Time Phasing By their nature, actions to reduce wind-shear hazards must be time phased. It should be possible to conduct an education and training campaign in 1 to 2 years. Improvements to the present system will probably take 3 to 5 years to implement. It will probably take 5 to 7 years to develop and deploy terminal Doppler radars at airports. Research on airborne remote sensors can be accelerated immediately, but it is unlikely to yield operational benefits in less than 7 to 10 years. Research Needs Meteorology A number of areas require further research to reduce the hazards of low-altitude wind shear. From a meteorological standpoint, more work is needed to fully understand all forms of wind shear. This should lead to better and more timely forecasts of the phenomena. This effort should include field studies of low-altitude wind shear, particularly those associated with convective clouds and thunderstorms. To help in this understanding, LLWSAS data should be recorded at all sites and analyzed, and existing data from previous research programs should be reexamined. At an appropriate time, another maj or field research program, such as JAWS, should be carried 3
out in the humid southeastern United States to complement that done in the dry midwestern plains. To improve the timeliness and ef fectiveness of present and future warning systems, research is needed on the automation of observations and their display and dissemination to users. An important component of this program would be an operational test project at a major airport that has a high likelihood of wind shear. Aircraft Performance and Operations With respect to aircraft performance and operations, additional analyses and simulation studies are needed on the control of both transport and general aviation aircraft in wind shear. Research is also needed on the effects of heavy rain, which often accompanies wind shear, on all aircraft. Simplified mathematical models of wind shear are needed for use in simulators, which are used to develop aircraft systems and to train pilots. Research is also essential on airborne detectors and on guidance and control systems and their cockpit displays. Work is needed on longer-range airborne warning systems, such as those based on Doppler radar or lidar, that can detect wind shear well ahead of an aircraft. Pilots should be informed on how best to fly their aircraft to escape an accidentally encountered wind shear. Simple instruments, such as angle-of-attack indicators, energy-rate sensors, or vertical-acceleration meters, which could be added to existing aircraft, should be studied as aids in coping with a severe wind-shear encounter. The committee has concluded that wind shear represents a hazard to all aircraft, ranging from small general aviation aircraft to swept-wing jet transports. Much more analysis is needed to bound the problem for the many types of aircraft that exist and their instrumentation. Moreover, general aviation pilots are typically less well trained than air transport pilots, and general aviation pilots have little or no access to advanced training simulators. They often operate from smaller and less well instrumented airports. The most practical and immediate solution appears to be an extensive education program to warn general aviation pilots of the hazards associated with low-altitude wind shear and to teach both avoidance and escape procedures. Finally, the committee recommends that the FAA establish an integrated and sustained program for coping with all aspects of wind shear--meteorological, technological, operational, and educational. 4
RECOMMENDAT IONS Specif ic recommendations are presented below under four broad categories: general, detection and prediction, aircraft performance and operations, and research. The numbering of the recommendations doe s no t s igni f y prior i ty . Genera 1 1. Need For an Integrated Wind-Shear Program To provide for the safety of the flying public, the FM and the aviation industry should address the many facets of the low-altitude wind-shear problem as a whole. The FAA should develop and implement a coherent and sustained program for coping with the educational, meteorological, technological, and operational aspects of low-altitude wind-shear haz ard s . 2. Wind-Shear Education Program The FAA and the industry should prepare and disseminate as widely as possible updated and authoritative information on wind shear. Informational materials should stress avoidance of wind shear and should describe flight control techniques for recovery from encounters. The information should encompass all of types of aircraft, with appropriate guidance for each class. It should include recommendations on the most effective means of training pilots. The FAA should revise and update its 1979 advisory circular (AC 00-50A) on wind shear and the Airman's Infurmution Minuet (AIM) to present the latest information, including detection techniques, alerting and warning procedures, effects of wind shear on aircraft performance, and procedures for recovery from wind-shear encounters. 3. Pilot/Controller Communications The FAA should promote the use of standardized terminology and improved communications between flight crews and control towers. A standardized system of pilot reports (PIREPs) should be developed for reporting low-altitude wind-shear encounters. PIREPs should be mandatory and should include a report of the location, severity, and nature of the shear encountered--in consistent, standardized terminology. Controllers should communicate such reports to all flight crews in the vicinity. In addition, techniques for the direct broadcast to pilots of wind-shear data from LLWSAS or other sensors should be investigated. 5
Wind- She ar De t ec t ion Sys tem Deve 1 opment The FAA should select a site to test direct and remote-sensing techniques in a complete sys tem for detecting low-altitude wind shear and for providing information to pilots and controllers and to test the use of the information in the air traffic control system. The test site should be at a major airport where wind shear conditions are relatively frequent. Detection and Prediction 5. The Low-Level Wind Shear Alert System (LLWSAS) LLWSAS is the only system available in the near term for detecting low-altitude wind shear on an operational basis and every ef fort should be made to assess and improve its performance. Opportunities include, but are not limited to, better signal processing, reduced spacing between and increased number of sensors, improved sensor response and improved wind-display techniques and criteria for issuing wind-shear warnings, and the possible use of ground-based pressure sensors to augment LLWSAS information. 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 basis for modification of current LLWSAS installations and for improved system performance for future installations. Depending on the New Orleans test results, the FM should modify existing LLWSAS s ys tems and ins ta 1 1 improved s ys tems at all high-traffic dens i ty airports with terminal automation systems ( 153 airports) where there is likelihood of the occurrence of dangerous wind shears. 6. Record and Analyze LLWSAS Data LLWSAS wind measurements should be recorded and analyzed to evaluate the system's performance and to learn more about the climatic properties of low-altitude wind shear. This should be done at all airports equipped with LLWSAS. 7. Use of Available Radar Data The existing network of weather radars, operated by the NWS, should be used more effectively to judge the likel ihood of wind-shear conditions. These radars detect rain showers, thunderstorms, and phenomena often associated with wind shear. Information from weather radars should be made available to air traffic controllers in a timely and easily understandable fashion. 6
8. Next Generation Weather Radar (NEXRAD) The next generation Doppler weather radar system (NEXRAD) should be developed and installed with al 1 pass ible speed. This long-range radar system will serve many national needs related to severe-weather detection, forecasting, and warning. For aviation the NEXRAD system can be used to detect and monitor weather situations along flight routes and, if located at or near some airports, to detect low-altitude wind shear or its precursors. Moreover, the Doppler radar will advance the rate of development of radar techniques for the detection of low- altitude wind shear and the development of dedicated Doppler terminal radars. 9. Airport Terminal Weather Radar The FAA should take immediate action to develop a pulsed Doppler radar system that can be used to observe weather conditions at and around airport terminals. This terminal radar system should be able to operate with a high degree of automation and to provide information on low-altitude wind shear, turbulence, and rainfall intensity. Such a radar must be capable of supplying information updated each minute and must have such features as ground-clutter cancellation and adequate spatial resolution. 10. Use of Airport Terminal Weather Radar Observations For terminal Doppler radar to be most useful to traffic controllers and pilots, a concerted effort should be devoted to developing proce- dures for analyzing, displaying, and using its observations. 11. Airborne Remote Sensors Research should continue on the use of airborne Doppler lidars and microwave Doppler radars as a means for detecting low-altitude wind shear. Aircraft Performance and Operations 12. Wind-Shear Effects on Flight Characteristics The FAA should sponsor analytical and simulator investigations to determine: 0 The wind-shear penetration and recovery capabilities of transport aircraft, based on various onboard detection, guidance, and control systems. o The effects of wind shear on various typical categories of general aviation aircraft and helicopters so that authoritative information on their response characteristics and piloting techniques in wind shear can be provided. 7
13 . Aircraf t Operating Procedures The FAA should ensure that air carriers and other commercial operators instruct flight crews on what to do if they inadvertently encounter a low-altitude wind shear during takeoff or landing. In addition, the FAA should encourage operators of jet aircraft to incorporate in their manuals the operating procedures recommended in its advisory circular on wind shear. Aircraft manufacturers should recommend configuration-change sequences (gear, flaps, power, spoilers, etc.) that provide the highest probability for recovery from a wind shear encounter. Pilots should be taught to exceed the normal maximum thrust limits and to go to emergency thrust when necessary. 14. Guidance and Control Aids Onboard sensors and guidance aids should be evaluated in a systematic manner to determine their merits for future development and for possible retrofit in existing aircraft. These include flight director modifications, ground speed/airspeed flight management systems, vertical-acceleration sensors, and energy-rate sensors. Angle-of-attack indicators should be added to the cockpit instrumen- tation of transport aircraft for use in maneuvering through wind shears. Angle of attack should be provided either as a separate variable or as an input to other command displays. Sensors should provide fl ight crews wi th a voice warning o f a hazardous wind shear . 15. Standardization of Wind-Shear Models The FAA should sponsor a program to develop and define standardized models of wind shear based on the latest meteorological data. These models are required for design and certification of aircraft subsystems and for use in training simulators. The FAA should include other government agencies, aircraft manufacturers, commercial operators, and any other interested parties in the program. 16. Certification of Onboard Systems The FAA should update its certification requirements for airborne wind-shear alerting, flight guidance, and automatic control systems. 17. Wind-Shear Simulation Training The FAA and the industry should cooperate to investigate new and innovative ways to make available the best possible simulation training for wind shear to ache largest pass ible number of pilots, including general aviation pilots . 8
Research 18. Effects of Heavy Rain how heavy rain affects the characteristics of aircraft. Particular ~ r - ., attention should be paid to the possible adverse effects of heavy rain on aircraft lift, performance, and controllability, including its effects on wind-shear detection and flight sensor systems. Investigations should continue on low-sDeed aerodynamic 19. The Nature of Low-Altitude Wind Shear More must be learned about the various kinds of wind shear and the meteorological conditions that cause or are associated with them. This knowledge is needed to reduce the hazards represented by low-altitude wind shear. Research should include additional field observations and the construction of theoretical models over the relevant scales--from about 1,000 feet to 10-20 miles and from minutes to hours. The existing body of data obtained by various research programs should be reexamined and augmented, at an appropriate time, by a field program in the humid southeastern United States. Analyses of the data obtained from the JAWS Project should be used to plan any new field investigation. Basic research into the origins of strong thunderstorm downdrafts and possible forecast methods should be an important component of any new program. 9
