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1 Introduction Low-altitude wind variability, or wind shear,* has long been recognized as a potential hazard to aircraft landing and taking off. Although wind shear can result from a number of basically different meteorological conditions, pilots have been trained to avoid thunderstorms in particular because of often associated severe wind variability and turbulence near the ground and aloft. It has recently been recognized that small, short-lived downdrafts, called microbursts, are serious hazards to aircraft during landings and takeoffs. In some microbursts the air carried downward strikes the ground and spreads out in a shal low layer--sometimes only a few hundred feet in thicknes s. The parent cloud from which the microburst descends is a convective one but one that has not necessarily grown to thunderstorm size and strength. Thunderstorm outflow and accompanying downdrafts, some of the scale and intensity that have recently been named microbursts and downbursts, were identified by the Thunderstorm Pro ject nearly 40 years ago (Byers and graham, 1949~. Such outflows and downdrafts were newly emphasized as the cause of some serious accidents after a quantitative analysis of the winds encountered by Eastern Airlines Flight 66 while landing at John F. Kennedy International Airport on June 24, 1975. Analysis of the flight recorder data from another aircraft operating in the immediate vicinity provided a wind model considered to be very similar to that encountered by EAL Flight 66. A detailed map of the wind-shear patterns at the time of the crash was constructed from an analysis of available data, including meteoro- logical satellite photographs and surface weather observations and measurements (Fujita, 1976; Lewellen et al., 1976~. The analysis provided valuable insight into the characteristics of violent downburst -Inless specified otherwise in this report, wind shear is the differ- ence of wind velocity at two points divided by the distance between the two points. 11

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cells within thunderstorms, the need to detect their presence as early as possible, and the need for immediate communication of warnings to air traffic controllers and flight crews in the vicinity. Incident/Accident Records \ In 197 7 the FAA conduc ted a s tudy o f NTSB repor t s on a ircraf t accidents and incidents related to low-altitude wind shear that occurred from 1964 through 1975 (Shrager, 1977~. More than 59,000 reports were reviewed, covering all classes of civil aircraft and flight operations. About one-third of the accidents or incidents, more than 19 ,000, occurred during terminal area operations. Only 25 accidents or incidents involving large aircraft (more than 12,500 pounds) were identified in which low-altitude wind shear could have been a contributing factor. Of these 25 cases, 23 occurred during approach or landing and only 2 during takeof f . Table 1 lists 27 U.S. aircraft accidents or incidents that occurred from 1964 to 1982 and that are at tributed to low-al titude wind shear. The list includes most of the 25 cases identified by the FAA. Some were omitted because, on further examination, they could not be attributed to wind shear. The table does include wind-shear-re lated accidents or incidents that have oc curred s ince 1976, including 2 during 1982. In 1981 general aviation aircraft numbered more than 200,000 and flew more than 40 million hours (compared with 3,973 aircraft and 8 million flight hours for air carriers). General aviation operations accounted for 662 fatal accidents from all causes, with 1,265 fatalities (FAA, 1981~. Informal accident cause/factor statistics from the NTSB for 1981 indicate that weather caused or was a related factor in 40 percent (289 cases) of the U.S. general aviation accidents. Of these, wind shear was reportedly the cause of one fatal accident and was a factor in two. It should be noted that the NTSB generally investigates only those general aviation accidents that result in a fatality, and not all of those attributed to weather were analyzed by trained meteorologists. Low-altitude wind variability may have been a factor in some of these. In 1975, NASA, in cooperation with the FAA, instituted the Aviation Safety Reporting System (ASRS), whereby safety-related incidents involving aircraft operations are submitted voluntarily and treated anonymously, with the expectation that potential flight safety problems may be identified and corrective action suggested. A total of 26 reports have been indexed as wind shear related out of nearly 21,600 reports received since May 1, 1978. Of these, 17 appear to involve wind shear as a primary factor. 12

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A recent study (Anderson and Clark, 1981) of the effects of wind shear on aircraft operations and flight safety in Australia, including an extensive survey of pilots, concluded that wind shear was a causal or contributory factor in numerous aircraft accidents in Australia and elsewhere and that inadequate knowledge of wind structure and of the resulting effects on aircraft operations constitutes a flight safety hazard. Furthermore, the term wind shear is subject to various - interpretations among pilots, and specific definitions are often misunderstood. Pilot judgments as to the aircraft types most suscep- tible to wind shear were not readily explicable in terms of aircraft size, landing speed, or wing loading. The use of standard terminology and improved training for pilots and air traffic controllers was recommended, along with research on optimal piloting techniques during wind-shear encounters. In the United Kingdom the Royal Aircraft Establishment has undertaken a program to extract wind-shear data from records obtained from 10 Boeing 747 aircraft operated throughout the world by British Airways. (Haynes, 1980; Woodfield and Woods, 1981~. This is a continuing effort to obtain wind information on strong wind-shear events during approach and landing. Time histories of wind velocities and aircraft reactions to interesting events are identified and analyzed. The results may lead to statistics on the probabilities of encountering wind shears and criteria for testing and evaluating autopilots and onboard wind-shear detection systems. The rarity and lack of a reliable statistical data base on wind- shear-related accidents, shear encounters, or even the frequency of occurrence of potentially hazardous wind shears does not diminish the importance or severity of the safety problem. The potentially catas- trophic consequences of an encounter during takeoff or approach and landing require that wind shear always be taken into account as a primary safety consideration when weather conditions are such that strong wind shears may be present. The widespread lack of appreciation among pilots, traffic controllers, and aircraft operations personnel of the seriousness of the possible safety hazards has exacerbated the problem. Reports by the NTSB of investigations of air carrier accidents at least partly attributable to wind shear have resulted in a series of specific safety recommendations by the NTSB to the FAA. These recom- mendations are routinely considered and acted on by the FAA and followed up by the NTSB. Together with other FAA activities, these have contributed to a compendium of FAA actions with respect to wind shear. The NTSB's report of the investigation of the Pan American World Airways Flight 759 accident that occurred on July 9, 1982, contained 14 recommendations for priority and longer-term action intended to improve safety in wind-shear weather conditions (NTSB, 1983). 13

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FAA Wind-Shear Program Activities In 19 71 the FAA inn' fated a program to work on the prob lem of wind shear in coordination with other organizations working in the field. Several areas of investigation were addressed, including wind-shear forecasting techniques and means of detecting the presence of wind shear with both ground-based and airborne instrumentation. A multiphased research and development program was undertaken to investigate and develop cockpit displays, instrumentation, and operational procedures for assisting a pilot in the event of a wind-shear encounter. The project involved development of wind-shear models and evaluation of cockpit instrumentation, various cockpit instrument panel display configurations, and flight-path management systems in moving-base simulations of the flight of various large transport airplanes in wind shear. The results have been published in a series of reports (e.g., Foy, 1979~. In 1976 a wind-shear detection system called the Low-Level Wind Shear Alert System (LLWSAS) was developed (Goff, 1980) , and instal- lations are now in operational use at 59 ma jor airports ~ see Table 2 ~ . Also, the FAA published an advisory circular (AC 00-50 ), entitled Low Level Wind Shear, dated April 18, 1976, intended to provide guidance for recognizing the possibilities of hazardous wind-shear situations and piloting techniques for recovery from wind-shear encounters. Detailed research on the nature and characteristics of downbursts, sponsored by the FAA together with the NWS, NSF, and NASA has been undertaken. Project NIMROD conducted by the University of Chicago in the north-central midwestern United States during 19 78-19 79 and the JAWS Project in the Denver area during the surfer of 1982 have provided extensive new knowledge on the meteorological characteristics of wind shear required for more realistic computer modeling of wind-shear fields for flight simulation, instrument design and development, and system certification. In May 1977 the FAA amended Part 121 of the Federal Aviation Regulations [FAR 121.601 (b) ~ to require air carriers to adopt an approved system for obtaining weather forecasts and reports of adverse weather conditions, including low-altitude wind shear, at each airport used in their operations. In support of this rule, FAA inspectors were directed to ensure that the air carriers provided pilot training for adverse weather operations, applying the information on wind-shear hazards contained in the FAA's Advisory Circular AC 00-50 . In 1979 the FAA published an updated advisory circular (AC 00-50A, dated 1/23/79 ~ and developed a pilot training film to provide detailed information, guidance, and training to cope with wind shear during takeoff or landing operations, based on newly acquired data. In May 1979 the FAA issued an advance notice of proposed rulemaking (NPRM 79-11 ) to invite public discussion and to solicit comments as to the 16

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TABLE 2 Location of Low-Level Wind Shear Alert System (LLWSAS) Installations IN OPERATION (59 UNITS) Albuquerque, NM Atlanta, GA Baltimore, MD Birmingham, AL Boston, MA Buffalo, NY Charlotte, NC Chicago (O'Hare), Cincinnati, OH Cleveland (Hopkins), OH Columbus, OH Dallas/Ft. Worth, TX Dayton, OH Denver, CO Des Moines, IA Detroit (Metro. ), MI Ft. Lauderdale (Ins. ) , FL Houston (Int.), TX Houston, TX TX TO Be INSTALLED (51 UNITS) Indianapolis (Int.), IN Jackson, MS Jacksonville, FL Kansas City (Int.), MO Knoxville, TN Las Vegas, NV Little Rock, AR Los Angeles, CA Louisville, KY Memphis (Int.), TN Miami, FL Milwaukee, WI Minneapolis (Int.), MN Mobile, AL Nashville, TN New Orleans, LA New York (Kennedy) NY New York (LaGuardia) Newark (Int.), NJ Norfolk, VA Oklahoma City, OK Omaha, NE Orlando (Int.), FL Philadelphia (Int.), PA Phoenix, AZ Pittsburgh (Int.), PA Raleigh-Durham, NC Roanoke, VA Rochester, NY St. Louis (Int.), MO Salt Lake City, UT San Antonio, TX San Juan, PR Sarasota, FL Tampa, FL Tulsa, OK Washington (Dulles), VA NY Washington, (National), VA W. Palm Beach, FL Wichita, KS Albany, NY Fayetteville, NC Montgomery, AL Asheville, NC Fort Smith, AR Pensacola, FL Augusta, GA Fort Myers, FL Peoria, IL Austin, TX Grand Rapids, MI Richmond, VA Baton Rouge, LA Green Bay, WI Rochester, MN Billings, MT Greensboro, NC San Francisco, CA Bristol, TN Greer, SC Savannah, FA Cedar Rapids, LA Honolulu Oahu, HI Shreveport, LA Charleston, SC Huntsville, AL Sioux City, LA Charleston, WV Lansing, MI Sioux Falls, SD Chattanooga, TN Lexington, KY Springfield (Capitol), IL Colo Spgs, CO Lincoln, NE Springfield, MO Columbia, SC Lubbock, TX Syracuse, NY Columbus, GA Madison, WI Tallahassee, FL Dallas-Love, TX Midland, TX Toledo, OH -~ Daytona Beach, PL Moline, IL Tuscon, AZ E1 Paso, TX Monroe, LA Windsor Locks, CT Source: FAA. 1983 17

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need to amend FAR 121 to require large air carrier aircraft to utilize wind-shear detection equipment or to take other actions to provide practical, effective, and reliable detection of hazardous wind shears. No regulatory action has yet been taken directly in response to this proposal. In this connection, however, the FAA has prepared an advisory circular presenting criteria for operational approval of airborne wind-shear alerting and flight guidance systems and wind-shear detection and avoidance systems. These proposed criteria, including presently available mathematical models of a variety of wind-shear and turbulence fields, are intended to permit FAA acceptance of concepts designed to enable pilots to recognize the presence of wind shear, to optimize their reactions, and to fully utilize the performance capabilities of their aircraf t to cope with a wind-shear hazard that may be encountered . The circular provides that the wind-shear models will be updated as new data become available. This advisory circular is currently under review, preparatory to its adoption. Air traff ic control procedures used by the FAA relative to wind shear include the use of meteorological forecasts, surface and upper-air weather and weather radar observations, voluntary pilot reports (PIREPs ~ of wind-shear encounters, and LLWSAS. 18