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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.
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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.
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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
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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.
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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.
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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.
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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.
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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 .
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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.
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Representative terms from entire chapter:
doppler radar
