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| Copyright © 2009. National Academy of Sciences. All rights reserved. Terms of Use and Privacy Statement |
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OCR for page 79
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
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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.
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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
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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
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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.
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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
Representative terms from entire chapter:
wind shears
