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Background
Since the beginning of manned
flight, low-altitude encounters with atmospheric
turbulence and gusts have been among the most challenging
safety issues facing aircraft operators, air traffic
controllers, and the aerospace engineering community.
The prediction, detection, and avoidance of potentially
hazardous wind conditions have been a high priority
technical target internationally. Major wind-induced
accidents caused by the inability of the pilots
to maintain aircraft performance and control have
historically plagued the entire spectrum of civil
aircraft types, including large commercial transports,
regional airliners, business jets, and small personal-owner
general aviation vehicles.
In the 1970s and 1980s, an
alarming number of fatal accidents in the United
States and abroad were attributed to the phenomenon
known as wind shear, defined as any rapid change
in wind direction or velocity. Severe wind shear
is a rapid change in wind direction or velocity
and causes horizontal velocity changes of at least
15 m/sec over distances of 1 to 4 km, or vertical
speed changes greater than 500 ft/min. About 540
fatalities and numerous injuries resulted from
wind-shear crashes involving 27 civil aircraft
between 1964 and 1994. Wind shear also caused numerous
near accidents in which the aircraft recovered
just before ground contact.
Research focused by industry,
government agencies, academia, and airlines on
this major threat to aviation safety in the 1980s
resulted in a vastly improved fundamental knowledge
of the atmospheric environment and the critical
properties associated with wind shears. In particular,
the experimental and analytical efforts of meteorologists,
coupled with analyses of piloting strategies during
wind-shear encounters, pilot training, and the
development of ground-based and airborne sensing
technology, paved the way for technical solutions
to mitigate this serious problem. Prior to this
concentrated research effort, a gust front, or
a leading edge of rain-cooled air, was widely believed
to be the main wind-shear threat presented by thunderstorms
to aircraft in takeoff or landing. A gust front
is formed along the leading edges of large domes
of rain-cooled air that result from cold downdrafts
from individual thunderstorm cells. At the leading
edge of this gust front, a dynamic clash occurs
between the cool outflowing air and the warmer
thunderstorm inflowing air and produces the familiar
wind shift, temperature drop, and gusty winds that
precede a thunderstorm.
However, extensive studies
of thunderstorm and related downburst phenomena
in the vicinity of airports by the late meteorological
researcher, T. Theodore Fujita of the University
of Chicago, played a key role in reversing the
incorrect implication of the gust front to aircraft
accidents. Fujita, who developed the tornado severity
scale that bears his name, had conducted extensive
studies on airline crashes. A key causal factor
in his analysis was the generation and effects
of a rapidly descending vertical column of air
formed when air at high altitudes quickly cools
due to the evaporation of ice, rain, or snow. Fujita
submitted that a concentrated, strong three-dimensional
outflow associated with the ground impact of the
downdraft was the real fatal hazard in aircraft
encounters. Although not totally technically correct
in details, a layman’s interpretation of
this physical phenomenon is the flow from a water-hose
nozzle directed straight at a driveway, producing
a spray of water in all directions. In this simplified
model, the impact pressure field causes the downflow
component to decelerate as air approaches the surface,
and the horizontal component of the wind to accelerate
outward from the impact center. But Fujita’s
theory of a critical vertical “downdraft”
in the mid-1970s was highly controversial at the
time. Subsequently, photographic evidence of the
phenomenon was obtained, and Fujita coined the
name “microburst” for it. Fujita defined
a microburst as a relatively small downburst whose
outward, damaging winds extend no more than 4 km
(2.2 nmi) over the surface. Radar meteorologists
have redefined a microburst as a divergent low-level
wind field with a velocity change of at least 15
knots over a distance between 1 and 4 km. The microburst
exhibits severe, low-altitude wind-shear gradients
that are experienced by a landing aircraft as rapid
changes in the relative wind vector, sometimes
to an extent that the performance capabilities
of the airplane are exceeded, which results in
ground impact. Roughly half of microbursts, as
defined by radar meteorologists, are truly hazardous
to aircraft.
Cross-sectional view of
microburst.
Aircraft landing in microburst
first experiences
headwinds followed by downdraft and tailwinds.
Another characteristic feature
of the microburst is air circulation in the form
of a vortex ring surrounding the downdraft core.
This vortex ring contains strong outflow winds
that contribute to the larger hazards caused by
horizontal shears and vertical winds over scales
between 1 and 4 km. Most microbursts last for a
few minutes, and generally less than 10 min. Microbursts
can occur anywhere convective weather conditions
(thunderstorms, rain showers, or virga) occur.
Virga is rain that evaporates before it reaches
the ground and is associated with a “dry”
microburst. The terms “microburst”
and “wind shear” are often used interchangeably
because the vast majority of dangerous wind shears
result from microbursts.
An aircraft flying through
a microburst may experience extremely hazardous
airspeed fluctuations. As the aircraft enters the
edge of the downburst outdraft, it initially encounters
an increased head wind. This head wind increases
the lift of the aircraft and, therefore, the altitude
of the aircraft. If the pilots are unaware that
this speed increase is caused by wind shear, they
are likely to react to correct the aircraft approach
angle by reducing engine power. The aircraft then
passes into the vertically descending microburst
core, where it encounters an abrupt change from
head winds to downflow winds, which results in
a loss of lift and altitude. Immediately thereafter,
the aircraft crosses into a region of tail winds.
This wind change reduces the relative airspeed
of the aircraft and further decreases lift, which
causes the aircraft to lose more altitude. Because
the aircraft is now flying on reduced power, it
is vulnerable to sudden losses of airspeed and
altitude. The pilots may be able to escape the
microburst by adding power to the engines, but
if the engine response time is not rapid or if
the shear is strong enough, the aircraft may crash.
Photograph of vortex ring
preceding downdraft core in microburst. Photograph
”1991 William Bunting.
Obviously, technology that
permits the detection and avoidance of severe wind-shear
conditions is a key element in the national air
transportation system. Working with industry, academia,
and the FAA, Langley researchers provided key concepts
and the validation of advanced airborne detection
systems that have been implemented by airlines
in the 1990s. As a result of these breakthrough
efforts, wind-shear accidents have been virtually
eliminated for large commercial transports.
Several excellent technical
summaries that form the basis for the following
brief discussion of Langley’s contributions
and technical leadership for this critical national
program are available, especially those by Roland
L. Bowles, P. Douglas Arbuckle, Michael S. Lewis,
David A. Hinton, Fred H. Proctor, and Lane E. Wallace
(see bibliography).
Langley Research and Development
Activities
At Langley, initial interests
in wind-shear studies were stimulated by a series
of tragic accidents during the early 1980s. The
national concern and technological challenges resulting
from these accidents urgently demanded the identification
of potential solutions that would eliminate future
occurrences of such horrible events.
The first accident involved
a Pan American Boeing 727 that attempted to depart
the New Orleans International Airport on July 9,
1982, during a severe thunderstorm. Witnesses observed
the aircraft climb after takeoff to an altitude
of about 100 ft when it began to descend, striking
some trees about 2,400 ft past the end of the runway,
crashing into a residential area, and demolishing
six houses. All 145 onboard the aircraft were killed,
as well as 8 people in the residential area. The
probable cause of the accident was identified as
“The airplane’s encounter during the
liftoff and initial climb phase of flight with
a microburst-induced wind shear which imposed a
downdraft and a decreasing head wind, the effects
of which the pilot would have had difficulty recognizing
and reacting to in time for the airplane’s
descent to be arrested before its impact into trees.
Contributing to the accident was the limited capability
of current ground-based low-level wind-shear detection
technology to provide definitive guides for controllers
and pilots for use in avoiding low-level wind-shear
encounters.” Unfortunately, the Low Level
Windshear Alert System (LLWAS) developed by the
FAA in 1976 did not provide adequate warning in
this particular accident. The early LLWASs were
more “gust front” detectors than microburst
detectors and had a sparse array of sensors. These
sensors were only installed around the airport,
not 1 or 2 miles away where needed. These LLWAS
sensors were also installed before the physical
characteristics of microbursts were common knowledge.
Following the accident, the
FAA contracted with the National Academy of Sciences
(NAS) to review the technological state of the
art in wind-shear alerting systems and to define
technical options that might be used to mitigate
and reduce wind-shear induced accidents. The NAS
participants in the study met at the Langley Research
Center in 1984, and participation in the ensuing
discussions stimulated Langley researcher Roland
L. Bowles and others to analyze and brainstorm
the wind-shear issue. Bowles and his peers in the
FAA began to plan a joint technical program to
address one of the major deficiencies cited in
the NAS report—the need for accelerated research
on airborne wind-shear detection systems—especially
forward-looking systems that could provide adequate
warning to the pilot before a wind shear was encountered.
Unfortunately, before Langley
could assemble enough momentum to aggressively
address airborne wind-shear detection, a second
horrifying accident was thrust into the national
spotlight. On August 2, 1985, a Delta Airlines
Lockheed L-1011 approaching the Dallas-Fort Worth
International Airport in a thunderstorm that included
heavy rain and lightning encountered a microburst.
The aircraft touched down in a field about 6,000
ft short of the runway, bounced, struck a car on
a highway, collided with two water tanks, broke
up, and burst into flames. The human toll was 137
fatalities. Subsequent analysis indicated that
the pilot was able to traverse the downdraft winds,
but the aircraft crashed as it tried to fly into
the outflow winds that contained high velocity
tail winds. The probable cause was stated as “The
flight crew’s decision to initiate and continue
the approach into a cloud which they observed to
contain visible lightning; the lack of specific
guidelines, procedures, and training for avoiding
and escaping from low-level wind shear; and the
lack of definitive, real-time wind-shear hazard
information. This resulted in the aircraft’s
encounter at low altitude with a microburst-induced,
severe wind shear from a rapidly developing thunderstorm
located on the final approach course.” The
ground-based LLWAS wind-shear warning system finally
detected the microburst a full 2 min after the
aircraft had crashed.
Immediately following the
accident, national pressures on Congress to provide
solutions to these traumatic accidents resulted
in a fact-finding visit by Congressman George Brown
of California to Langley for a briefing on wind-shear
research. At the time, the most directly relevant
NASA research was being conducted in the area of
piloted simulation technology, involving cloud-scale
modeling. Jeremiah F. Creedon, then Director for
Flight Systems, briefed Brown on Langley’s
potential plans for a technical attack on the problem.
An enthusiastic endorsement by Brown of the concepts
and capabilities offered by Langley resulted from
this briefing. Brown carried his impressions of
the briefing back to Congress where he played a
key role in pursuing support for wind-shear research.
Subsequently, in 1986, the FAA announced a National
Integrated Windshear Plan, which included Langley
as the lead organization for airborne wind-shear
detection research under a joint NASA and FAA Airborne
Windshear Program. At Langley, wind-shear research
was organized under the leadership of Roland L.
Bowles. Bowles and his team quickly formulated
analyses, simulations, laboratory tests, and flight
tests that would help the FAA reach the objective
of certifying predictive wind-shear detection systems
for installation on all commercial aircraft. The
program consisted of three main elements: hazard
characterization, sensor technology, and flight
management systems. This joint program was later
expanded in 1990 to include the integration of
both airborne and ground-based detection technology.
Hazard Characterization
One critical and fundamental
element of Langley’s Wind-Shear Program was
a major effort aimed at understanding the detailed
characteristics and relative hazard of microbursts.
The use of Langley’s supercomputer capability
had already been instrumental in the development
of a mesoscale numerical weather model by Langley
contractor, Michael L. Kaplan, and others in the
early 1980s. This sophisticated weather model became
a useful research tool in understanding large severe
storms events, and it also became the concept for
the National Weather Service to develop a numerical
weather model that became operational in the period.
A more detailed model of wind-shear phenomena was
developed by Fred H. Proctor and his associates.
Known as the Terminal Area Simulation System (TASS),
this three-dimensional, time-dependent model included
representation of liquid and ice microphysics.
The effects of condensation, evaporation, freezing,
and sublimation in the atmosphere and their impact
on atmospheric winds could be numerically simulated
by this impressive tool to promote an understanding
of microburst formation and structure. Using TASS,
Proctor could actually simulate the time-dependent
life cycle of a convective storm, including microbursts
that might develop at subsequent times. Data sets
generated from this model were eventually used
by the FAA in its certification process for onboard
wind-shear sensors. In 1993, Proctor and Bowles
were awarded NASA Langley’s prestigious award
for the best technical paper, the H. J. E. Reid
Award, for a case study of a Denver wind-shear
incident using the TASS model.
Throughout the program, TASS
was applied to numerous actual microburst cases
and demonstrated the ability to produce simulation
results that agreed closely with observations.
The new capabilities provided by TASS were used
in a multitude of analysis objectives: an understanding
of microburst events, reconstructing missing information
from actual observations, evaluating microburst
sensor capabilities, and providing answers regarding
flight management strategies during microburst
encounters.
In addition to defining the
detailed atmospheric characteristics produced by
microbursts, the Langley researchers addressed
the key issue of defining the relative hazard associated
with microburst encounters. A brilliant approach
to this problem was conceived and implemented by
Bowles. He examined the overall performance capabilities
of aircraft during a microburst encounter and subsequently
derived a metric he named the “F-Factor,”
which quantifies the loss in aircraft performance
capability that would be experienced during a specific
wind shear. The nondimensional F-Factor is based
on a consideration of the weight, thrust, and drag
of the aircraft as well as the effects of local
velocities in the wind shear on the specific excess
thrust (thrust minus drag divided by weight) required
to maintain steady flight conditions due to wind
variations in a microburst. For example, if a representative
aircraft was capable of a specific excess thrust
value of 0.15 at a flight condition of interest,
a wind shear with intensity greater than an F-Factor
of 0.15 would exceed the maximum performance capability
of the airplane. When encountering such a wind
shear, the airplane would lose airspeed, altitude,
or both, regardless of pilot inputs. The typical
transport aircraft traveling at 150 knots and encountering
a wind shear with an F-Factor of 0.15 over 1 nmi
(24 sec) would lose 911 ft of altitude if recovery
action was not taken. Refinements and the ultimate
development of the F-Factor principle by Bowles,
Michael S. Lewis, and David A. Hinton included
a consideration of the length of time over which
the aircraft is exposed to the wind shear to produce
a refined definition of the F-Factor averaged over
1 km (about 15 sec of exposure at typical jet transport
low-altitude airspeeds).
The breakthrough analysis
and derivation of the F-Factor by Bowles is regarded
by many as the key contribution of NASA in the
taming of the wind-shear threat. As discussed in
a later section, the F-Factor provided enabling
analyses and assessments for advanced airborne
wind-shear sensors and is now used as a FAA-mandated
tool in the development and commercial sales of
wind-shear sensors. In recognition of his outstanding
contributions in wind-shear research, Roland Bowles
was awarded an R&D 100 Award (1993), the Langley
H. J. E. Reid Award (1993), and the AIAA Engineer
of the Year Award (1994).
The scope of the Langley
program to characterize wind-shear hazards also
involved other aspects of operations in thunderstorms.
For example, stimulated by concerns over the unknown
effects of the extremely heavy rainfall that is
typically experienced during wind-shear conditions,
researcher R. Earl Dunham, Jr., led work to determine
experimentally the impact of heavy rain on aerodynamic
characteristics (especially lift) of representative
transport airfoils. In this unique study, wind-tunnel
tests were initially conducted in the Langley 14-
by 22-Foot Subsonic Tunnel using water spray bars
in the tunnel to determine if simulated rain particles
would degrade the aerodynamic performance of representative
subscale wing models. Subsequently, a “car
wash” test section was constructed along
the track of the Langley Aircraft Landing Dynamics
Facility (ALDF), and a large-scale instrumented
wing-flap model was propelled through simulated
rain to obtain results more representative of actual
aircraft conditions. Results of the wind-tunnel
and ALDF testing showed significant degradation
in the maximum lift, and a marked decrease in the
stall angle of attack under extremely heavy rain
conditions (greater than about 1,000 mm/hr). Results
of the tests were then used by Dan D. Vicroy in
a theoretical analysis of wind-shear encounters
where it was determined that climb performance
reductions equivalent to an F-Factor of about 0.01
would be experienced. A major conclusion of these
activities, however, was that for the vast majority
of wind-shear encounters, heavy rain was an insignificant
effect. The extremely large rain rates needed to
impact aircraft lift are extremely rare in nature.
Sensor Technology
While Langley researched
the airborne detection technology, the FAA undertook
an aircrew training program that focused on wind-shear
recognition and procedures for recovering from
its effects. The FAA also led the development of
advanced ground-based wind-shear detection instruments,
including the Terminal Doppler Weather Radar (TDWR)
now being installed at major U.S. airports. Developed
by the Raytheon Corporation, the TDWR can accurately
measure wind velocities in terminal areas and generate
real-time aircraft hazard displays that are updated
every minute.
Langley’s focus, however,
was on the development of airborne systems capable
of predicting the presence of wind shear in a forward-looking
mode. In response to the terrible accidents of
the early 1980s, the FAA in 1988 directed that
all commercial aircraft have onboard wind-shear
detection systems by the end of 1993. Three airlines—American,
Northwest, and Continental—received exemptions
until the end of 1995 in order to install and test
emerging predictive wind-shear sensors rather than
reactive systems that do not report the condition
until an airplane already has encountered it. The
reactive system processes data from standard aircraft
instruments to determine the presence of wind shear.
The reactive system, therefore, only advises a
pilot of a wind-shear event, which allows an increase
in engine power and possibly escape of the hazard;
however, the airplane might not be capable of recovering
from a severe wind shear at that point. Langley
concentrated on a predictive system in the cockpit
that would provide 10 to 40 sec of advance warning;
thereby, the pilot would be able to determine the
proper maneuver, add power for flight stability,
or avoid the wind-shear area altogether. A Wind-Shear
Program Office was established about the time that
Langley realized that multiple sensors had to be
flight-tested for assessments.
Langley TSRV research aircraft
at Orlando during wind-shear flight research efforts.
Pushed by the 1995 implementation
decision imposed by the FAA wind-shear implementation
plan and augmented by technology application experts
from the Research Triangle Institute, the Langley
team initiated the development and assessments
of three different types of microburst sensors.
The one that ultimately became the first in airline
service was the Doppler microwave radar, which
sends a radio wave ahead of the aircraft to bounce
off raindrops in the thunderstorm and return to
the instrument. Computerized measurement of the
Doppler shift (the difference in wavelength frequency
between the outbound wave and the returning signal)
provides an indication of wind-shear velocity.
A second type of system known as Lidar, for Light
Detection and Ranging, operates under the same
Doppler shift principle but employs a laser beam
instead of a radio wave. A third type, a passive
infrared sensor, is based on the fact that a microburst,
usually cooler than the surrounding air, can theoretically
be detected by infrared measurement of the temperature
differential ahead of the airplane.
Langley’s task was
essentially to build a technology base that would
enable manufacturers to develop their own commercially
viable, proprietary systems. The enormous job began
with characterizing the wind-shear hazard and determining
the warning time required. Extensive computational
simulations by Langley’s Fred H. Proctor,
using his personal experience and the TASS computer
models (which had been thoroughly validated by
actual observations), documented the structure,
strength, and evolution of microbursts. This work
established the basic specifications for sensors
and enabled development of algorithms for rejecting
ground “clutter” that could confuse
sensor signals. As would be expected, the Bowles
F-Factor was a key factor in these analyses. All
this knowledge gave manufacturers a broad knowledge
base about how to extract wind-shear information
from a sensor signal, how to process the data against
hazard criteria, and how to alert flight crews
to valid threats while rejecting “nuisance”
indicators.
By 1991, 5 years into Langley’s
wind-shear sensor development program, the technology
had advanced to the point where validation of the
sensors required actual flight tests in wind-shear
conditions. For that challenging job, Langley outfitted
its unique Boeing 737 flying laboratory. Formally
known as the Transport Systems Research Vehicle
(TSRV), this particular 737 was the first production
aircraft of the Boeing 737-100 jetliner series.
This unique aircraft was extensively modified by
Langley and equipped with a rear research cockpit
in what would have been the forward section of
the passenger cabin for studies of advanced flight
displays and technology. Although it had been used
for over 20 years in very significant aeronautical
research at Langley, the wind-shear program was
arguably the most important technical project for
the TSRV. The aft flight deck was used as a command
post to monitor ground radar uplinks and airborne
sensors and to fly the airplane during maneuvers
to intercept a microburst. In the rear cockpit,
a moving-map display, with radar-derived microburst
icons, was used as an efficient tool for setting
up the straight-in approaches needed to allow radar
and lidar scanning before entry. Before microburst
entry, the forward deck took control and manually
flew the microburst penetration.
Three types of forward-looking
wind-shear sensors were evaluated during the flight
tests: a modified doppler radar transmitter from
Rockwell International, Collins Air Transport Division
(Langley developed the research signal-processing
algorithms and hardware for the wind-shear application);
a doppler lidar by Lockheed Corporation, Missiles
and Space Division, United Technologies Optical
Systems, Inc., and Lassen Research; and an infrared
detector by Turbulence Prediction Systems. Emedio
Bracalente, Langley, led the Airborne Radar Development
Group.
Laser beams used to align
lidar and infrared detection systems on TSRV.
Langley selected two field
sites for the joint NASA and FAA flight test program:
one at Orlando, Florida, and the other at Denver,
Colorado. Both areas were noted for frequent microbursts
in summertime, but it was anticipated that microbursts
in the Orlando area would be predominantly “wet,”
whereas a major portion of the microbursts at Denver
would be “dry.” The flight test plan
was challenging. The researchers had to anticipate
when and where a microburst would form based on
radar and other meteorological data because the
life cycle of a microburst (and its parent thunderstorm)
is shorter than the time it would take the aircraft
to be boarded and airborne. When ground controllers
predicted a potential wind shear, the Langley crew
would scramble, take off, fly directly toward and
into the microburst, observe and record the sensor
findings, then validate them by cross-checking
with ground radar data and with data from an airborne
reactive system for measuring wind-shear velocities
in situ. Orlando flights were supported by a TDWR
operated by the Lincoln Laboratory at the Massachusetts
Institute of Technology. At Denver, a research
radar of similar capability was operated by the
National Center for Atmospheric Research. For safety
purposes, the TSRV was flown into wind shear at
speeds higher than those of a normal jetliner approach
and at altitudes greater than takeoff and landing
levels (speeds of 240–260 mph and altitudes
of 750–1,500 ft). Other safety factors employed
during the deployment were to avoid flying into
areas with radar reflectivity greater than 50 dBz
(45 dBz in Denver), and F-Factors greater than
0.15. Over the summers of 1991 and 1992, the Langley
team conducted 130 flights and experienced 75 wind-shear
events. The airplane flew through heavy rains and
dust clouds, near hail and frequent lightning,
all in proximity to major airports without any
safety incidents. The results of the test program
demonstrated that Doppler radar systems offered
the greatest promise for early introduction to
airline service. The Langley forward-looking Doppler
radar detected wind shear consistently and at longer
ranges than other systems, and it was able to provide
20 to 40 sec warning of upcoming microbursts. Some
of the predictive sensors showed good correlation
with data from ground radars and the onboard reactive
systems.
TSRV approaching thunderstorm
for microburst studies.
The personal professionalism,
dedication, and individual contributions of the
TSRV wind-shear flight test team were shining examples
of this outstanding Langley contribution to aircraft
of the 1990s. Michael S. Lewis led these flight
efforts in his role as Deputy Program Manager,
augmented by an enthusiastic team that included
electronic technicians led by Artie D. Jessup;
flight operations led by TSRV crew chief Michael
Basnett; and research pilots Lee H. Person, Jr.,
and Kenneth R. Yenni.
Flight Management Systems
Yet another key element in
the NASA and FAA Airborne Wind-Shear Program was
directed at defining operational concepts to minimize
or mitigate the hazards associated with microbursts.
Research efforts included defining aircraft wind-shear
recovery strategies, using ground-based radar information
on the flight deck as an alternative for airborne
weather radar, and determining the most effective
crew information and procedures. David A. Hinton
led numerical and piloted simulations to determine
the pilot control strategy that would result in
a minimum aircraft energy loss when a microburst
was encountered. Hinton examined several methods
of controlling aircraft energy, and all findings
indicated that the factor that most strongly affected
a microburst wind-shear recovery was the time the
recovery was initiated. In the studies, the average
recovery altitude for all strategies only varied
by about 20 ft; however, the average recovery altitude
varied by almost 300 ft when the initiation time
of the recovery was advanced by 5 sec.
Hinton also led Langley’s
studies of the use of information provided by the
FAA airport TDWR in the cockpit. Major challenges
for this approach included the impact of time delays
in view of the changing dynamics of microburst
characteristics between updates from the TDWR (each
minute) and correcting for the differences between
the height of the ground-based radar beam and the
altitude of the airplane. During flight tests of
the Langley TSRV Boeing 737 aircraft in 1991 and
1992, uplinked TDWR data were used to locate microbursts
and maneuver the TSRV to penetrate the event. However,
the results of Hinton’s study indicated that
the ground-based TDWR information was more appropriate
for microburst awareness and advisories rather
than as a flight deck wind-shear hazard alerting
system.
Langley conducted or sponsored
several important studies aimed at issues regarding
the most effective crew-alerting information and
responses during wind-shear encounters. These detailed
studies addressed issues such as how much advanced
warning was required, should lateral maneuvers
be attempted for escape, and should existing wind-shear
pilot training be modified for forward-looking
sensor systems. The scope of studies included numerical
and piloted simulations using microburst models
and candidate cockpit display formats. Another
important issue identified by Langley involved
the interpretation of valid forward-looking wind-shear
system alerts as nuisance alerts. This concern
involved the fact that pilots, having been alerted
by the system, might conclude that the system had
issued a false alarm if the microburst penetration
was uneventful.
Following these research
studies, the technology development effort of NASA
for airborne wind-shear detection systems was essentially
complete, but the Langley group continued working
in a consulting capacity on the matter of FAA certification.
No certification standards existed—they had
to be invented and the Langley researchers now
represented the most knowledgeable body in the
world of wind-shear expertise. Langley worked with
the FAA and industry to develop a set of standards
for certification of wind-shear sensors. Collectively,
the standards define the hazard, the cockpit interface
and alerts to be given to flight crews, a suggested
methodology for certification, and the requisite
sensor performance levels. Langley research was
the basis for most of the specifications. In addition
to providing guidance for development of specifications,
TASS simulations provided the microburst data sets
required for certification testing. This set consists
of a range of possible events (dry, wet, large
scale, small diameter, multicore) that an aircraft
may encounter.
A final wind-shear related
Langley contribution occurred immediately after
the NASA and FAA program ended. On July 2, 1994,
a USAir DC-9-31 crashed following a missed approach
at the Charlotte-Douglas International Airport.
The accident resulted in 37 fatalities of the 57
onboard. As an altitude of 350 ft was reached during
the go-around, the aircraft rapidly began to descend.
The aircraft then collided with trees and a private
residence and broke up. The accident investigation
stated that the probable causes were (1) the decision
of the flight crew to continue an approach into
severe convective activity that was conducive to
a microburst, (2) the failure of the flight crew
to recognize a wind-shear situation in a timely
manner, (3) the failure of the flight crew to establish
and maintain the proper airplane attitude and thrust
setting necessary to escape the wind shear, and
(4) the lack of real-time adverse weather and wind-shear
hazard information dissemination from air traffic
control, all of these factors led to an encounter
with and failure to escape from a microburst-induced
wind shear that was produced by a rapidly developing
thunderstorm located at the approach end of the
runway. Contributing to the accident were (1) the
lack of air traffic control procedures that would
have required the controller to display and issue
radar weather information to the pilots, (2) the
failure of the Charlotte tower supervisor to properly
advise and ensure that all controllers were aware
of and reporting the reduction in visibility and
the low-level wind-shear alerts that had occurred
in multiple quadrants, (3) the inadequate remedial
actions by USAir to ensure adherence to standard
operating procedures, and (4) the inadequate software
logic in the wind-shear warning system of the airplane
that did not provide an alert upon entry into the
wind shear.
In response to an NTSB request
for assistance, a Langley team of Fred H. Proctor,
Emedio Bracalente, and Steve Harrah, together with
George Switzer and Charles Britt of Research Triangle
Institute, used the TASS model to simulate the
microburst and then used radar simulation to show
what a wind-shear radar would have seen if onboard
the aircraft. The Charlotte microburst was one
of the smallest and most intense that the Langley
team had ever seen (just under 1 km but with very
high F-Factor); this was a worst-case situation
because it gave the pilots no reaction time. The
primary conclusion of the study was that the accident
may have been avoided if the aircraft had been
equipped with a wind-shear radar.
Applications
In addition to the usual
practice of disseminating technical papers to effect
technology transfer, NASA and the FAA jointly sponsored
five well-attended national wind-shear conferences
beginning in 1987 and ending in 1993. The Airborne
Wind-Shear Program was based on one of the most
effective methods of technology transfer—the
participation of potential manufacturers, industry,
and regulatory agencies to track the development
and assessments of wind-shear detectors. Three
major avionics manufacturers (Allied Signal, Westinghouse
Electronic Systems Group, and Rockwell Collins
Commercial Avionics) sent engineering teams to
Langley to meet directly with the radar engineering
personnel and follow the developmental effort of
Langley step by step. The three companies each
requested and were provided Langley wind-shear
simulations, which they used extensively in developing
their own commercial systems. Langley personnel
and contractors also participated in several government-industry
efforts to develop standards for forward-looking
wind-shear detection systems.
On September 1, 1994, Allied
Signal Bendix RDR-4B became the first predictive
wind-shear system to gain FAA certification for
airline operations. The RDR-4B forward-looking
radar was the product of the decade-long FAA, NASA,
industry, and academia research program spearheaded
by Langley that developed the technology base to
enable commercial manufacture of the Allied Signal
and other wind-shear detection-prediction systems.
Many of Langley’s technology
concepts have been incorporated into industry’s
implementation of advanced wind-shear detection
systems. For example, the F-Factor became a regulatory
parameter. According to FAA regulations, wind-shear
warnings must be given for F-Factors of 0.13 and
greater. FAA regulations determine the combinations
of altitude, airspeed, distance from wind shear,
and F-Factor that result in an advisory, caution,
or warning to the flight crew.
Three major U.S. airlines
(United, Northwest, and Continental) subsequently
selected the RDR-4B; collectively, they ordered
more than 1,000 units. The technology is also being
extended to foreign airlines, and among those who
have purchased the RDR-4B are Swissair, Alitalia,
Iberia, Gulf Air, and Kuwait Airways.
On November 30, 1994, Continental
Airlines Flight 1637, a Boeing 737-300 jetliner,
took off from Washington (D.C.) National Airport
bound for Cleveland. It was a routine, regularly
scheduled flight, but to aviation safety officials
all over the world it was something more—it
was a historic moment that marked the introduction
to commercial airline service of an onboard cockpit
instrument for detecting and predicting wind shear
in a forward-looking mode.
In the mid- and late-1990s,
avionics companies rapidly provided advanced wind-shear
detection systems. By June, 1996, vendors such
as Allied Signal/Bendix (now Honeywell), Rockwell
Collins, and Westinghouse Electric produced certified
forward-looking wind-shear radar systems. Over
2,000 orders had been placed for the systems from
foreign and domestic carriers as well as the U.S.
Air Force. These wind-shear detection systems issue
microbursts warnings within a specific distance
(e.g., 0.25 nmi above or below the flight path
of the aircraft) and within a specific angular
sector (e.g., -30∞) of the aircraft heading.
A warning icon is displayed on the radar display,
and an aural warning is also issued by a voice
synthesizer. Wind-shear alerts are typically inhibited
during takeoff from the time the aircraft reaches
100 knots airspeed until it is 50 ft above ground
level (AGL) to discourage avoidance maneuvers during
this critical flight time. Similarly, alerts are
inhibited during approach when the aircraft is
below 50 ft AGL. Alerts are never given above a
maximum altitude (e.g., 1,500 ft AGL).
It took almost a decade to
bring the predictive wind-shear system from concept
to commercial availability, but aviation experts
say that was a remarkably brief period when the
complexity of the phenomenon is considered. The
program stands as a model of cooperative endeavor
by a broad segment of the U.S. aviation community,
including government agencies, aircraft manufacturers,
sensor manufacturers, airlines, research organizations,
and academia.
The contributions of the
NASA Langley Research Center to this national success
story stand out among the thousands of legendary
technological accomplishments of the Center. Cited
as “NASA at its best” by NASA’s
Aeronautics Advisory Committee, the NASA and FAA
Airborne Wind-Shear Research Program was nominated
in 1994 for the Nation’s prestigious Robert
J. Collier Trophy in recognition of the most significant
aerospace accomplishment.
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