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OCR for page 51
Aircraft Performance and Operations
This chapter discusses the performance and flight characteristics
of airplanes as distinguished from other forms of aircraft, such as
helicopters and airships. Most of the analysis cannot be generalized
and may not apply to aircraft other than airplanes. Although the
hazards of wind shear to helicopters could be large, there are
insufficient statistics to determine the extent of the problem.
Similarly, the frequency of airship operations is so low that it does
not merit special attention at this time.
Wind-Shear Warnings
Operational information on wind shear originates from
meteorological forecasts, pilot reports (PIREPs ) , and/or detection by
Low-Level Wind Shear Alerting Systems (LLWSAS ~ at those airports that
have such equipment (currently 59 ~ . Meteorological forecasts are the
least useful for predicting downbursts because of their short lifetime
and random occurrence and location. These forecasts, at best, warn
flight crews and controllers of conditions conducive to generating
downbursts and wind-shear activity. This information, despite its
lack of small-scale detail, alerts pilots to the possibility of a wind
shear encounter, reducing their recognition and reaction times.
Forecas ts and PIREPs of encounters are the only sources of wind shear
information for the thousands of airports that lack LLWSAS.
Pilots are not required to submit reports of wind-shear
encounters. PIREPs are voluntary weather reports, broadcast by pilots
to towers, departure control, approach control, flight service
stations, or to an air-route traffic control center to warn of
encountered or observed weather phenomena . The Airman 's Information
Manual (AIM) tells pilots when and how to report wind shears and other
hazardous-weather information. Published quarterly by the FAA, it
provides fl ight crews with teas ic £1 ight information and air traffic
control (ATC) procedures for use in the National Airspace System. In
particular, AIM's Section 523 urges pilots to report wind shear
encounters and specifies a particular format to ensure that enough
information is furnished to make the report useful. Excerpts from the
AIM, Section 523, are contained in Appendix A of this volume.
51
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The FAA Air Traffic Controller's Handbook contains instructions
for the use of PIREPs and LLWSAS information, including the issuance
of wind shear advisories to pilots. The instructions contained in
Section 7, are minimal. Under Section 6 the subject of wind-shear
advisories is dealt with at some length for those airports that are
equipped with LLWSAS. The handbook notes that "LLWSAS is designed to
detect possible low-altitude wind shear conditions around the
periphery of the airport and that it does not detect wind shear beyond
that limitation."
The FAA Facility Operation and Administration Handbook (paragraph
1222, dated September 2, 1982) gives the facility chief the option of
using the centerfield wind information: "if operationally feasible,
facility chiefs may elect to designate wind information derived from
remote sensors located near runway thresholds as the wind to be issued
to arriving aircraft rather than from the centerfield source, except
that the centerfield source will be used during outages of the remote
sensor/s." Such procedures as described above require the local
facility chief to issue a letter to airmen explaining to arriving
aircraft the origin of wind information, if it is obtained from a
peripheral sensor. However, controllers are not required to specify
the source of remote-wind data used when issuing these data to
arriving aircraft, except when an alert occurs.
The training material of the Controller Training Academy at
Oklahoma City does not incorporate the best-available information on
wind shear. Wind shear is treated under the heading ''Turbulence" and
comprises, perhaps, three pages of text. Additional, more up-to-date
information should be included and given more emphasis in training
materials a
In response to NTSB Safety Recommendations issued March 25, 1983
(NTS8, 1983), which addressed the need for improvements in the LLWSAS
system and procedures for its use, the F. M is in the process of
amending its handbook for controllers. Similarly, the F. M plans to
emphasize in the AIM the importance of pilots reporting wind-shear
occurrences promptly. The FAA is preparing additional material for
pilots and controllers in the form of advisory circulars and bulletins.
At busy airports, controllers must communicate vital LLWSAS data
and other weather information while occupied with their primary task
of guiding aircraft and ensuring traffic separation. Not only is it
difficult to interpose weather information, in the fast-paced flow of
traffic directives, it is also important that the wind shear
information be transmitted promptly and clearly without the likelihood
f or mis int erpre tat ion .
If not given in a standardized format, PIREP can exacerbate the
difficulty of the controllers' task. A recent FAA staff study of the
FAA weather program in October 1982 (unpubl ished) recognized the
52
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problem of handling and distributing PIREPs and recommended revising
the current procedures to improve their proper processing and
d istribution.
This situation calls for an alternative means of transmitting
critical information accurately and directly to pilots. During 1977
the FM conducted a study of how to best provide pilots with
information on potentially hazardous weather conditions. Among other
things, the FM looked at the cost of a system that would allow a
meteorologic t at the Center Weather Service Unit (CWSU ~ to tape a
hazardous weather advisory covering the center 's geographic area of
responsibility. CWSU meteorologists at the ATC center either now have
or could acquire al 1 the information they need to prepare a
transcribed broadcast of reported or predicted low-altitude wind
shears and of any other hazardous weather occurring in or forecasted
for the geographical boundaries of the center, including air carrier
terminals and other airports. In the proposed system, meteorologists
would upda te the taped advisory as required, which would be
continuously broadcasted on a discrete frequency. The FAA reported on
March 23, 1978 that such a system would require four discrete
frequencies in the 25-kilohertz range and 66 transmitters to
adequately cover the continental airspace for all aircraft operating
above 18,000 feet. The cost of such a system was estimated to be
$1 mil 1 ion .
As noted earlier, the FAA is developing equipment that will enable
meteorologists at their CWSUs to transmit automatically weather
messages to towers and control facilities . However, in the near term,
pending completion of an automatic data up-link, perhaps the proposed
system or some variation, is worthy of another look.
In summary, all equipment and procedures now in place can be
improved and re fined to make warnings of poss ible wind-shear hazards
more effective. It is essential that the aviation system exploit
these capabilities to the utmost while more advanced warning systems,
such as NEXRAD, terminal radars, airborne radars, and other airborne
detection/warning concepts, are being developed.
Cockpit Procedures and Training
FAA Regulat ions
The Federal Aviation Regulations (FAR) applicable to air carriers
and certain other categories of operators require these operators to
develop and use approved procedures to advise pilots of severe weather
conditions, including possible thunderstorms and frontal systems that
may cause low-altitude wind-shear conditions at departure and
destination airports; flight procedures for operating in potentially
hazardous weather conditions; approved airman training programs
53
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covering all aspects of normal and emergency operations; and manuals
including "instructions and information necessary to allow the
personnel concerned to perform their duties and responsibilities with
a high degree of safety." [FAR 121.135(a)~13~. This is in accordance
with the Federal Aviation Act of 1958 [Sec. 601(b)], which prescribes
that: "the Administrator shall give full consideration to the duty
resting upon air carriers to perform their services with the highest
possible degree of safety in the public interest."
_ ¢~ __= a_ ___ A__ _ for airplane type
certification by the FAA need not include a discussion of wind-shear
effects on the airplane. The flight operations manual for each
airplane used by an airline, however, is required to include a
detailed discussion of piloting procedures to be followed in the event
of a wind-shear encounter during takeoff or landing. In addition, air
carriers are required to include procedures for coping with wind shear
as part of their FAA-approved training programs. These operational
and training programs are monitored in the field by FAA operations
inspectors, who have the responsibility to evaluate the adequacy of
the air carriers' training programs and line operations.
The airplane flight manna 1 required
The FAA issued Advisory Circular AC 00-50A, entitled Low Level
Wind Shear, on January 13, 1979. It is the primary source of FAA
guidance to pilots for recognizing the more significant meteorological
phenomena that may cause wind-shear hazards. It also contains
procedures for pilots to use in detecting the presence of wind shears
and for flying airplanes safely in the event of an encounter. Since
its publication in 1979, much more has been learned about the
characteristics and hazards of wind shear, especially from Projects
NIMROD and JAWS. Advisory Circular AC 00-50A should be revised and
updated to include new information. In particular, the FAA should
describe the structure of downbursts and enhance the discussion of
airplane performance and piloting techniques in wind-shear conditions.
Operating Procedures
Airline flight operations manuals were reviewed by the committee
as to the procedures specified for use by airline flight crews in the
event of low-altitude wind-shear encounters. All of the instructions
appear to be based on the recommendations in FAA Advisory Circular
AC 00-50A, and on recommendations developed by the Boeing Company, and
published in the January 1977 and January 1979 issues of Boeing
Airliner magazine (Higgins and Roosme, 1977; Higgins and Patterson,
1979~. But there is considerable variance among airlines in the
extent of coverage and in details on flight operations procedures.
Some manuals provide very extensive instructions, while others are far
less complete. Additional emphasis needs to be placed on the potential
severity of and hazards from downbursts and strong wind shears and the
importance of early recognition of and immediate reaction to them. In
addition, the manuals should discuss flying at high angles of attack
54
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or at stick-shaker speed* and the need for timely PIREPs phrased in
standard terminology. It is noted that some airlines include material
relevant to wind shear in their training manuals rather than in their
flight operations manuals. In these cases, the training materials and
flight operations manual materials are complementary.
Airline flight operations manuals are organized differently from
one company to another, as are their methods of presenting procedures
for dealing with wind shears. Typically, all procedures related to
takeoff and initial climb appear in one section of the manual and all
procedures applicable to approach and landing appear in another.
One major airline presents extensive material on wind shear in its
flight operations manual. It includes the following warnings in the
part of the manual dealing with takeoff (normal operations):
If significant wind shear is suspected, consider the alternatives
of taking off in a different direction or delaying the takeoff
until conditions are more favorable. If shear is suspected, use
full takeoff thrust; do not use reduced thrust. If the takeoff is
not obstacle limited, a speed in excess of V2+10** may be used
for the initial climb to provide additional protection from
decreasing headwinds or downdrafts. If significant wind shear or
downdrafts are encountered at low altitude after takeoff and
airspeed has decreased to below normal climb speed, apply go-around
thrust and adjust pitch attitude to climb out at the existing
airspeed. Do not lower the nose in an attempt to regain speed
until reaching a safe altitude. If ground contact is imminent,
use the procedure for avoiding imminent ground contact.
A similar warning appears in the section of the flight operations
manual dealing with landing (normal operations):
If wind shear is encountered on final approach, do not hesitate to
go around if the approach profile and airspeed cannot be
restabilized. It cannot be emphasized too strongly that a
go-around is often the profess tonal p i lo t's best course of ac Lion.
If ground contact is imminent, use the procedure for avoiding
imminent ground c ont ac t.
The emergency section of the same manual contains the following:
*The "s tick shaker" is a device that vibrates the control column to
provide stall warning prior to reaching stall angle of attack.
**V2 is defined as the engine-out takeoff safety speed. Normal
all-engine initial-climb target speeds vary from V2~10 to V2+20
knots, depending on the model.
55
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AVOIDING IMMINENT GROUND CONTACT
In the event of imminent contact with the ground, such as during
an extreme wind shear or downdraft encounter or unintentional
flight toward terrain, it may be necessary to use all available
airplane energy by trading airspeed for altitude to avoid or
soften impact.
Simultaneously increase pitch attitude and apply thrust, if
necessary, to the limit of forward throttle movement. The initial
rotation should be accomplished sharply, and pitch attitude should
then be adjusted to achieve a rate of airspeed decay sufficient to
arrest the descent and to climb. Maintain these conditions until
reaching a safe height or until the stick shaker activates,
whichever occurs first. If the stick shaker activates, lower the
nose sufficiently to stop further airspeed decay, maintain
attitude and thrust, and continue climb with the stick shaker
activated. When safe conditions are achieved, initiate recovery
of airspeed. Be aware that in almost all cases, pitch attitudes
in excess of 20 degrees will be required in this maneuver. Rate
of airspeed decay should not be so great as to decelerate
significantly below stick shaker speed.
There are some potential problems with a pullup to stick shaker
speed that should be considered:
0 If the pullup is accomplished too soon (significantly prior
to imminent ground con-t-act) climb rate is actually
decreased, since climb rate at stick-shaker speed is less
than at V2, V2 + 10, or Vprog
o If the pullup is accomplished too soon, and ground contact
still occurs, there is no airspeed cushion to use for a
"flare" to soften impact.
0 If airspeed decay rate during the pullup is too great, speed
will decrease through the stick-shaker speed, and the
airplane could stall.
When at stick-shaker speed, any turbulence or additional
shear could cause the airplane to stall.
O It is possible that the effects of heavy rain could cause an
increase in the airplane's stall speed. This could cause
the airplane to stall before reaching stick-shaker speed.
Nevertheless, if ground contact is indeed imminent, this
maneuver represents a "last chance" effort to avoid or soften
contact with terrain. Early recognition of the flight
*Vprog. refers to a preselected or commanded speed, which may be
entered into a flight director or autothrottle.
56
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condition, either through GPWS [Ground Proximity Warning System]
alert or crew awareness of the possibility of wind shear, should
allow use of the normal go-around procedure which would avoid the
necessity of using the extreme measures described above.
Any time engine limits are exceeded in these circumstances, the
possibility of severe engine damage exists and a landing at the
nearest suitable airport may be required.
This discussion of operating procedures during a wind-shear
encounter is among the most comprehensive treatments found in manuals
used by airlines operating under Part 121 of the FAR. It does not
appear that similarly detailed procedures are common among the Part
135 commuter and air taxi or the general aviation communities.
No U.S. aircraft operators--airline, commuter/air taxi, or general
aviation--are known to have established operational limitations
governing the takeoff and landing decision by the pilot-in-command
based solely on reported low-altitude wind-shear levels. LLWSAS
information is generally treated as advisory in nature. It is clear
that the 15-knot vector difference threshold that triggers an LLWSAS
alert is not regarded as a serious operational hazard except in the
case of a takeoff limited by runway length or by obstacles in the
departure flight path. Airline operating policies and procedures
typically require pilots to assess all relevant factors in deciding
whether to take off or to continue an approach to landing in the event
of a potential wind-shear exposure. The wind-shear level indicated by
LLWSAS is only one of many factors to be cons idered.
Training Programs
A review of airline training programs reveals that, as required by
the FAA, FAR 121 air carriers train fl ight crews on the nature of wind
shear and operational procedures required to cope with its potential
threat. But the exact nature and timing of the training vary widely
from airline to airline. All airlines appear to cover the subject
extensively in ground training. As with operations manuals, the
material is based largely on excerpts from the FAA's advisory circular,
Boeing Airliner articles, and the technical literature.
The FAA has a similar wind-shear ground training requirement in
FAR 135, for operators subject to those regulations. However, the
committee was unable to assess the scope of coverage of wind-shear
hazards in ground training programs of FAR 135 operators. There is no
specific requirement for wind-shear training for general aviation
pilots other than the very general "aeronautical knowledge" requirement
in FAR 61, applicable to issuance of pilot certificates and ratings.
The FAA has no spec if ic requirement for flight training of pilots
on procedures to be followed in the event of an inadvertent wind-shear
encounter. The FARs applicable to training have evolved from a premise
57
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that all necessary flight training--for all classes of operators--can
be conducted in flight in an airplane. Requirements in the FARs
governing use of simulators to conduct flight training are permissive.
Simulators may be substituted for airplane training, but there is no
requirement for simulator training for any class of carrier, not even
for air carriers operating under FAR 121.
Inflight training for wind-shear encounters is not feasible. In
view of the random and rare occurrences of wind shear in nature and
because of the threat posed to flight safety by exposure to severe
wind shears at low altitude, air carriers operating under Part 121
conduct such flight training in simulators. These devices are more
economical to operate than airplanes and provide superior training
capability for a range of abnormal and emergency training problems,
including severe wind-shear encounters. However, only those few
"advanced" simulators approved under F. M 's Phase II and Phase III
criteria in Appendix H of FAR 121 are required to incorporate
"representative three-dimensional wind-shear dynamics based on
airplane-related data."
FAR 135 operators typically conduct a much smaller amount of
flight training in simulators. Also, relatively little flight
training in simulators is conducted within the general aviation
community, although corporate and other operators of sophisticated
multiengine turbojets use simulators extensively for training.
It is unlikely that simulators will become universally available
for flight training in the foreseeable future--not even for air
carriers operating under FAR 121. Their use is limited in par t by the
FM requirements that s imulators meet demanding technical specif ica-
tions (FAR 121.407; FAR 121, Appendix H; FAR 135 .335; Advisory
Circular AC 120-40~. Many aircraft, including older transport-category
types, have never had an adequate simulator data package developed to
enable a simulator to be programmed such that it can meet applicable
criteria for FAA approval.
The FM has fostered and regulated simulator training since the
introduction several decades ago of regulations governing the use of
simulators in air carrier training programs. As simulators have been
improved, the FAA has systematically recognized and credited their use
in pilot training, checking, and certification programs. Advisory
Circular AC 120-40, Airplane Simulator and Visual System Evaluation,
contains criteria for the approval of simulators. However, these
criteria are conservative in the sense that they require simulators
with capabilities that make them very complex and expensive to obtain
and to operate, in order to be approved as a training device for pilots
operating under FAR 121 and 135. The F. M does not specifically
recognize simulators as a substitute for airplane training of general
aviation pilots, although the FM has granted several exemptions for
this purpose.
58
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Wind-Shear Modeling for Training Simulators
Advanced simulators currently used for pilot training and checking
are required to be capable of full-mission, pilot-in-the-loop
simulation. They are required to have appropriate visual display and
actual cockpit force-feel systems, including instrumentation recording
systems, and appropriate dynamic modeling of the wind shear conditions
to be encountered.
Accurate portrayal of downbursts and wind-shear conditions and
related aircraft responses in a simulator with motion base and visual
displays presents a complex computer modeling problem. It requires
extensive and expensive computation capabilities, available only in
costly advanced simulators with large memory capacities, fast compu-
tation capabilities, and real-time cockpit controls and displays and
response t imes necessary for the realis tic portrayal of wind-shear
encount ers.
Results of recent wind-shear measurements in the JAWS Project
indicated that the wind-shear models now being used by the airline
industry for training, for airborne control and display system
development, and for certification purposes do not accurately portray
actual wind-shear situations. Most models in use today are based on
data developed for the FAA during 1976-1979 (Foy, 1979~.
The most accurate portrayal of wind-shear fields are the four-
dimensional, time-varying models based on JAWS data being proposed by
NASA's Langley Research Center. These models are of interest and
importance in research. However, their use of time dependence greatly
increases computational requirements, which can easily exceed the
capabilities of the computers used in simulators.
It is possible to use a somewhat simplified representation of wind
shears for some purposes without significantly compromising the
results. In these cases, very real benefits in time and effort plus
substantial savings in costs can be achieved. The increasing
complexity and costs associated with the latest developments in
computer modeling and simulation may make it prohibitively expensive
for many potential users and may serve to stifle the application of
new technology and safety advances. Studies should be undertaken to
determine when simplified wind-shear models are acceptable for purposes
of system design, evaluation and certification, and for training in
simulators.
The FAA has proposed procedures for approving airborne wind shear
systems (Draft Advisory Circular AC 120-XX,
~ ). This proposed advisory circular describes
wind fields considered acceptable for various specific applications.
They were developed from accident reconstructions, meteorological
data, and other sources. The FAA plans to update these wind field
definitions as new information from JAWS and other studies becomes
59
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available. The new def initions should include "severe" wind profiles
that may exceed the performance capability of a specific airplane.
Development of these new wind-shear models will take a considerable
period of time and much effort. It is essential that industry
standards for implementation and use of the resulting models be
established to minimize different interpretations of the data. This
should be undertaken as a joint government-industry effort. The FAA
should publish the results for use in simulator training as well as
for evaluation and approval of airborne systems. If possible, these
wind-field models should be standardized for specific applications to
ensure consistency among users. Particularly promising is a simple
empirical model that defines a three-dimensional wind field that may
be adapted for use in simulator applications to represent thunderstorm
outflow phenomena of the type associated with recent aircraft
accidents. This model is being developed at NASA's Ames Research
Center.
Per formance in Wind Shear
Airplane Response to Wind Shear
Airplanes generate the aerodynamic forces that make fl ight
possible by means of airspeed, which is the velocity of an airplane
relative to the surrounding air mass. Thus, a change in velocity of
the surrounding a ir, or a wind shear, will cause a change in the
aerodynamic forces on an airplane. However, all airplanes have some
degree of speed stability and, as a consequence, once they have been
disturbed by a wind shear, they will try to return to their original
velocity relative to that of the new air mass. This process may take
a half a minute or more depending on the airplane and the size of the
wind shear (assuming the pilot takes no corrective actions).
When an airplane encounters a wind shear, there will be changes in
components of wind along each of the airplane's axes of motion. The
longitudinal axis runs along the center of the fuselage and the wind
component along this axis is a headwind or tailwind. The vertical
axis has its associated up- or downdrafts, and the lateral axis has
its associated crosswinds from the left or right. Each of these wind
components will produce a different response based on the airplane's
aerodynamic configuration. A brief description of how an airplane
responds to each of these wind shears along individual axes will
contribute to an understanding of the wind-shear problem. The
airplane's response to each of these wind-shear components assumes no
corrective action by the pilot and that the wind is steady before the
plane enters and after it leaves the shear field. In actuality,
wind-shear components exist in three dimensions, and the net effect of
flying through such a wind field imposes disturbances in all six
degrees of freedom of the aircraft. Much of the ensuing discussion
centers on the control-fixed response. The closed-loop (piloted)
responses will, of course, be very different and will represent actual
cases that must be studied.
60
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Longitudinal Wind Shear. A longitudinal wind shear that increases
i. - , . .
an airplane's airspeed can arise either from an increase in a headwind
or a decrease in a tailwind. In either case, an increased airspeed
caused by a shear will cause an increase in lift and drag. The air-
plane pitches up and climbs, while its pitch stability causes it to
reduce its angle of attack and decrease its lift as it starts to
recoverer to its original trim condition. In the free-response case,
after s everal osc it let ions, the a irplane wi 11 res tab 1 ize at i ts
original airspeed on a flight path that is parallel but displaced
above its original one, and its new speed relative to the ground will
have decreased by the magnitude of the wind shear. A reduction in
engine thrus t is required to regain the original inertial f light path.
A decreasing tailwind and an increasing headwind both provide an
apparent increase in airplane performance. However, the two cases
present different problems to the pilot attempting to fly a glideslope
since the decreasing tailwind case, with its higher ground speed,
re quir e s 1 e s s power t o fo 1 1 ow th e pr oper appr oach pa th .
The opposite situation for a longitudinal wind shear occurs when
there is an increasing tailwind or decreasing headwind. In this case,
airspeed decreases and lift and drag are reduced. The airplane
pitches down and descends, while the airplane 's pitch stability causes
it to increase its angle of attack to recover lift. Eventually, the
airplane will restabilize at its trimmed airspeed on a flight path
parallel to the original one but displaced below it and at a higher
ground speed than before the wind-shear encounter. The shear has
caused a reduction in altitude, which has been converted into
increased ground speed at the original airspeed. To restore the
altitude loss, energy must be added to the airplane in the form of
increased engine thrust.
Updrafts. An updraft disturbs an airplane by increasing its angle
of attack. This increased angle of attack increases li ft and drag,
which cause the airplane to climb and decelerate. The increased lift
causes the airplane to pitch nose-down to reduce the angle of attack
and to recover its original value. Again, after several oscillations,
the airplane wit 1 return to its original angle of attack and airspeed
relative to the air mass, but it will be climbing relative to its
original inertial flight path.
The opposite situation, a downdraft, decreases an airplane's angle
of attack, thus reducing lift and causing it to sink. The decrease in
lift causes a decrease in drag and a nose-up pitch to restore angle of
attack. Eventually, the airplane will settle out at the original
airspeed and angle of attack but will descend inertially within the
a or mas s .
Updraf ts impart energy to an a irpl ane making it c l imb; downdraf ts
absorb energy, making it sink. Pilots must reduce power in updrafts
61
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o
-
cr
7
8
us -200
-100
it:
~ -300
o
111
a'
by:
TOTAL HOR IZONTAL TAI LWIND SHEAR (knots)
10 20
-400
-500
40 50
NOTE: Altitude Loss on Vertical Scale is for Recovery to
Level Flight with the Aircraft accelerating
60 70 80
~ ~ '1'~
:~
90 1 00
-
Maximum
_ Capabil ity
with
,' Warning
, A/ +
, / Gu Ida nce
x ~~
1
ASSUMPTI ONS
Initial Speed VREF + 10
Constant 30 Deg. Flaps and Gear Down
, I 1 1
FIGURE 14 Transport Aircraft Flare Capability in Wind Shear from a
Three-Degree Approach Path.
v
>~
go-around and flare performance for an aircraft with a system providing
a warning of a shear encounter. It assumes an appropriately trained
pilot who follows wind-shear guidance and command indicators. Here,
the power application and nose-up attitude control are initiated at
the end of 2 seconds and, as in the previous case, power application
is continued until full power and incipient stick-shaker conditions
are attained and the flare maneuver completed. The constant,
incipient-stick-shaker angle-of-attack portion of the flare is a very
important assumption in the dynamics of the flare maneuver.
The asymptote at the top of the upper curves in Figure 14 shows an
approximately 80-foot altitude loss. The first 30 feet represent the
2-second delay in recognizing the shear encounter, and the remaining
50 feet are lost during a straightforward flare. For wind shear
conditions along the asymptote line, the airplane has sufficient
performance to penetrate without further loss in altitude, providing
the pilot uses the airplane's pull-up capabilities up to the
stick-shaker angle of attack.
For more severe shears an aircraft cannot maintain altitude
after it has reached stick-shaker condition. In these cases, as it
loses altitude the airplane accelerates and develops a normal load
factor, which provides the flare. In Figure 14, at 8 knots/second
68
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shear gradient and with an 8-second penetration time, there is a
64-knot change in horizontal tailwind. The curve shows a loss of 235
feet in altitude in this maneuver for the maximum performance case.
Figure 14 considers only horizontal tailwind shears. The addition
of downdrafts is difficult to present in a generalized form.
Follow-up studies should consider combined horizontal wind shears and
downdrafts with realistic combinations of shear gradients. Figure 14,
which presents the results of a simplistic analysis of a very complex
problem, shows that under 500 feet above ground level (AGL) the
probability of surviving a severe wind shear is greatly enhanced by an
immediate recognition and response to the shear. A corollary is that
warning devices and equipment that improve the pilot 's ability to
respond in this manner also contribute significantly to the
probability of success. Thus, pilots must rapidly recognize a severe
low-altitude wind shear on approach and immediately execute a missed
approach maneuver. This means immediate application of maximum thrust
and rotation to arrest the descent of the aircraft.
As a last resort, in a severe wind shear the airplane may have to
be flown at or slightly below stick-shaker (or other stall-warning)
angle of attack until it leaves the shear area and its performance
margins are restored. At that point, it should be possible to climb
and accelerate to normal climb speeds.
Few nonmilitary aircraft are equipped with an angle-of-attack
display. Therefore, stick-shaker (or other stall-warning) angle of
attack, which is 2° to 3° below the angle of attack for maximum
lift coefficient, becomes the upper limit of rotation on aircraft
lacking special instrumentation.
The combination of energy trade and use of maximum thrus t gives
pita ts the tees t chance of preventing ground contact i f they encounter
a severe shear without warning at low altitude. Pilots must recognize
that flight at stick-shaker angle of attack entails significant risks
that can be justified only if the aircraft encounters severe shear at
very low altitude.
Pilots should react to shears above 500 feet as rapidly and
positively as described above, but in this case the extra altitude
makes it possible to initially rotate to less severe angles of attack,
better preserving speed and lift safety margins.
General Aviation Aircraft
While extensive research has been conducted on the characteristics
of wind-shear encounters of conventional jet transport airplanes,
there is only a small amount of analysis (Lehman et al. , 1977) for the
large variety of general aviation airplanes, which includes corporate,
commuter, and personal aircraft. First, there appear to be few
documented cases of wind-shear-related accidents of general aviation
airplanes. Second, accidents involving these airplanes are rarely
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investigated as aggressively as those of air carrier aircraft.
However, the hazard of low-altitude wind-shear accidents is present
f or genera 1 avi a t ion a ircr a f t .
The to 1 lowing s impl if fed analys is provides some 1 imited ins ight
into the basic factors influencing the ability of general aviation
airplanes to adjust to wind shears. It also indicates an avenue of
further study that should be pursued. The analysis assumes that an
airplane takes 20-30 seconds to tranverse a downburst, during which
period the wind-shear velocities are constant. A measure of the
severity of the shear, then, is the peak velocity difference across
the microburst. Airplane performance capability can be represented as
the ability to increase speed per nautical mile traveled. On this
basis, comparisons can be made among the various classes of airplanes.
Table 4 presents acceleration capability at the best angle of
climb speed in the clean configuration for general aviation airplanes
and for a heavily loaded three-engine jet transport in the takeoff
configuration, based on airplane flight manual performance data. It
shows that many airplanes thought to have low performance capabilities
have surprisingly high values of acceleration potential compared with
jet transports. While this appears to be a very favorable factor for
light general aviation aircraft penetrating wind shears, it is by no
means the only factor to be considered with regard to the hazards of
severe wind-shear encounters. Also, the acceleration potential values
presented in Table 4 are based on a static, instantaneous performance
analysis and are only indicative of the relative acceleration capabil-
ities of the aircraft types listed. A similar criterion is described
by Frost (1983~.
TABLE 4 Acceleration Capability at Best Angle of Climb Speed
Rate Acceleration Performance
Aircraft Type Airspeed of Climb Margin Factor, ~V/^ X
V(KN) R/C(FPM) (KN)/(SEC) (KN)/(NMI)
Trainer 62 780 2 .37 138
Basic 4-Place 62 660 2.00 116
High-Per formanc e
Single Engine 83 1050 2.38 103
Light Twin 78 1140 2.75 127
Cabin Twin 88 1380 2.95 121
Light Turboprop 105 1560 2.79 496
H igh-Per formance
Turboprop 106 2100 3.72 126
Light Turbo fan 150 2040 2 .56 61
High-Performance Jet 200 4200 3.95 71
High-Performance
Turboprop 180 3300 3.45 69
Typical 3-Engine Je t
Transport at T.O. 160 2300 2.7 61
. . . . . .
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The following additional points should be explored before definite
conclusions can be reached on the relative capabilities of general
aviation aircraft and jet transports in penetrating severe wind shears.
1. General aviation aircraft usually have lower takeoff and
landing approach speeds than do commercial jets.
Consequently, a given total wind shear represents a higher
percentage of the airplane's flight speed. This, of course,
makes it more difficult to penetrate a given wind shear.
2. Since the minimum operating speeds during takeoff and landing
approach are defined as a percentage of the FAR stall speed,
general aviation aircraft with low stall speeds will have
smaller speed margins than do jet transports. Their ability
to absorb an airspeed loss in a wind shear would be less than
airplanes with higher stall speeds. However, general aviation
aircraft generally operate at proport ionately larger speed
margins above their minimum operating speeds compared with
transport aircraft. Their slower penetration speeds would
allow pilots more time for recognition of and response to wind
shears.
3. Propeller-driven general aviation aircraft have the benefit of
the propeller slipstream over the wing, which provides an
added margin of lift not available to jets.
4. Some jet-powered general aviation aircraft have very high
thrust-to-weight ratios. This is a very favorable factor in
penetrating adverse wind shears.
5. Slow-flying general aviation aircraft will be more sensitive
to updrafts and downdrafts than are jet transports, since such
drafts produce a larger angle-of-attack change on a
slower-flying airplane.
6. General aviation aircraft with small wing spans have less roll
damping and are more active in roll than large jet
transports. Lateral control could require a large increase in
pilot attention in a severe shear that could deteriorate the
pilot's flight-path-tracking performance.
Thus, there is no justification of an assumption that general
aviation aircraft are less susceptible to wind shear than transport
aircraft. More research is needed on representative general aviation
aircraft to determine their vulnerability to wind shear. NASA has
conducted wind tunnel tests on a series of full-scale general aviation
airplanes, and this would provide a data base for the required
aerodynamic characteristics. The Ames Research Center has performed
related simulator investigations of wake vortex encounters on various
categories of airplanes, using available aerodynamic data for
representative airplanes. The aerodynamic data base that permits a
detailed piloted simulation is generally available or could be
estimated, and it should be suitable for analytical and simulator
investigations of general aviation airplanes in wind shear.
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Guidance and Control Aids
The threat to safe aircraft operation posed by wind-shear effects,
as distinct from high levels of turbulence, occurs principally near
the ground, when an aircraft is taking off or landing. During these
phases of flight, the pilot's workload could be greatly al leviated
with guidance and control aids. Documented dangerous encounters with
wind shear have lasted for half a minute or less, implying transient
forcing of the closed-loop phugoid motion, which provides the basic
mechanism for interchanging kinetic and potential energy, i.e., for
trading speed and altitude. Guidance and control aids could improve
pilot capability in effectively combating wind shear by helping to
manage the aircraft's energy level, either automatically or by helping
to perform the necessary maneuvers. Other critical elements in the
cockpit for coping with wind shear include:
o Warning that severe wind shear may be encountered.
O Recognition that a wind shear has been encountered .
O Prompt initiation of the proper control actions to deal with
the effects of wind shear.
o Exercise of continuous control actions during an encounter so
as to succes s ful ly go-around, complete the takeof f, or
successfully land or execute ground contact under circumstances
that will maximize the chances for survival.
All of these elements need to be addressed to improve flight safety in
the presence of wind-shear hazards.
In the context of practical flight operation, guidance and control
aids must allow an aircraft to complete its nominal flight phase as
often as possible, reserving the decision to abort the approach or
delay the takeof f as infrequently as poss ible and for only the more
severe shear encounters. The "go/no-go" decision must be accurate and
must be based on all available information: meteorological, LLWSAS,
contra l tower reports, and as a last resort onboard warning or
guidance information if a shear is encountered .
Wind-shear guidance and control aids must be capable of using the
full performance of the aircraft. As distinct from normal conditions,
severe wind shears can drive an aircraf t to "the edge of the f light
envelope" and beyond. Consequently, these aids mus t be optimal in the
sense of generating maximum performance while maintaining safety
marg in S .
F1 ight guidance displays provided to pilots range from very
primit ive one s on light general aviation a ircraf t to very
sophisticated displays on advanced commercial transports that
incorporate wind-shear guidance features. In between these extremes
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are the bulk of aircraft that are not adequately equipped with
guidance systems to cope with a severe wind-shear encounter. In fact,
some flight directors may provide misleading guidance information by
commanding nose-down pitch to recover speed in an increasing tailwind,
rather than anticipating a loss in altitude and commanding a pitch-up
maneuver. Manufacturers should review their systems to determine if
misleading commands exist and to warn pilots of this possibility. It
is imperative that wind-shear guidance and control systems provide
margins in their designs for turbulence and rain effects when these
effects have been determined.
Specifications for appropriate onboard systems depend on the type
of aircraft as well as on the circumstances of installation (new or
retrofit). The components of wind-shear guidance and control aids may
include sensors, ground-air communication links, cockpit control
devices, computers, and displays.
Systems can be assembled from off-the-shelf components; in fact,
wind-shear warning devices are currently being sold, but only the most
advanced fl ight directors contain some degree of wind-shear command
capability to provide the required margin of safety and a minimum of
schedule interruptions. More research and development must be done,
particularly on ground and airborne "predictive" sensors.
Airborne Warning Sys tems
All airborne radar warning systems based on current technology
will likely suffer from the ground-return (clutter) problem. This
problem stems from the need for an aircraft on approach to scan the
descending fl ight-path volume ahead for evidence of wind-shear
activity. A low-altitude look-down capability (ahead of the aircraft)
probably is not feasible in the near future because of associated
development problems. However, research should be continued on
promising technologies that can sense wind shear before it is
encountered. The payoff for a practical device would be high.
Existing airborne weather radar, and lidar and IR sys tems under
development, can look up the proposed flight path without attendant
ground-c lut t er re t urn . I f such radar had a Doppl er capab i 1 ity, i t
could allow pilots to detect severe low-altitude wind shear in the
critical portion of the proposed takeof f path. This information could
influence the pilot to delay takeof f until the observed or suspected
wind shear had dissipated and, through PIREPs, provide a warning to
approaching a ircraf t .
Because pilots bear the ultimate responsibility for ensuring safe
flight, the information presented to them regarding aircraft state and
wind-shear environment must be appropriate, cone ise, and complete . It
should be presented in such a way as to enable pilots to make the
right decisions. As a minimum, pilots need both a wind-shear warning
and a wind-shear command display. The warning is needed to identify a
possible fl ight-critical event and a confirmation that abnormal
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aircraft response and cockpit displays can be expected. Pilots should
be apprised of the correct command strategy, either to control
manually or to monitor the automatic system. command information is
required both for aborting and for completing the intended flight
phase.
Devices. State-of-the-art equipment appears to be sufficiently well
developed that no research is needed specifically for wind-shear
components. However, consideration should be given to nonstandard
application of flight controls such as spoilers and flaps.
Algorithms. The key to effective use of available components is
the logic that binds them together. Existing flight computers can
implement anti-wind-shear logic, processing, as necessary, a multitude
of inputs and outputs. Onboard computers could carry mathematical
models of aircraft performance, nonlinear control logic, and optimal
estimation algorithms. These can be used to ensure that the aircraft
and the pilot perform at their best in a wind shear.
Manual and Automatic Control Systems
Guidance and control systems can be designed to generate the
critical functions and displays required to cope with most wind-shear
encounters. The automatic flight control systems on advanced
transport aircraft have the capability to satisfactorily perform many
of these functions automatically. Such systems and their associated
displays constitute a near-term approach to providing adequate
guidance and control aids for dealing with wind shear. An example of
such a system is discussed in Appendix B. It should be noted that
advanced transport aircraft in service today have these capabilities.
~ . Extensive
evaluations must be performed to ensure that automatic flight control
systems operate satisfactorily during all foreseeable situations.
Detailed nonlinear models of automatic flight control systems are
investigated on a simulator in a large variety of wind-shear
conditions to validate the systems. These evaluations include, for
instance, sensitivity analyses with discrete wind-shear models to
ensure that landing performance does not vary excessively when wind
levels are increased. Finally, flight testing in the terminal area is
performed with the express purpose of demonstrating satisfactory
performance in the most severe winds available and to validate earlier
analytical and simulator test results. These winds, however, do not
approach the severity of those evaluated by analysis or simulation.
This work is part of FAA's certification process for airplanes and
their subsystems.
Based on the demonstrated capability of systems such as the one
described in Appendix B. current technology and existing guidance and
control systems have the inherent capacity to cope with unexpected
wind shears up to some level of severity, reflecting the basic
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aircraft performance limits. However, newer, more complete wind-shear
models have not yet been evaluated in connection with such guidance
and control systems. This needs to be done to more fully determine
acceptable system performance for wind shears used for system design.
Furthermore, research should be conducted to ensure that the
operational envelope of flight safety provided by automatic systems is
near the maximum attainable.
Separate Wind-Shear Indication and Alerting Displays. Many
existing autopilot and/or flight director sys tems contain the basic
sensors and can be upgraded to a wind-shear-certified level, but such
modifications would undoubtedly be costly and time-consuming. This
would be especially true for automatic system modifications, which
would have to be multiple-redundant to achieve dispatch reliability.
Therefore, consideration should be given to wind-shear-specific
di splays that ran be added to existing cockpit instrumentation. A
study sponsored by the FAA (Foy, 1977) explored a number of such
possibilities. Most of the reasonably successful displays measured
ground speed, which could eas i ly be compared wi th airspeed to deduce
wind speed and which was additionally used as the teas is for commands
on the fast/slow indicator. Some also used energy rate or
acceleration margin for a warning and go-around advisory. However,
their relative success was airplane dependent, being considerably
higher for the DC-10 and Boeing 727 than for the Boeing 707, perhaps,
although not so stated, because of the additional engine-out
performance margin of a 3-engine compared with a 4-engine airplane.
Successful penetrations for the 3-engine aircraft group were
approximately 70-80 percent, whereas succesful penetrations for the
4-engine Boeing 707 dropped to approximately 40 percent. None of the
systems permitted the pilot to cope successfully with severe wind
shear at takeoff. The relatively poor penetration performance during
takeoff was about the same as for the baseline (standard instruments)
condition.
Additional studies were conducted by Bray at NASA's Ames Research
Center (Foy, 1979~. This effort led to a head-up display (HUD)
presentation that used complementary-filtered horizontal inertial
acceleration and rate-of-change of airspeed and vertical inertial
acceleration and rate-of-change of barometric climb rate. The system
provided s tatus information with a signal corresponding to potential
fl ight path serving as the primary alerting cue. It also presented
the angle of attack, developed from measured pitch attitude and
computed flight-path angle. The pilots exposed to the HUD thought it
was a very good learning tool and that it appeared to improve
shear-penetration performance. However, there were a significant
number of unsuccessful shear penetrations recorded in the studies.
~ _ . Perhaps encouraged by the
degree of wind-shear penetration improvement reported by some of the
studies discussed above, various avionics manufacturers have begun
marketing products specifically designed to improve pilot performance
in wind shears . Of three systems that have reached hardware s sage,
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those by Smiths Industries and SFENA incorporate new information in an
existing display, while Safe Flight is producing a separate
instrument. Other devices may be available but were not made known to
the committee.
The Smiths system utilizes air data computer outputs of vertical
speed and airspeed, differentiating the latter and combining the
result with vertical speed to give energy rate. The energy-rate
information is displayed on a second needle incorporated into the
vertical speed indicator. An energy rate that falls below vertical
speed (climb rate) implies a requirement for added thrust and vice
versa.
The Safe Flight system uses an aircraft's existing airspeed and
angle-of-attack information from conventional sensors, along with
horizontal and vertical accelerometers supplied by their computer to
calculate wind-shear components and alert the pilot. At least one
airline intends to retrofit its fleet with this device if planned
simulations are successful and "if the Government authorizes everyday
f 1 eetwide use" (New York Times, June 6, 1983~.
Angle-of-attack sensors have long been available commercially and
are considered by many to be an obvious and highly desirable aid in
the proper utilization of an aircraft's maximum climb performance, so
necessary to the successful transit of severe wind shear.
An energy-rate sensor described at NASA's Langley Research Center
(Ostroff, 1983) is an interesting instrument that, because of its
relative simplicity, may be added to existing general aviation
aircraft. The probe is simply a hollow round tube with an aft-facing
hole near its closed tip. The tube is mounted on the side of the
airplane near the nose, protruding at an angle 20° forward of a line
perpendicular to the surface. An interior line connects the probe to
a climb-rate transducer through a restrictor and a filter volume .
Increases in either speed or altitude cause a decreased pressure at
the sensing hole (and vice versa), which is transformed to an
energy-rate indication by the resulting "leakage" flow.
Although it is not described as an anti-wind-shear device, the
Hunt ing ton Air Speed Direc tor (Kidd, 1983 ~ could improve the
probabilities of a safe wind-shear penetration. The device measures a
differential pressure that is proportional to the product of dynamic
pressure and angle of attack. It has been concluded that changes in
this quantity are proportional, in turn, to vertical acceleration, or
roughly to the energy acceleration, which provides a direct indication
of hazardous downdrafts and tailwinds.
Applicability and Utility. Assuming that the aforementioned
systems give correct command and display information to a pilot during
all types of wind-shear encounters, they would be a useful addition to
present transport and general aviation aircraft to reduce the hazard
posed by wind shear s . However, the extent of their individual
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usefulness for particular aircraft has yet to be established, as does
their routine usability. Many airline transport pilots strongly
resist the imposition of yet another instrument into their already
crowded scan pattern unless it reduces their workload.
To the extent that such aids are considered crucial or critical to
flight, they must be certified and their reliability to established
standards demonstrated if they are to be required for aircraft
dispatch. Reliability implies low failure rates; however, the greater
concern is the probability of malfunction during a wind-shear
encounter. This is the product of the individual probabilities of
malfunction and the probability of encounter, both of which should be
much less than one.
Requirements. On May 3, 1979, the F. M issued an Advance Notice of
Proposed Rulemaking (NPRM 79-11, Docket No. 19110~. This notice
discussed FAA research and development on wind shear, and it requested
comments and recommendations to assist the agency in determining what,
if any, regulatory proposals should be developed to amend FAR 121 to
require wind-shear detection equipment or other onboard systems to
assist in coping with hazardous wind shears. The cogent period
closed on August 3, 1979. The FAA docket file includes 35 responses
from industry, government organizations, and the public.
At the time the notice was issued, the results of the FAA
s imulator research program were incomplete and not widely circulated.
The consensus of the comments on the NPRM appeared to be that research
and deve lopment work should cont inue on ground and airborne wind-shear
systems to gain better understanding of the problems and to develop
practical solutions prior to any regulatory action by the FAA. No
final action has been taken on the NPRM 79-11 and the docket is still
open. In view of the advances in anti-wind-shear technology, it may
be timely to ask again those questions posed in NPRM 79-11 and to
cons ider whether regulatory ac tior1 by the FAA is now appropriate to
require anti-wind-shear equipment for air carrier aircraft.
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Representative terms from entire chapter:
wind shears
