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EXAMPLE-
1. Denver Tower, Cessna 1234 encountered wind shear,
loss of 20 knots at 400.
2. Tulsa Tower, American 721 encountered wind shear on
final, gained 25 knots between 600 and 400 feet followed
by loss of 40 knots between 400 feet and surface.
Pilots using Inertial Navigation Systems should
report the wind and altitude both above and below the
shear layer.
EXAMPLE-
Miami Tower, Gulfstream 403 Charlie encountered an
abrupt wind shear at 800 feet on final, max thrust required.
Pilots who are not able to report wind shear in these
specific terms are encouraged to make reports in
terms of the effect upon their aircraft.
24. Clear Air Turbulence (CAT) PIREPs
24.1 Clear air turbulence (CAT) has become a very
serious operational factor to flight operations at all
levels and especially to jet traffic flying in excess of
15,000 feet. The best available information on this
phenomenon must come from pilots via the PIREP
procedures. All pilots encountering CAT conditions
are urgently requested to report time, location, and
intensity (light, moderate, severe, or extreme) of the
element to the FAA facility with which they are
maintaining radio contact. If time and conditions
permit, elements should be reported according to the
standards for other PIREPs and position reports. See
TBL GEN 3.5-10, Turbulence Reporting Criteria
Table.
25. Microbursts
25.1 Relatively recent meteorological studies have
confirmed the existence of microburst phenomena.
Microbursts are small-scale intense downdrafts
which, on reaching the surface, spread outward in all
directions from the downdraft center. This causes the
presence of both vertical and horizontal wind shears
that can be extremely hazardous to all types and
categories of aircraft, especially at low altitudes. Due
to their small size, short life-span, and the fact that
they can occur over areas without surface precipita-
tion, microbursts are not easily detectable using
conventional weather radar or wind shear alert
systems.
25.2 Parent clouds producing microburst activity
can be any of the low or middle layer convective
cloud types. Note however, that microbursts
commonly occur within the heavy rain portion of
thunderstorms, and in much weaker, benign-appear-
ing convective cells that have little or no precipitation
reaching the ground.
25.3 The life cycle of a microburst as it descends in
a convective rain shaft is seen in FIG GEN 3.5-8,
Evolution of a Microburst. An important consideration
for pilots is the fact that the microburst intensifies for
about 5 minutes after it strikes the ground.
25.4 Characteristics of microbursts include:
25.4.1 Size. The microburst downdraft is typically
less than 1 mile in diameter as it descends from the
cloud base to about 1,000-3,000 feet above the
ground. In the transition zone near the ground, the
downdraft changes to a horizontal outflow that can
extend to approximately 2 1
/2 miles in diameter.
25.4.2 Intensity. The downdrafts can be as strong
as 6,000 feet per minute. Horizontal winds near the
surface can be as strong as 45 knots resulting in a
90-knot shear (headwind to tailwind change for a
traversing aircraft) across the microburst. These
strong horizontal winds occur within a few hundred
feet of the ground.
25.4.3 Visual Signs. Microbursts can be found
almost anywhere that there is convective activity.
They may be embedded in heavy rain associated with
a thunderstorm or in light rain in benign- appearing
virga. When there is little or no precipitation at the
surface accompanying the microburst, a ring of
blowing dust may be the only visual clue of its
existence.
30 AUG 07
AIP
United States of America
GEN 3.5-43
15 MAR 07
Federal Aviation Administration Nineteenth Edition
FIG GEN 3.5-8
Evolution of a Microburst
5 -in -2 in 5 in 10 in
HEIGHT (feet)
10,000
5,000
WIND SPEED
20 10-nots
20 nots
SCALE (miles)
0 1 2 3
Vertical cross section of the evolution of a microburst wind field. T is the time of initial divergence at
the surface. The shading refers to the vector wind speeds. Figure adapted from Wilson et al., 1984,
Microburst Wind Structure and Evaluation of Doppler Radar for Wind Shear Detection, DOT/FAA
Report No. DOT/FAA/PM-84/29, National Technical Information Service, Springfield, VA 37 pp.
25.4.4 Duration. An individual microburst will
seldom last longer than 15 minutes from the time it
strikes the ground until dissipation. The horizontal
winds continue to increase during the first 5 minutes
with the maximum intensity winds lasting approxi-
mately 2-4 minutes. Sometimes microbursts are
concentrated into a line structure and, under these
conditions, activity may continue for as long as
1_hour. Once microburst activity starts, multiple
microbursts in the same general area are not
uncommon and should be expected.
30 AUG 07
AIP
United States of America
GEN 3.5-44
15 MAR 07
Federal Aviation Administration Nineteenth Edition
FIG GEN 3.5-9
Microburst Encounter During Takeoff
NOTE-
A microburst encounter during takeoff. The airplane first encounters a headwind and experiences increasing performance
(1), this is followed in short succession by a decreasing headwind component (2), a downdraft (3), and finally a strong
tailwind (4), where 2 through 5 all result in decreasing performance of the airplane. Position (5) represents an extreme
situation just prior to impact. Figure courtesy of Walter Frost, FWG Associates, Inc., Tullahoma, Tennessee.
25.5 Microburst wind shear may create a severe
hazard for aircraft within 1,000 feet of the ground,
particularly during the approach to landing and
landing and take-off phases. The impact of a
microburst on aircraft which have the unfortunate
experience of penetrating one is characterized in
FIG GEN 3.5-9. The aircraft may encounter a
headwind (performance increasing), followed by a
downdraft and a tailwind (both performance
decreasing), possibly resulting in terrain impact.
30 AUG 07
AIP
United States of America
GEN 3.5-45
15 MAR 07
Federal Aviation Administration Nineteenth Edition
FIG GEN 3.5-10
25.6 Detection of Microbursts, Wind Shear, and
Gust Fronts
25.6.1 FAA's Integrated Wind Shear Detection
Plan
25.6.1.1 The FAA currently employs an integrated
plan for wind shear detection that will significantly
improve both the safety and capacity of the majority
of the airports currently served by the air carriers.
This plan integrates several programs, such as the
Integrated Terminal Weather System (ITWS),
Terminal Doppler Weather Radar (TDWR), Weather
System Processor (WSP), and Low Level Wind Shear
Alert Systems (LLWAS) into a single strategic
concept that significantly improves the aviation
weather information in the terminal area.
(See FIG GEN 3.5-10.)
25.6.1.2 The wind shear/microburst information and
warnings are displayed on the ribbon display
terminal (RBDT) located in the tower cabs. They are
identical (and standardized) to those in the LLWAS,
TDWR and WSP systems, and designed so that the
controller does not need to interpret the data, but
simply read the displayed information to the pilot.
The RBDTs are constantly monitored by the
controller to ensure the rapid and timely dissemina-
tion of any hazardous event(s) to the pilot.
30 AUG 07
AIP
United States of America
GEN 3.5-46
15 MAR 07
Federal Aviation Administration Nineteenth Edition
FIG GEN 3.5-11
25.6.1.3 The early detection of a wind shear/micro-
burst event, and the subsequent warning(s) issued to
an aircraft on approach or departure, will alert the
pilot/crew to the potential of, and to be prepared for,
a situation that could become very dangerous!
Without these warnings, the aircraft may NOT be able
to climb out of or safely transition the event, resulting
in a catastrophe. The air carriers, working with the
FAA, have developed specialized training programs
using their simulators to train and prepare their pilots
on the demanding aircraft procedures required to
escape these very dangerous wind shear and/or
microburst encounters.
25.6.1.4 Low Level Wind Shear Alert System
(LLWAS)
a) The LLWAS provides wind data and software
processes to detect the presence of hazardous wind
shear and microbursts in the vicinity of an airport.
Wind sensors, mounted on poles sometimes as high
as 150 feet, are (ideally) located 2,000 - 3,500 feet,
but not more than 5,000 feet, from the centerline of
the runway. (See FIG GEN 3.5-11.)
b) The LLWAS was fielded in 1988 at 110 airports
across the nation. Many of these systems have been
replaced by new terminal doppler weather radar
(TDWR) and weather systems processor (WSP)
technology. Eventually all LLWAS systems will be
phased out; however, 39 airports will be upgraded to
the LLWAS-NE (Network Expansion) system,
which employs the very latest software and sensor
technology. The new LLWAS-NE systems will not
only provide the controller with wind shear warnings
and alerts, including wind shear/microburst detection
at the airport wind sensor location, but will also
provide the location of the hazards relative to the
airport runway(s). It will also have the flexibility and
capability to grow with the airport as new runways are
built. As many as 32 sensors, strategically located
around the airport and in relationship to its runway
configuration, can be accommodated by the
LLWAS-NE network.
30 AUG 07
AIP
United States of America
GEN 3.5-47
15 MAR 07
Federal Aviation Administration Nineteenth Edition
FIG GEN 3.5-12
25.6.1.5 Terminal Doppler Weather Radar
(TDWR)
a) TDWRs are being deployed at 45 locations
across the U.S. Optimum locations for TDWRs are
8_to 12 miles from the airport proper, and designed to
look at the airspace around and over the airport to
detect microbursts, gust fronts, wind shifts, and
precipitation intensities. TDWR products advise the
controller of wind shear and microburst events
impacting all runways and the areas 1
/2 mile on either
side of the extended centerline of the runways and to
a distance of 3 miles on final approach and 2 miles on
departure. FIG GEN 3.5-12 is a theoretical view of
the runway and the warning boxes that the software
uses to determine the location(s) of wind shear or
microbursts. These warnings are displayed (as
depicted in the examples in subparagraph e) on the
ribbon display terminal located in the tower cabs.
b) It is very important to understand what TDWR
DOES NOT DO:
1) It_DOES NOT warn of wind shear outside of
the alert boxes (on the arrival and departure ends of
the runways).
2) It_DOES NOT detect wind shear that is
NOT a microburst or a gust front.
3) It_DOES NOT detect gusty or cross wind
conditions.
4) It _DOES NOT detect turbulence.
However, research and development is continuing on
these systems. Future improvements may include
such areas as storm motion (movement), improved
gust front detection, storm growth and decay,
microburst prediction, and turbulence detection.
c) TDWR also provides a geographical situation
display (GSD) for supervisors and traffic manage-
ment specialists for planning purposes. The GSD
displays (in color) 6 levels of weather (precipitation),
gust fronts and predicted storm movement(s). This
data is used by the tower supervisor(s), traffic
management specialists, and controllers to plan for
runway changes and arrival/departure route changes
in order to reduce aircraft delays and increase airport
capacity.
25.6.1.6 Weather Systems Processor (WSP)
a) The WSP provides the controller, supervisor,
traffic management specialist, and ultimately the
pilot, with the same products as the terminal doppler
weather radar at a fraction of the cost. This is
accomplished by utilizing new technologies to access
the weather channel capabilities of the existing
ASR-9 radar located on or near the airport, thus
eliminating the requirements for a separate radar
location, land acquisition, support facilities, and the
associated communication landlines and expenses.
30 AUG 07
AIP
United States of America
GEN 3.5-48
15 MAR 07
Federal Aviation Administration Nineteenth Edition
b) The WSP utilizes the same RBDT display as the
TDWR and LLWAS, and, like the TDWR, has a GSD
for planning purposes by supervisors, traffic
management specialists, and controllers. The WSP
GSD emulates the TDWR display; i.e., it also depicts
6 levels of precipitation, gust fronts and predicted
storm movement, and like the TDWR, GSD is used
to plan for runway changes and arrival/departure
route changes in order to reduce aircraft delays and to
increase airport capacity.
c) This system is currently under development and
is operating in a developmental test status at the
Albuquerque, New Mexico, airport. When fielded,
the WSP is expected to be installed at 34 airports
across the nation, substantially increasing the safety
of flying.
25.6.1.7 Operational Aspects of LLWAS, TDWR,
and WSP
To demonstrate how this data is used by both the
controller and the pilot, 3 ribbon display examples
and their explanations are presented:
a) MICROBURST ALERTS
EXAMPLE-
This is what the controller sees on his/her ribbon display
in the tower cab.
27A MBA 35K- 2MF 250 20
NOTE-
(See FIG GEN 3.5-13 to see how the TDWR/WSP
determines the microburst location).
This is what the controller will say when issuing the
alert.
PHRASEOLOGY-
RUNWAY 27 ARRIVAL, MICROBURST ALERT, 35 KT
LOSS 2 MILE FINAL, THRESHOLD WINDS 250 AT 20.
In plain language, the controller is telling the pilot
that on approach to runway 27, there is a microburst
alert on the approach lane to the runway, and to
anticipate or expect a 35-knot loss of airspeed at
approximately 2 miles out on final approach (where
the aircraft will first encounter the phenomena). With
that information, the aircrew is forewarned, and
should be prepared to apply wind shear/microburst
escape procedures should they decide to continue the
approach. Additionally, the surface winds at the
airport for landing runway 27 are reported as
250_degrees at 20 knots.
NOTE-
Threshold wind is at pilot's request or as deemed
appropriate by the controller.
b) WIND SHEAR ALERTS
EXAMPLE-
This is what the controller sees on his/her ribbon display
in the tower cab.
27A WSA 20K- 3MF 200 15
NOTE-
(See FIG GEN 3.5-14 to see how the TDWR/WSP
determines the wind shear location).
This is what the controller will say when issuing the
alert.
PHRASEOLOGY-
RUNWAY 27 ARRIVAL, WIND SHEAR ALERT, 20 KT
LOSS 3 MILE FINAL, THRESHOLD WINDS 200 AT 15.
In plain language, the controller is advising the
aircraft arriving on runway 27 that at 3 miles out the
pilot should expect to encounter a wind shear
condition that will decrease airspeed by 20 knots and
possibly the aircraft will encounter turbulence.
Additionally, the airport surface winds for landing
runway 27 are reported as 200 degrees at 15 knots.
NOTE-
Threshold wind is at pilot's request or as deemed
appropriate by the controller.
30 AUG 07
AIP
United States of America
GEN 3.5-49
15 MAR 07
Federal Aviation Administration Nineteenth Edition
FIG GEN 3.5-13
FIG GEN 3.5-14
30 AUG 07
AIP
United States of America
GEN 3.5-50
15 MAR 07
Federal Aviation Administration Nineteenth Edition
FIG GEN 3.5-15
c) MULTIPLE WIND SHEAR ALERTS
EXAMPLE-
This is what the controller sees on his/her ribbon display
in the tower cab.
27A WSA 20K+ RWY 250 20
27D WSA 20K+ RWY 250 20
NOTE-
(See FIG GEN 3.5-15 to see how the TDWR/WSP
determines the gust front/wind shear location).
This is what the controller will say when issuing the
alert.
PHRASEOLOGY-
MULTIPLE WIND SHEAR ALERTS.
RUNWAY 27 ARRIVAL, WIND SHEAR ALERT, 20 KT
GAIN ON RUNWAY;
RUNWAY 27 DEPARTURE, WIND SHEAR ALERT, 20 KT
GAIN ON RUNWAY, WINDS 250 AT 20.
EXAMPLE-
In this example, the controller is advising arriving and
departing aircraft that they could encounter a wind shear
condition right on the runway due to a gust front
(significant change of wind direction) with the possibility
of a 20 knot gain in airspeed associated with the gust front.
Additionally, the airport surface winds (for the runway in
use) are reported as 250 degrees at 20 knots.
25.6.1.8 The Terminal Weather Information for
Pilots System (TWIP)
a) With the increase in the quantity and quality of
terminal weather information available through
TDWR, the next step is to provide this information
directly to pilots rather than relying on voice
communications from ATC. The National Airspace
System has long been in need of a means of delivering
terminal weather information to the cockpit more
efficiently in terms of both speed and accuracy to
enhance pilot awareness of weather hazards and to
reduce air traffic controller workload. With the TWIP
capability, terminal weather information, both
30 AUG 07
AIP
United States of America
GEN 3.5-51
15 MAR 07
Federal Aviation Administration Nineteenth Edition
alphanumerically and graphically, is now available
directly to the cockpit on a test basis at 9 locations.
b) TWIP products are generated using weather
data from the TDWR or the Integrated Terminal
Weather System (ITWS) testbed. TWIP products are
generated and stored in the form of text and character
graphic messages. Software has been developed to
allow TDWR or ITWS to format the data and send the
TWIP products to a database resident at Aeronautical
Radio, Inc. (ARINC). These products can then be
accessed by pilots using the ARINC Aircraft
Communications Addressing and Reporting System
(ACARS) data link services. Airline dispatchers can
also access this database and send messages to
specific aircraft whenever wind shear activity begins
or ends at an airport.
c) TWIP products include descriptions and
character graphics of microburst alerts, wind shear
alerts, significant precipitation, convective activity
within 30 NM surrounding the terminal area, and
expected weather that will impact airport operations.
During inclement weather; i.e., whenever a predeter-
mined level of precipitation or wind shear is detected
within 15 miles of the terminal area, TWIP products
are updated once each minute for text messages and
once every 5 minutes for character graphic messages.
During good weather (below the predetermined
precipitation or wind shear parameters) each message
is updated every 10 minutes. These products are
intended to improve the situational awareness of the
pilot/flight crew, and to aid in flight planning prior to
arriving or departing the terminal area. It is important
to understand that, in the context of TWIP, the
predetermined levels for inclement versus good
weather has nothing to do with the criteria for
VFR/MVFR/IFR/LIFR; it only deals with precipita-
tion, wind shears, and microbursts.
26. PIREPs Relating to Volcanic Ash Activity
26.1 Volcanic eruptions which send ash into the
upper atmosphere occur somewhere around the world
several times each year. Flying into a volcanic ash
cloud can be exceedingly dangerous. At least two
B747s have lost all power in all four engines after
such an encounter. Regardless of the type aircraft,
some damage is almost certain to ensue after an
encounter with a volcanic ash cloud.
26.2 While some volcanoes in the U.S. are
monitored, many in remote areas are not. These
unmonitored volcanoes may erupt without prior
warning to the aviation community. A pilot observing
a volcanic eruption who has not had previous
notification of it may be the only witness to the
eruption. Pilots are strongly encouraged to transmit a
PIREP regarding volcanic eruptions and any
observed volcanic ash clouds.
26.3 Pilots should submit PIREPs regarding volca-
nic activity using the Volcanic Activity Reporting
form (VAR) as illustrated in FIG GEN 3.5-30. (If a
VAR form is not immediately available, relay enough
information to identify the position and type of
volcanic activity.)
26.4 Pilots should verbally transmit the data required
in items 1 through 8 of the VAR as soon as possible.
The data required in items 9 through 16 of the VAR
should be relayed after landing, if possible.
27. Thunderstorms
27.1 Turbulence, hail, rain, snow, lightning, sus-
tained updrafts and downdrafts, and icing conditions
are all present in thunderstorms. While there is some
evidence that maximum turbulence exists at the
middle level of a thunderstorm, recent studies show
little variation of turbulence intensity with altitude.
27.2 There is no useful correlation between the
external visual appearance of thunderstorms and the
severity or amount of turbulence or hail within them.
Also, the visible thunderstorm cloud is only a portion
of a turbulent system whose updrafts and downdrafts
often extend far beyond the visible storm cloud.
Severe turbulence can be expected up to 20 miles
from severe thunderstorms. This distance decreases
to about 10 miles in less severe storms. These
turbulent areas may appear as a well-defined echo on
weather radar.
27.3 Weather radar, airborne or ground-based, will
normally reflect the areas of moderate to heavy
precipitation. (Radar does not detect turbulence.) The
frequency and severity of turbulence generally
increases with the areas of highest liquid water
content of the storm. NO FLIGHT PATH THROUGH
AN AREA OF STRONG OR VERY STRONG
RADAR ECHOES SEPARATED BY 20-30 MILES
OR LESS MAY BE CONSIDERED FREE OF
SEVERE TURBULENCE.
30 AUG 07
AIP
United States of America
GEN 3.5-52
15 MAR 07
Federal Aviation Administration Nineteenth Edition
27.4 Turbulence beneath a thunderstorm should not
be minimized. This is especially true when the
relative humidity is low in any layer between the
surface and 15,000 feet. Then the lower altitudes may
be characterized by strong out-flowing winds and
severe turbulence.
27.5 The probability of lightning strikes occurring to
aircraft is greatest when operating at altitudes where
temperatures are between -5 C and +5 C. Lightning
can strike aircraft flying in the clear in the vicinity of
a thunderstorm.
27.6 Current weather radar systems are able to
objectively determine precipitation intensity. These
precipitation intensity areas are described as “light,”
“moderate,” “heavy,” and “extreme.”
REFERENCE-
Pilot/Controller Glossary Term- Precipitation Radar Weather
Descriptions.
EXAMPLE-
Alert provided by an ATC facility to an aircraft:
(aircraft identification) EXTREME precipitation between
ten o'clock and two o'clock, one five miles. Precipitation
area is two five miles in diameter.
EXAMPLE-
Alert provided by an AFSS/FSS:
(aircraft identification) EXTREME precipitation two zero
miles west of Atlanta V-O-R, two five miles wide, moving
east at two zero knots, tops flight level three niner zero.
28. Thunderstorm Flying
28.1 Above all, remember this: never regard any
thunderstorm lightly, even when radar observers
report the echoes are of light intensity. Avoiding
thunderstorms is the best policy. Following are some
Do's and Don'ts of thunderstorm avoidance:
28.1.1 Don't land or takeoff in the face of an
approaching thunderstorm. A sudden gust front of
low-level turbulence could cause loss of control.
28.1.2 Don't attempt to fly under a thunderstorm
even if you can see through to the other side.
Turbulence and wind shear under the storm could be
disastrous.
28.1.3 Don't fly without airborne radar into a cloud
mass containing scattered embedded thunderstorms.
Scattered thunderstorms not embedded usually can
be visually circumnavigated.
28.1.4 Don't trust the visual appearance to be a
reliable indicator of the turbulence inside a
thunderstorm.
28.1.5 Do avoid by at least 20 miles any
thunderstorm identified as severe or giving an intense
radar echo. This is especially true under the anvil of
a large cumulonimbus.
28.1.6 Do clear the top of a known or suspected
severe thunderstorm by at least 1,000 feet altitude for
each 10 knots of wind speed at the cloud top.
However, the altitude capability of most aircraft
make it unlikely that the aircraft will be able to clear
the storm top.
28.1.7 Do circumnavigate the entire area if the area
has 6/10 thunderstorm coverage.
28.1.8 Do remember that vivid and frequent
lightning indicates the probability of a severe
thunderstorm.
28.1.9 Do regard as extremely hazardous any
thunderstorm that tops 35,000 feet or higher whether
the top is visually sighted or determined by radar.
28.2 If you cannot avoid penetrating a thunderstorm,
before entering the storm, you should do the
following:
28.2.1 Tighten your safety belt, put on your shoulder
harness if you have one, and secure all loose objects.
28.2.2 Plan and hold your course to take you through
the storm in a minimum time.
28.2.3 To avoid the most critical icing, establish a
penetration altitude below the freezing level or above
the level of -15 C.
28.2.4 Verify that pitot heat is on and turn on
carburetor heat or jet engine anti-ice. Icing can be
rapid at any altitude and cause almost instantaneous
power failure and/or loss of airspeed indication.
28.2.5 Establish power settings for turbulence
penetration airspeed recommended in your aircraft
manual.
28.2.6 Turn up cockpit lights to highest intensity to
lessen danger of temporary blindness from lightning.
28.2.7 If using automatic pilot, disengage altitude
hold mode and speed hold mode. The automatic
altitude and speed controls will increase maneuvers
of the aircraft thus increasing structural stresses.
30 AUG 07
AIP
United States of America
GEN 3.5-53
15 MAR 07
Federal Aviation Administration Nineteenth Edition
28.2.8 If using airborne radar, tilt the antenna up and
down occasionally. This will permit you to detect
other thunderstorm activity at altitudes other than the
one being flown.
28.3 Following are some Do's and Don'ts during the
thunderstorm penetration:
28.3.1 Do keep your eyes on your instruments.
Looking outside the cockpit can increase danger of
temporary blindness from lightning.
28.3.2 Don't change power settings; maintain
settings for the recommended turbulence penetration
airspeed.
28.3.3 Don't attempt to maintain constant altitude;
let the aircraft “ride the waves.”
28.3.4 Don't turn back once you are in the
thunderstorm. A straight course through the storm
most likely will get you out of the hazards more
quickly. In addition, turning maneuvers increase
stress on the aircraft.
29. Wake Turbulence
29.1 General
29.1.1 Every aircraft generates a wake while in
flight. Initially, when pilots encountered this wake in
flight, the disturbance was attributed to “prop wash.”
It is known, however, that this disturbance is caused
by a pair of counterrotating vortices trailing from the
wing tips. The vortices from larger aircraft pose
problems to encountering aircraft. For instance, the
wake of these aircraft can impose rolling moments
exceeding the roll control authority of the encounter-
ing aircraft. Further, turbulence generated within the
vortices can damage aircraft components and
equipment if encountered at close range. The pilot
must learn to envision the location of the vortex wake
generated by larger (transport category) aircraft and
adjust the flight path accordingly.
29.1.2 During ground operations and during takeoff,
jet engine blast (thrust stream turbulence) can cause
damage and upsets if encountered at close range.
Exhaust velocity versus distance studies at various
thrust levels have shown a need for light aircraft to
maintain an adequate separation behind large turbojet
aircraft. Pilots of larger aircraft should be particularly
careful to consider the effects of their “jet blast” on
other aircraft, vehicles, and maintenance equipment
during ground operations.
29.2 Vortex Generation
29.2.1 Lift is generated by the creation of a pressure
differential over the wing surface. The lowest
pressure occurs over the upper wing surface and the
highest pressure under the wing. This pressure
differential triggers the roll up of the airflow aft of the
wing resulting in swirling air masses trailing
downstream of the wing tips. After the roll up is
completed, the wake consists of two counter rotating
cylindrical vortices. Most of the energy is within a
few feet of the center of each vortex, but pilots should
avoid a region within about 100 feet of the vortex
core. (See FIG GEN 3.5-16.)
29.3 Vortex Strength
29.3.1 The strength of the vortex is governed by the
weight, speed, and shape of the wing of the generating
aircraft. The vortex characteristics of any given
aircraft can also be changed by extension of flaps or
other wing configuring devices as well as by change
in speed. However, as the basic factor is weight, the
vortex strength increases proportionately. Peak
vortex tangential speeds up to almost 300 feet per
second have been recorded. The greatest vortex
strength occurs when the generating aircraft is
HEAVY, CLEAN, and SLOW.
29.3.2 Induced Roll
29.3.2.1 In rare instances, a wake encounter could
cause inflight structural damage of catastrophic
proportions. However, the usual hazard is associated
with induced rolling moments which can exceed the
roll control authority of the encountering aircraft. In
flight experiments, aircraft have been intentionally
flown directly up trailing vortex cores of larger
aircraft. It was shown that the capability of an aircraft
to counteract the roll imposed by the wake vortex
primarily depends on the wing span and counter-con-
trol responsiveness of the encountering aircraft.
29.3.2.2 Counter-control is usually effective and
induced roll minimal in cases where the wing span
and ailerons of the encountering aircraft extend
beyond the rotational flow field of the vortex. It is
more difficult for aircraft with short wing span
(relative to the generating aircraft) to counter the
imposed roll induced by vortex flow. Pilots of
short-span aircraft, even of the high-performance
type, must be especially alert to vortex encounters.
(See FIG GEN 3.5-17.)
29.3.2.3 The wake of larger aircraft requires the
respect of all pilots.
30 AUG 07
AIP
United States of America
GEN 3.5-54
15 MAR 07
Federal Aviation Administration Nineteenth Edition
29.4 Vortex Behavior
29.4.1 Trailing vortices have certain behavioral
characteristics which can help a pilot visualize the
wake location and thereby take avoidance precau-
tions.
29.4.1.1 Vortices are generated from the moment
aircraft leave the ground, since trailing vortices are a
by-product of wing lift. Prior to takeoff or touchdown
pilots should note the rotation or touchdown point of
the preceding aircraft. (See FIG GEN 3.5-18.)
29.4.1.2 The vortex circulation is outward, upward
and around the wing tips when viewed from either
ahead or behind the aircraft. Tests with large aircraft
have shown that the vortices remain spaced a bit less
than a wing span apart, drifting with the wind, at
altitudes greater than a wing span from the ground. In
view of this, if persistent vortex turbulence is
encountered, a slight change of altitude and lateral
position (preferably upwind) will provide a flight
path clear of the turbulence.
29.4.1.3 Flight tests have shown that the vortices
from larger (transport category) aircraft sink at a rate
of several hundred feet per minute, slowing their
descent and diminishing in strength with time and
distance behind the generating aircraft. Atmospheric
turbulence hastens breakup. Pilots should fly at or
above the preceding aircraft's flight path, altering
course as necessary to avoid the area behind and
below the generating aircraft. However, vertical
separation of 1,000 feet may be considered safe.
(See FIG GEN 3.5-19.)
FIG GEN 3.5-16
Wake Vortex Generation
FIG GEN 3.5-17
Wake Encounter Counter Control
COUNTER
CONTROL
FIG GEN 3.5-18
Wake Ends/Wake Begins
Touchdown Rotation
Wake Ends Wake Begins
30 AUG 07
AIP
United States of America
GEN 3.5-55
15 MAR 07
Federal Aviation Administration Nineteenth Edition
FIG GEN 3.5-19
Vortex Flow Field
AVOID
Nominally 500-1000 Ft.
Sink Several Ft.,/Rate
Hundred Min.
FIG GEN 3.5-20
Vortex Movement Near Ground - No Wind
No Wind
3K 3K
29.4.1.4 When the vortices of larger aircraft sink
close to the ground (within 100 to 200 feet), they tend
to move laterally over the ground at a speed of 2 or
3_knots. (See FIG GEN 3.5-20.)
29.4.1.5 There is a small segment of the aviation
community that have become convinced that wake
vortices may “bounce” up to twice their nominal
steady state height. With a 200-foot span aircraft, the
“bounce” height could reach approximately 200 feet
AGL. This conviction is based on a single
unsubstantiated report of an apparent coherent
vortical flow that was seen in the volume scan of a
research sensor. No one can say what conditions
cause vortex bouncing, how high they bounce, at
what angle they bounce, or how many times a vortex
may bounce. On the other hand, no one can say for
certain that vortices never “bounce.” Test data have
shown that vortices can rise with the air mass in which
they are embedded. Wind shear, particularly, can
cause vortex flow field “tilting.” Also, ambient
thermal lifting and orographic effects (rising terrain
or tree lines) can cause a vortex flow field to rise.
Notwithstanding the foregoing, pilots are reminded
that they should be alert at all times for possible wake
vortex encounters when conducting approach and
landing operations. The pilot has the ultimate
responsibility for ensuring appropriate separations
and positioning of the aircraft in the terminal area to
avoid the wake turbulence created by a preceding
aircraft.
30 AUG 07
AIP
United States of America
GEN 3.5-56
15 MAR 07
Federal Aviation Administration Nineteenth Edition
FIG GEN 3.5-21
Vortex Movement Near Ground - with Cross Winds
6K
(3K + 3K)
3K Wind
0 (3K - 3K)
FIG GEN 3.5-22
Vortex Movement in Ground Effect - Tailwind
Light Quartering
Tailwind
x
Tail Wind
Touchdown Point
29.4.2 A crosswind will decrease the lateral
movement of the upwind vortex and increase the
movement of the downwind vortex. Thus a light wind
with a cross-runway component of 1 to 5 knots could
result in the upwind vortex remaining in the
touchdown zone for a period of time and hasten the
drift of the downwind vortex toward another runway.
(See FIG GEN 3.5-21.) Similarly, a tailwind condi-
tion can move the vortices of the preceding aircraft
forward into the touchdown zone. THE LIGHT
QUARTERING TAILWIND REQUIRES MAXI-
MUM CAUTION. Pilots should be alert to larger
aircraft upwind from their approach and takeoff flight
paths. (See FIG GEN 3.5-22.)
30 AUG 07
AIP
United States of America
GEN 3.5-57
15 MAR 07
Federal Aviation Administration Nineteenth Edition
29.5 Operations Problem Areas
29.5.1 A wake encounter can be catastrophic. In
1972 at Fort Worth, Texas, a DC-9 got too close to a
DC-10 (two miles back), rolled, caught a wingtip,
and cartwheeled coming to rest in an inverted
position on the runway. All aboard were killed.
Serious and even fatal general aviation accidents
induced by wake vortices are not uncommon.
However, a wake encounter is not necessarily
hazardous. It can be one or more jolts with varying
severity depending upon the direction of the
encounter, weight of the generating aircraft, size of
the encountering aircraft, distance from the generat-
ing aircraft, and point of vortex encounter. The
probability of induced roll increases when the
encountering aircraft's heading is generally aligned
with the flight path of the generating aircraft.
29.5.2 AVOID THE AREA BELOW AND
BEHIND THE GENERATING AIRCRAFT,
ESPECIALLY AT LOW ALTITUDE WHERE
EVEN A MOMENTARY WAKE ENCOUNTER
COULD BE HAZARDOUS. This is not easy to do.
Some accidents have occurred even though the pilot
of the trailing aircraft had carefully noted that the
aircraft in front was at a considerably lower altitude.
Unfortunately, this does not ensure that the flight path
of the lead aircraft will be below that of the trailing
aircraft.
29.5.3 Pilots should be particularly alert in calm
wind conditions and situations where the vortices
could:
29.5.3.1 Remain in the touchdown area.
29.5.3.2 Drift from aircraft operating on a nearby
runway.
29.5.3.3 _Sink into the takeoff or landing path from a
crossing runway.
29.5.3.4 Sink into the traffic pattern from other
airport operations.
29.5.3.5 Sink into the flight path of VFR aircraft
operating on the hemispheric altitude 500 feet below.
29.5.4 Pilots of all aircraft should visualize the
location of the vortex trail behind larger aircraft and
use proper vortex avoidance procedures to achieve
safe operation. It is equally important that pilots of
larger aircraft plan or adjust their flight paths to
minimize vortex exposure to other aircraft.
29.6 Vortex Avoidance Procedures
29.6.1 Under certain conditions, airport traffic
controllers apply procedures for separating IFR
aircraft. If a pilot accepts a clearance to visually
follow a preceding aircraft, the pilot accepts
responsibility for separation and wake turbulence
avoidance. The controllers will also provide to VFR
aircraft, with whom they are in communication and
which in the tower's opinion may be adversely
affected by wake turbulence from a larger aircraft, the
position, altitude and direction of flight of larger
aircraft followed by the phrase “CAUTION - WAKE
TURBULENCE.” After issuing the caution for wake
turbulence, the airport traffic controllers generally do
not provide additional information to the following
aircraft unless the airport traffic controllers know the
following aircraft is overtaking the preceding
aircraft. WHETHER OR NOT A WARNING OR
INFORMATION HAS BEEN GIVEN, HOWEVER,
THE PILOT IS EXPECTED TO ADJUST AIR-
CRAFT OPERATIONS AND FLIGHT PATH AS
NECESSARY TO PRECLUDE SERIOUS WAKE
ENCOUNTERS. When any doubt exists about
maintaining safe separation distances between
aircraft during approaches, pilots should ask the
control tower for updates on separation distance and
aircraft groundspeed.
29.6.2 The following vortex avoidance procedures
are recommended for the various situations:
29.6.2.1 Landing Behind a Larger Aircraft_-
Same Runway. Stay at or above the larger aircraft's
final approach flight path - note its touchdown point
- land beyond it.
29.6.2.2 Landing Behind a Larger Aircraft_-
When a Parallel Runway is Closer Than
2,500_Feet. Consider possible drift to your runway.
Stay at or above the larger aircraft's final approach
flight path_-_note its touchdown point.
29.6.2.3 Landing Behind a Larger Aircraft_-
Crossing Runway. Cross above the larger aircraft's
flight path.
29.6.2.4 Landing Behind a Departing Larger
Aircraft_-_Same Runway. Note the larger aircraft's
rotation point_-_land well prior to rotation point.
29.6.2.5 Landing Behind a Departing Larger
Aircraft_-_Crossing Runway. Note the larger
aircraft's rotation point_-_if past the
intersection_-_continue the approach_-_land prior to
30 AUG 07
AIP
United States of America
GEN 3.5-58
15 MAR 07
Federal Aviation Administration Nineteenth Edition
the intersection. If larger aircraft rotates prior to the
intersection, avoid flight below the larger aircraft's
flight path. Abandon the approach unless a landing is
ensured well before reaching the intersection.
29.6.2.6 Departing Behind a Larger Aircraft.
Note the larger aircraft's rotation point_-_rotate prior
to larger aircraft's rotation point_-_continue climb
above the larger aircraft's climb path until turning
clear of the larger aircraft's wake. Avoid subsequent
headings which will cross below and behind a larger
aircraft. Be alert for any critical takeoff situation
which could lead to a vortex encounter.
29.6.2.7 Intersection Takeoffs_-_Same Runway.
Be alert to adjacent larger aircraft operations,
particularly upwind of your runway. If intersection
takeoff clearance is received, avoid subsequent
headings which will cross below a larger aircraft's
path.
29.6.2.8 Departing or Landing After a Larger
Aircraft Executing a Low Approach, Missed
Approach, Or Touch-and-go Landing. Because
vortices settle and move laterally near the ground, the
vortex hazard may exist along the runway and in your
flight path after a larger aircraft has executed a low
approach, missed approach, or a touch-and-go
landing, particular in light quartering wind condi-
tions. You should ensure that an interval of at least
2_minutes has elapsed before your takeoff or landing.
29.6.2.9 En Route VFR (Thousand-foot Altitude
Plus 500 Feet). Avoid flight below and behind a
large aircraft's path. If a larger aircraft is observed
above on the same track (meeting or overtaking)
adjust your position laterally, preferably upwind.
29.7 Helicopters
29.7.1 In a slow hover-taxi or stationary hover near
the surface, helicopter main rotor(s) generate
downwash producing high velocity outwash vortices
to a distance approximately three times the diameter
of the rotor. When rotor downwash hits the surface,
the resulting outwash vortices have behavioral
characteristics similar to wing tip vortices produced
by fixed-wing aircraft. However, the vortex
circulation is outward, upward, around, and away
from the main rotor(s) in all directions. Pilots of small
aircraft should avoid operating within three rotor
diameters of any helicopter in a slow hover-taxi or
stationary hover. In forward flight, departing or
landing helicopters produce a pair of strong,
high-speed trailing vortices similar to wing tip
vortices of larger fixed-wing aircraft. Pilots of small
aircraft should use caution when operating behind or
crossing behind landing and departing helicopters.
29.8 Pilot Responsibility
29.8.1 Government and industry groups are making
concerted efforts to minimize or eliminate the
hazards of trailing vortices. However, the flight
disciplines necessary to ensure vortex avoidance
during VFR operations must be exercised by the pilot.
Vortex visualization and avoidance procedures
should be exercised by the pilot using the same degree
for concern as in collision avoidance.
29.8.2 Wake turbulence may be encountered by
aircraft in flight as well as when operating on the
airport movement area.
29.8.3 Pilots are reminded that in operations
conducted behind all aircraft, acceptance of instruc-
tions from ATC in the following situations is an
acknowledgment that the pilot will ensure safe
takeoff and landing intervals and accepts the
responsibility of providing his/her own wake
turbulence separation:
29.8.3.1 Traffic information.
29.8.3.2 Instructions to follow an aircraft.
29.8.3.3 The acceptance of a visual approach
clearance.
29.8.4 For operations conducted behind heavy
aircraft, ATC will specify the word “heavy” when this
information is known. Pilots of heavy aircraft should
always use the word “heavy” in radio communica-
tions.
29.8.5 Heavy and large jet aircraft operators should
use the following procedures during an approach to
landing. These procedures establish a dependable
baseline from which pilots of in-trail, lighter aircraft
may reasonably expect to make effective flight path
adjustments to avoid serious wake vortex turbulence.
29.8.5.1 Pilots of aircraft that produce strong wake
vortices should make every attempt to fly on the
established glidepath, not above it; or, if glidepath
guidance is not available, to fly as closely as possible
to a “3-1” glidepath, not above it.
EXAMPLE-
Fly 3,000 feet at 10 miles from touchdown, 1,500 feet at
5_miles, 1,200 feet at 4 miles, and so on to touchdown.
30 AUG 07
AIP
United States of America
GEN 3.5-59
15 MAR 07
Federal Aviation Administration Nineteenth Edition
29.8.5.2 Pilots of aircraft that produce strong wake
vortices should fly as closely as possible to the
approach course centerline or to the extended
centerline of the runway of intended landing as
appropriate to conditions. |
|
IP卡
狗仔卡
发表于 2008-12-19 23:16:29
显身卡