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c) These SIGMETs are considered “widespread”
because they must be either affecting or be forecasted
to affect an area of at least 3,000 square miles at any
one time. The International SIGMET is issued for
12_hours for volcanic ash events, 6 hours for
hurricanes and tropical storms, and 4 hours for all
other events. Like the domestic SIGMETs, Interna-
tional SIGMETs are also identified by an alphabetic
designator from Alpha through Mike and are
numbered sequentially until that weather phenome-
non ends. The criteria for an International SIGMET
are:
1) Thunderstorms occurring in lines, embedded
in clouds, or in large areas producing tornadoes or
large hail.
2) Tropical cyclones.
3) Severe icing.
4) Severe or extreme turbulence.
5) Dust storms and sandstorms lowering
visibilities to less than 3 miles.
6) Volcanic ash.
EXAMPLE-
Example of an International SIGMET:
WSNT06 KKCI 022014
SIGA0F
KZMA KZNY TJZS SIGMET FOXTROT 3 VALID
022015/030015 KKCI- MIAMI OCEANIC FIR NEW
YORK OCEANIC FIR SAN JUAN FIR FRQ TS WI AREA
BOUNDED BY 2711N6807W 2156N6654W 2220N7040W
2602N7208W 2711N6807W. TOPS TO FL470. MOV NE
15KT. WKN. BASED ON SAT AND LTG OBS.
MOSHER
3.9.3.6 AIRMET (WA)
a) AIRMETs (WAs) are advisories of significant
weather phenomena but describe conditions at
intensities lower than those which require the
issuance of SIGMETs. AIRMETs are intended for
dissemination to all pilots in the preflight and en route
phase of flight to enhance safety. AIRMET Bulletins
are issued on a scheduled basis every 6 hours
beginning at 0145 UTC during Central Daylight
Time and at 0245 UTC during Central Standard Time.
Unscheduled updates and corrections are issued as
necessary. Each AIRMET Bulletin contains any
current AIRMETs in effect and an outlook for
conditions expected after the AIRMET valid period.
AIRMETs contain details about IFR, extensive
mountain obscuration, turbulence, strong surface
winds, icing, and freezing levels.
b) There are three AIRMETs: Sierra, Tango, and
Zulu. After the first issuance each day, scheduled or
unscheduled bulletins are numbered sequentially for
easier identification.
1) AIRMET Sierra describes IFR conditions
and/or extensive mountain obscurations.
2) AIRMET Tango describes moderate turbu-
lence, sustained surface winds of 30 knots or greater,
and/or nonconvective low-level wind shear.
3) AIRMET Zulu describes moderate icing and
provides freezing level heights.
EXAMPLE-
Example of AIRMET Sierra issued for the Chicago FA
area:
CHIS WA 121345
AIRMET SIERRA UPDT 3 FOR IFR AND MTN OBSCN
VALID UNTIL 122000.
AIRMET IFR...SD NE MN IA MO WI LM MI IL IN KY
FROM 70NW RAP TO 50W RWF TO 50W MSN TO GRB
TO MBS TO FWA TO CVG TO HNN TO TRI TO ARG TO
40SSW BRL TO OMA TO BFF TO 70NW RAP
OCNL CIG BLW 010/VIS BLW 3SM FG/BR. CONDS
ENDG 15Z-17Z.
AIRMET MTN OBSCN...KY TN
FROM HNN TO TRI TO CHA TO LOZ TO HNN
MTNS OCNL OBSC CLDS/PCPN/BR. CONDS ENDG TN
PTN AREA 18Z- 20Z..CONTG KY BYD 20Z..ENDG 02Z.
EXAMPLE-
Example of AIRMET Tango issued for the Salt Lake City
FA area:
SLCT WA 121345
AIRMET TANGO UPDT 2 FOR TURB VALID UNTIL
122000.
AIRMET TURB...NV UT CO AZ NM
FROM LKV TO CHE TO ELP TO 60S TUS TO YUM TO
EED TO RNO TO LKV OCNL MOD TURB BLW FL180
DUE TO MOD SWLY/WLY WNDS. CONDS CONTG BYD
20Z THRU 02Z.
AIRMET TURB...NV WA OR CA CSTL WTRS
FROM BLI TO REO TO BTY TO DAG TO SBA TO 120W
FOT TO 120W TOU TO BLI
OCNL MOD TURB BTWN FL180 AND FL400 DUE TO
WNDSHR ASSOCD WITH JTSTR. CONDS CONTG BYD
20Z THRU 02Z.
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Federal Aviation Administration Nineteenth Edition
EXAMPLE-
Example of AIRMET Zulu issued for the San Francisco
FA area:
SFOZ WA 121345
AIRMET ZULU UPDT 2 FOR ICE AND FRZLVL VALID
UNTIL 122000.
AIRMET ICE...WA OR ID MT NV UT
FROM YQL TO SLC TO WMC TO LKV TO PDT TO YDC
TO YQL
LGT OCNL MOD RIME/MXD ICGICIP BTWN FRZLVL
AND FL220. FRZLVL 080-120. CONDS CONTG BYD
20Z THRU 02Z.
AIRMET ICE...WA OR
FROM YDC TO PDT TO LKV TO 80W MFR TO ONP TO
TOU TO YDC
LGT OCNL MOD RIME/MXD ICGICIP BTWN FRZLVL
AND FL180. FRZLVL 060-080. CONDS CONTG BYD
20Z THRU 02Z.
FRZLVL...WA...060 CSTLN SLPG 100 XTRM E.
OR...060-070 CASCDS WWD. 070-095 RMNDR.
NRN CA...060-100 N OF A 30N FOT-40N RNO LN SLPG
100-110 RMNDR.
3.9.3.7 Severe Weather Watch Bulletins (WWs)
and Alert Messages (AWWs)
a) WWs define areas of possible severe thunder-
storms or tornado activity. The bulletins are issued by
the Storm Prediction Center (SPC) in Norman, OK.
WWs are unscheduled and are issued as required.
b) A severe thunderstorm watch describes areas of
expected severe thunderstorms. (Severe thunder-
storm criteria are 3
/4-inch hail or larger and/or wind
gusts of 50 knots or greater.)
c) A tornado watch describes areas where the
threat of tornadoes exists.
d) In order to alert the WFOs, CWSUs, FSSs, and
other users, a preliminary notification of a watch
called the Alert Severe Weather Watch bulletin
(AWW) is sent before the WW. (WFOs know this
product as a SAW).
EXAMPLE-
Example of an AWW:
MKC AWW 011734
WW 75 TORNADO TX OK AR 011800Z-020000Z
AXIS..80 STATUTE MILES EAST AND WEST OF A
LINE..60ESE DAL/DALLAS TX/ - 30 NW ARG/ WALNUT
RIDGE AR/
..AVIATION COORDS.. 70NM E/W /58W GGG - 25NW
ARG/
HAIL SURFACE AND ALOFT..1
3
/4 INCHES. WIND
GUSTS..70 KNOTS. MAX TOPS TO 450. MEAN WIND
VECTOR 24045.
e) Soon after the AWW goes out, the actual watch
bulletin itself is issued. A WW is in the following
format:
1) Type of severe weather watch, watch area,
valid time period, type of severe weather possible,
watch axis, meaning of a watch, and a statement that
persons should be on the lookout for severe weather.
2) Other watch information; i.e., references to
previous watches.
3) Phenomena, intensities, hail size, wind speed
(knots), maximum cumulonimbus (CB) tops, and
estimated cell movement (mean wind vector).
4) Cause of severe weather.
5) Information on updating Convective Outlook
(AC) products.
EXAMPLE-
Example of a WW:
BULLETIN - IMMEDIATE BROADCAST REQUESTED
TORNADO WATCH NUMBER 381
STORM PREDICTION CENTER NORMAN OK
556 PM CDT MON JUN 2 1997
THE STORM PREDICTON CENTER HAS ISSUED A
TORNADO WATCH FOR PORTIONS OF NORTHEAST
NEW MEXICO TEXAS PANHANDLE
EFFECTIVE THIS MONDAY NIGHT AND TUESDAY
MORNING FROM 630 PM UNTIL MIDNIGHT CDT.
TORNADOES...HAIL TO 2
3
/4 INCHES IN DIAME-
TER...THUNDERSTORM WIND GUSTS TO 80
MPH...AND DANGEROUS LIGHTNING ARE POSSIBLE
IN THESE AREAS.
THE TORNADO WATCH AREA IS ALONG AND 60
STATUTE MILES NORTH AND SOUTH OF A LINE
FROM 50 MILES SOUTHWEST OF RATON NEW
MEXICO TO 50 MILES EAST OF AMARILLO TEXAS.
REMEMBER...A TORNADO WATCH MEANS CON-
DITIONS ARE FAVORABLE FOR TORNADOES AND
SEVERE THUNDERSTORMS IN AND CLOSE TO THE
WATCH AREA. PERSONS IN THESE AREAS SHOULD
BE ON THE LOOKOUT FOR THREATENING WEATH-
ER CONDITIONS AND LISTEN FOR LATER STATE-
MENTS AND POSSIBLE WARNINGS.
OTHER WATCH INFORMATION...CONTINUE...
WW_378...WW 379...WW 380
DISCUSSION...THUNDERSTORMS ARE INCREASING
OVER NE NM IN MOIST SOUTHEASTERLY UPSLOPE
FLOW. OUTFLOW BOUNDARY EXTENDS EASTWARD
INTO THE TEXAS PANHANDLE AND EXPECT STORMS
TO MOVE ESE ALONG AND NORTH OF THE
BOUNDARY ON THE N EDGE OF THE CAP. VEERING
WINDS WITH HEIGHT ALONG WITH INCREASGING
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MID LVL FLOW INDICATE A THREAT FOR SUPER-
CELLS.
AVIATION...TORNADOES AND A FEW SEVERE THUN-
DERSTORMS WITH HAIL SURFACE AND ALOFT TO
2_3 /4 INCHES. EXTREME TURBULENCE AND SUR-
FACE WIND GUSTS TO 70 KNOTS. A FEW CUMULO-
NIMBI WITH MAXIMUM TOPS TO 550. MEAN STORM
MOTION VECTOR 28025.
f) Status reports are issued as needed to show
progress of storms and to delineate areas no longer
under the threat of severe storm activity. Cancellation
bulletins are issued when it becomes evident that no
severe weather will develop or that storms have
subsided and are no longer severe.
g) When tornadoes or severe thunderstorms have
developed, the local WFO office will issue the
warnings covering those areas.
3.9.3.8 Center Weather Advisories (CWAs)
a) CWAs are unscheduled inflight, flow control,
air traffic, and air crew advisory. By nature of its short
lead time, the CWA is not a flight planning product.
It is generally a nowcast for conditions beginning
within the next two hours. CWAs will be issued:
1) As a supplement to an existing SIGMET,
Convective SIGMET or AIRMET.
2) When an Inflight Advisory has not been
issued but observed or expected weather conditions
meet SIGMET/AIRMET criteria based on current
pilot reports and reinforced by other sources of
information about existing meteorological condi-
tions.
3) When observed or developing weather
conditions do not meet SIGMET, Convective
SIGMET, or AIRMET criteria; e.g., in terms of
intensity or area coverage, but current pilot reports or
other weather information sources indicate that
existing or anticipated meteorological phenomena
will adversely affect the safe flow of air traffic within
the ARTCC area of responsibility.
b) The following example is a CWA issued from
the Kansas City, Missouri, ARTCC. The “3” after
ZKC in the first line denotes this CWA has been
issued for the third weather phenomena to occur for
the day. The “301” in the second line denotes the
phenomena number again (3) and the issuance
number (01) for this phenomena. The CWA was
issued at 2140Z and is valid until 2340Z.
EXAMPLE-
ZKC3 CWA 032140
ZKC CWA 301 VALID UNTIL 032340
ISOLD SVR TSTM over KCOU MOVG SWWD 10
KTS ETC.
4. Categorical Outlooks
4.1 Categorical outlook terms describing general
ceiling and visibility conditions for advance planning
purposes are used only in area forecasts. They are
defined as follows:
4.1.1 LIFR (Low IFR)_-_Ceiling less than 500 feet
and/or visibility less than 1 mile.
4.1.2 IFR_-_Ceiling 500 to less than 1,000 feet and/or
visibility 1 to less than 3 miles.
4.1.3 MVFR (Marginal VFR)_-_Ceiling 1,000 or
3,000 feet and/or visibility 3 to 5 miles inclusive.
4.1.4 VFR_-_Ceiling greater than 3,000 feet and
visibility greater than 5 miles; includes sky clear.
4.2 The cause of LIFR, IFR, or MVFR is indicated
by either ceiling or visibility restrictions or both. The
contraction “CIG” and/or weather and obstruction to
vision symbols are used. If winds or gusts of 25 knots
or greater are forecast for the outlook period, the word
“WIND” is also included for all categories, including
VFR.
EXAMPLE-
LIFR CIG-low IFR due to low ceiling.
IFR FG-IFR due to visibility restricted by fog.
MVFR CIG HZ FU-marginal VFR due both to ceiling and
to visibility restricted by haze and smoke.
IFR CIG RA WIND-IFR due both to low ceiling and to
visibility restricted by rain; wind expected to be 25 knots or
greater.
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5. Telephone Information Briefing Service
(TIBS)_
5.1 TIBS, provided by automated flight service
stations (AFSSs), is a continuous recording of
meteorological and aeronautical information, avail-
able by telephone. Each AFSS provides at least four
route and/or area briefings. In addition, airspace
procedures and special announcements (if applica-
ble) concerning aviation interests are also available.
Depending upon user demand, other items may be
provided; i.e., METAR observations, terminal airport
forecasts, winds aloft, and temperatures aloft
forecasts.
6. Inflight Weather Broadcasts
6.1 Weather Advisory Broadcasts. ARTCCs'
broadcast a Severe Weather Forecast Alert (AWW),
Convective SIGMET, or CWA alert once on all
frequencies, except emergency, when any part of the
area described is within 150 miles of the airspace
under their jurisdiction. These broadcasts contain
SIGMET or CWA identification and a brief
description of the weather activity and general area
affected.
EXAMPLE-
Attention all aircraft, SIGMET Delta Three, from Myton to
Tuba City to Milford, severe turbulence and severe clear
icing below one zero thousand feet. Expected to continue
beyond zero three zero zero zulu.
EXAMPLE-
Attention all aircraft, Convective SIGMET Two Seven
Eastern. From the vicinity of Elmira to Phillipsburg.
Scattered embedded thunderstorms moving east at one
zero knots. A few intense level five cells, maximum tops four
five zero.
EXAMPLE-
Attention all aircraft, Kansas City Center weather advisory
one zero three. Numerous reports of moderate to severe
icing from eight to niner thousand feet in a three zero mile
radius of St. Louis. Light or negative icing reported from
four thousand to one two thousand feet remainder of
Kansas City Center area.
NOTE-
Terminal control facilities have the option to limit the
AWW, Convective SIGMET, SIGMET, or CWA broadcast
as follows: local control and approach control positions
may opt to broadcast SIGMET or CWA alerts only when
any part of the area described is within 50 miles of the
airspace under their jurisdiction.
6.2 Hazardous Inflight Weather Advisory Ser-
vice (HIWAS). This is a continuous broadcast of
inflight weather advisories including summarized
AWWs, SIGMETs, Convective SIGMETs, CWAs,
AIRMETs, and urgent PIREPs. HIWAS has been
adopted as a national program and will be
implemented throughout the conterminous U.S. as
resources permit. In those areas where HIWAS is
commissioned, ARTCC, Terminal ATC, and AFSS/
FSS facilities have discontinued the broadcast of
inflight advisories. HIWAS is an additional source of
hazardous weather information which makes these
data available on a continuous basis. It is not,
however, a replacement for preflight or inflight
briefings or real-time weather updates from Flight
Watch (EFAS). As HIWAS is implemented in
individual center areas, the commissioning will be
advertised in the Notices to Airmen publication.
6.2.1 Where HIWAS has been implemented, a
HIWAS alert will be broadcast on all except
emergency frequencies once upon receipt by ARTCC
and terminal facilities which will include an alert
announcement, frequency instruction, number, and
type of advisory updated; e.g., AWW, SIGMET,
Convective SIGMET, or CWA.
EXAMPLE-
Attention all aircraft. Hazardous weather information
(SIGMET, Convective SIGMET, AIRMET, urgent pilot
weather report (UUA), or Center Weather Advisory
(CWA)), (number or numbers) for (geographical area)
available on HIWAS, flight watch, or flight service
frequencies.
6.2.2 In HIWAS ARTCC areas, AFSSs/FSSs will
broadcast a HIWAS update announcement once on all
except emergency frequencies upon completion of
recording an update to the HIWAS broadcast.
Included in the broadcast will be the type of advisory
update; e.g., AWW, SIGMET, Convective SIGMET,
or CWA.
EXAMPLE-
Attention all aircraft. Hazardous weather information for
(geographical area) available from flight watch or flight
service.
6.2.3 HIWAS availability is shown on IFR En Route
Low Altitude Charts and VFR Sectional Charts. The
symbol depiction is identified in the chart legend.
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7. Flight Information Services (FIS)
7.1 FIS. Aviation weather and other operational
information may be displayed in the cockpit through
the use of FIS. FIS systems are of two basic types:
Broadcast only systems (called FIS-B) and two-way
request/reply systems. Broadcast system components
include a ground- or space-based transmitter, an
aircraft receiver, and a portable or installed cockpit
display device. Two-way systems utilize transmitter/
receivers at both the ground- or space-based site and
the aircraft.
7.1.1 Broadcast FIS (i.e., FIS-B) allows the pilot to
passively collect weather and other operational data
and to display that data at the appropriate time. In
addition to textual weather products such as Aviation
Routine Weather Reports (METARs)/Aviation Se-
lected Special Weather Reports (SPECIs) and
Terminal Area Forecasts (TAFs), graphical weather
products such as radar composite/mosaic images,
temporary flight restricted airspace and other
NOTAMs may be provided to the cockpit. Two-way
FIS services permit the pilot to make specific weather
and other operational information requests for
cockpit display. A FIS service provider will then
prepare a reply in response to that specific request and
transmit the product to that specific aircraft.
7.1.2 FIS services are available from four types of
service providers:
7.1.2.1 A private sector FIS provider operating
under service agreement with the FAA using
broadcast data link over VHF aeronautical spectrum
and whose products have been reviewed and accepted
by the FAA prior to transmission. (Products and
services are defined under subparagraph 7.3.)
7.1.2.2 Through an FAA operated service using a
broadcast data link on the ADS-B UAT network.
(Products and services are defined under subpara-
graph 7.4.)
7.1.2.3 Private sector FIS providers operating under
customer contracts using aeronautical spectrum.
7.1.2.4 Private sector FIS providers operating under
customer contract using methods other than
aeronautical spectrum, including Internet data-tothe-cockpit service providers.
7.1.3 FIS is a method of receiving aviation weather
and other operational data in the cockpit that
augments traditional pilot voice communication with
FAA's Flight Service Stations (FSSs), ATC facilities,
or Airline Operations Control Centers (AOCCs). FIS
is not intended to replace traditional pilot and
controller/flight service specialist/aircraft dispatcher
pre-flight briefings or inflight voice communica-
tions. FIS; however, can provide textual and
graphical background information that can help
abbreviate and improve the usefulness of such
communications. FIS enhances pilot situational
awareness and improves safety.
7.1.4 To ensure airman compliance with Federal
Aviation Regulations, manufacturer's operating
manuals should remind airmen to contact ATC
controllers, FSS specialists, operator dispatchers, or
airline operations control centers for general and
mission critical aviation weather information and/or
NAS status conditions (such as NOTAMs, Special
Use Airspace status, and other government flight
information). If FIS products are systemically
modified (for example, are displayed as abbreviated
plain text and/or graphical depictions), the modifica-
tion process and limitations of the resultant product
should be clearly described in the vendor's user
guidance.
7.2 Operational Use of FIS. Regardless of the type
of FIS system being used, several factors must be
considered when using FIS:
7.2.1 Before using FIS for inflight operations, pilots
and other flight crewmembers should become
familiar with the operation of the FIS system to be
used, the airborne equipment to be used, including its
system architecture, airborne system components,
coverage service volume and other limitations of the
particular system, modes of operation and indications
of various system failures. Users should also be
familiar with the specific content and format of the
services available from the FIS provider(s). Sources
of information that may provide this specific
guidance include manufacturer's manuals, training
programs and reference guides.
7.2.2 FIS should not serve as the sole source of
aviation weather and other operational information.
ATC, AFSSs and, if applicable, AOCC VHF/HF
voice remain as a redundant method of communicat-
ing aviation weather, NOTAMs, and other operation-
al information to aircraft in flight. FIS augments these
traditional ATC/FSS/AOCC services and, for some
products, offers the advantage of being displayed as
graphical information. By using FIS for orientation,
the usefulness of information received from
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conventional means may be enhanced. For example,
FIS may alert the pilot to specific areas of concern
that will more accurately focus requests made to FSS
or AOCC for inflight updates or similar queries made
to ATC.
7.2.3 The airspace and aeronautical environment is
constantly changing. These changes occur quickly
and without warning. Critical operational decisions
should be based on use of the most current and
appropriate data available. When differences exist
between FIS and information obtained by voice
communication with ATC, FSS, and/or AOCC (if
applicable), pilots are cautioned to use the most
recent data from the most authoritative source.
7.2.4 FIS aviation weather products (e.g., graphical
ground-based radar precipitation depictions) are not
appropriate for tactical avoidance of severe weather
such as negotiating a path through a weather hazard
area. FIS supports strategic weather decision making
such as route selection to avoid a weather hazard area
in its entirety. The misuse of information beyond its
applicability may place the pilot and aircraft in
jeopardy. In addition, FIS should never be used in lieu
of an individual pre-flight weather and flight
planning briefing.
7.2.5 FIS NOTAM products, including Temporary
Flight Restriction (TFR) information, are advisoryuse information and are intended for situational
awareness purposes only. Cockpit displays of this
information are not appropriate for tactical naviga-
tion - pilots should stay clear of any geographic area
displayed as a TFR NOTAM. Pilots should contact
FSSs and/or ATC while en route to obtain updated
information and to verify the cockpit display of
NOTAM information.
7.2.6 FIS supports better pilot decision making by
increasing situational awareness. Better decisionmaking is based on using information from a variety
of sources. In addition to FIS, pilots should take
advantage of other weather/NAS status sources,
including, briefings from Flight Service Stations,
FAA's en route “Flight Watch” service, data from
other air traffic control facilities, airline operation
control centers, pilot reports, as well as their own
observations.
7.3 FAA FISDL (VHF) Service. The FAA's
FISDL (VHF datalink) system is a VHF Data Link
(VDL) Mode 2 implementation that provides pilots
and flight crews of properly equipped aircraft with a
cockpit display of certain aviation weather and flight
operational information. This information may be
displayed in both textual and graphical formats. The
system is operated under a service agreement with the
FAA, using broadcast data link on VHF aeronautical
spectrum on two 25 KHz spaced frequencies
(136.450 and 136.475 MHz). The FAA FISDL
(VHF) service is designed to provide coverage
throughout the continental U.S. from 5,000 feet AGL
to 17,500 feet MSL, except in areas where this is not
feasible due to mountainous terrain. Aircraft
operating near transmitter sites may receive useable
FISDL signals at altitudes lower than 5,000 feet
AGL, including on the surface in some locations,
depending on transmitter/aircraft line of sight
geometry. Aircraft operating above 17,500 feet MSL
may also receive useable FISDL signals under certain
circumstances.
7.3.1 FAA FISDL (VHF) service provides, free of
charge, the following basic text products:
7.3.1.1 Aviation Routine Weather Reports
(METARs).
7.3.1.2 Aviation Selected Special Weather Reports
(SPECIs).
7.3.1.3 Terminal Area Forecasts (TAFs), and their
amendments.
7.3.1.4 Significant Meteorological Information
(SIGMETs).
7.3.1.5 Convective SIGMETs.
7.3.1.6 Airman's Meteorological Information
(AIRMETs).
7.3.1.7 Pilot Reports (both urgent and routine)
(PIREPs); and,
7.3.1.8 Severe Weather Forecast Alerts and Warn-
ings (AWWs/WW) issued by the NOAA Storm
Prediction Center (SPC).
7.3.2 The format and coding of these text products
are described in Advisory Circular AC-00-45,
Aviation Weather Services, and FIG GEN 3.5-23
and FIG GEN 3.5-24, Key to Aerodrome Forecast
(TAF) and Aviation Routine Weather Report
(METAR).
7.3.3 Additional products, called “Value-Added
Products,” are also available from the vendor on a
paid subscription basis. Details concerning the
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content, format, symbology and cost of these
products may be obtained from the vendor.
7.4 FAA's Flight Information Service-Broadcast
(FIS-B) Service. FIS-B is a ground broadcast
service provided through the FAA's Universal Access
Transceiver (UAT) “ADS-B Broadcast Services”
network. The UAT network is an ADS-B data link
that operates on 978 MHz. The FAA FIS-B system
provides pilots and flight crews of properly equipped
aircraft with a cockpit display of certain aviation
weather and flight operational information. The
FAA's FIS-B service is being introduced in certain
regional implementations within the NAS (e.g., in
Alaska and in other areas of implementation).
7.4.1 FAA's UAT FIS-B provides the initial products
listed below with additional products planned for
future implementation. FIS-B reception is line of
sight and can be expected within 200 NM (nominal
range) of each ground transmitting site. The
following services are provided free of charge.
7.4.1.1 Text: Aviation Routine Weather Reports
(METARs).
7.4.1.2 Text: Special Aviation Reports (SPECIs).
7.4.1.3 Text: Terminal Area Forecasts (TAFs), and
their amendments.
7.4.1.4 Graphic: NEXRAD precipitation maps.
7.4.2 The format and coding of the above text
weather-related products are described in Advisory
Circular AC-00-45, Aviation Weather Services, and
FIG GEN 3.5-23 and FIG GEN 3.5-24, Key to
Aerodrome Forecast (TAF) and Aviation Routine
Weather Report (METAR).
7.4.3 Details concerning the content, format, and
symbology of the various data link products provided
may be obtained from the specific avionics
manufacturer.
7.5 Non-FAA FISDL Systems. Several commer-
cial vendors also provide customers with FIS data
over both the aeronautical spectrum and on other
frequencies using a variety of data link protocols. In
some cases, the vendors provide only the commu-
nications system that carries customer messages,
such as the Aircraft Communications Addressing and
Reporting System (ACARS) used by many air carrier
and other operators.
7.5.1 Operators using non-FAA FIS data for inflight
weather and other operational information should
ensure that the products used conform to FAA/NWS
standards. Specifically, aviation weather and NAS
status information should meet the following criteria:
7.5.1.1 The products should be either FAA/NWS
“accepted” aviation weather reports or products, or
based on FAA/NWS accepted aviation weather
reports or products. If products are used which do not
meet this criteria, they should be so identified. The
operator must determine the applicability of such
products to their particular flight operations.
7.5.1.2 In the case of a weather product which is the
result of the application of a process which alters the
form, function or content of the base FAA/NWS
accepted weather product(s), that process, and any
limitations to the application of the resultant product,
should be described in the vendor's user guidance
material.
7.5.2 An example would be a NEXRAD radar
composite/mosaic map, which has been modified by
changing the scaling resolution. The methodology of
assigning reflectivity values to the resultant image
components should be described in the vendor's
guidance material to ensure that the user can
accurately interpret the displayed data.
8. Weather Observing Programs
8.1 Manual Observations. Aviation Routine
Weather Reports (METAR) are taken at more than
600 locations in the U.S. With only a few exceptions,
these stations are located at airport sites and most are
staffed by FAA or NWS personnel who manually
observe, perform calculations, and enter the
observation into the distribution system. The format
and coding of these observations are contained in
FIG GEN 3.5-23.
8.2 Automated Weather Observing System
(AWOS)
8.2.1 Automated weather reporting systems are
increasingly being installed at airports. These
systems consist of various sensors, a processor, a
computer-generated voice subsystem, and a trans-
mitter to broadcast local, minute-by-minute weather
data directly to the pilot.
NOTE-
When the barometric pressure exceeds 31.00 inches Hg.,
see Section ENR 1.7, Altimeter Setting Procedures.
8.2.2 The AWOS observations will include the
prefix “AUTO” to indicate that the data are derived
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from an automated system. Some AWOS locations
will be augmented by certified observers who will
provide weather and obstruction to vision informa-
tion in the remarks of the report when the reported
visibility is less than 3 miles. These sites, along with
the hours of augmentation, are published in the
Airport/Facility Directory. Augmentation is identi-
fied in the observation as “OBSERVER WEATH-
ER.” The AWOS wind speed, direction and gusts,
temperature, dew point, and altimeter setting are
exactly the same as for manual observations. The
AWOS will also report density altitude when it
exceeds the field elevation by more than 1,000 feet.
The reported visibility is derived from a sensor near
the touchdown of the primary instrument runway.
The visibility sensor output is converted to a visibility
value using a 10-minute harmonic average. The
reported sky condition/ceiling is derived from the
ceilometer located next to the visibility sensor. The
AWOS algorithm integrates the last 30 minutes of
ceilometer data to derive cloud layers and heights.
This output may also differ from the observer sky
condition in that the AWOS is totally dependent upon
the cloud advection over the sensor site.
8.2.3 Referred to as AWOS, these real-time systems
are operationally classified into four basic levels:
8.2.3.1 AWOS-A: only reports altimeter setting.
NOTE-
Any other information is advisory only.
8.2.3.2 AWOS-l: usually reports altimeter setting,
wind data, temperature, dew point, and density
altitude.
8.2.3.3 AWOS-2 provides the information provided
by AWOS-l, plus visibility.
8.2.3.4 AWOS-3 provides the information provided
by AWOS-2, plus cloud/ceiling data.
8.2.4 The information is transmitted over a discrete
VHF radio frequency or the voice portion of a local
NAVAID. AWOS transmissions on a discrete VHF
radio frequency are engineered to be receivable to a
maximum of 25 NM from the AWOS site and a
maximum altitude of 10,000 feet AGL. At many
locations, AWOS signals may be received on the
surface of the airport, but local conditions may limit
the maximum AWOS reception distance and/or
altitude. The system transmits a 20- to 30-second
weather message updated each minute. Pilots should
monitor the designated frequency for the automated
weather broadcast. A description of the broadcast is
contained in paragraph 8.3, Automated Weather
Observing System (AWOS) Broadcasts. There is no
two-way communication capability. Most AWOS
sites also have a dial-up capability so that the
minute-by-minute weather messages can be ac-
cessed via telephone.
8.2.5 AWOS information (system level, frequency,
phone number) concerning specific locations is
published, as the systems become operational, in the
Airport/Facility Directory and, where applicable, on
published Instrument Approach Procedure (IAP)
charts. Selected individual systems may be incorpo-
rated into nationwide data collection and dissemina-
tion networks in the future.
8.3 Automated Weather Observing System
(AWOS) Broadcasts. Computer-generated voice is
used in AWOS to automate the broadcast of the
minute-by-minute weather observations. In addi-
tion, some systems are configured to permit the
addition of an operator-generated voice message;
e.g., weather remarks, following the automated
parameters. The phraseology used generally follows
that used for other weather broadcasts. Following are
explanations and examples of the exceptions.
8.3.1 Location and Time. The location/name and
the phrase “AUTOMATED WEATHER OBSERVA-
TION” followed by the time are announced.
8.3.1.1 If the airport's specific location is included in
the airport's name, the airport's name is announced.
EXAMPLE-
“Bremerton National Airport automated weather
observation one four five six zulu.”
“Ravenswood Jackson County Airport automated weather
observation one four five six zulu.”
8.3.1.2 If the airport's specific location is not
included in the airport's name, the location is
announced followed by the airport's name.
EXAMPLE-
“Sault Ste. Marie, Chippewa County International Airport
automated weather observation.”
“Sandusky, Cowley Field automated weather
observation.”
8.3.1.3 The word “TEST” is added following
“OBSERVATION” when the system is not in
commissioned status.
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EXAMPLE-
“Bremerton National Airport automated weather
observation test one four five six zulu.”
8.3.1.4 The phrase “TEMPORARILY INOPERA-
TIVE” is added when the system is inoperative.
EXAMPLE-
“Bremerton National Airport automated weather
observing system temporarily inoperative.”
8.3.2 Ceiling and Sky Cover
8.3.2.1 Ceiling is announced as either “CEILING”
or “INDEFINITE CEILING.” The phrases “MEA-
SURED CEILING” and “ESTIMATED CEILING”
are not used. With the exception of indefinite ceilings,
all automated ceiling heights are measured.
EXAMPLE-
“Bremerton National Airport automated weather
observation one four five six zulu, ceiling two thousand
overcast.”
“Bremerton National Airport automated weather
observation one four five six zulu, indefinite ceiling two
hundred.”
8.3.2.2 The word “CLEAR” is not used in AWOS
due to limitations in the height ranges of the sensors.
No clouds detected is announced as, “No clouds
below XXX” or, in newer systems as, “Clear below
XXX” (where XXX is the range limit of the sensor).
EXAMPLE-
“No clouds below one two thousand.”
“Clear below one two thousand.”
8.3.2.3 A sensor for determining ceiling and sky
cover is not included in some AWOS. In these
systems, ceiling and sky cover are not announced.
“SKY CONDITION MISSING” is announced only if
the system is configured with a ceilometer, and the
ceiling and sky cover information is not available.
8.3.3 Visibility
8.3.3.1 The lowest reportable visibility value in
AWOS is “less than 1
/4.” It is announced as
“VISIBILITY LESS THAN ONE QUARTER.”
8.3.3.2 A sensor for determining visibility is not
included in some AWOSs. In these systems, visibility
is not announced. “VISIBILITY MISSING” is
announced only if the system is configured with a
visibility sensor and visibility information is not
available.
8.3.4 Weather. In the future, some AWOSs are to
be configured to determine the occurrence of
precipitation. However, the type and intensity may
not always be determined. In these systems, the word
“PRECIPITATION” will be announced if precipita-
tion is occurring, but the type and intensity are not
determined.
8.3.5 Remarks. If remarks are included in the
observation, the word “REMARKS” is announced
following the altimeter setting. Remarks are
announced in the following order of priority:
8.3.5.1 Automated “remarks.”
a) Variable visibility.
b) Density altitude.
8.3.5.2 Manual input remarks. Manual input
remarks are prefaced with the phrase “OBSERVER
WEATHER.” As a general rule the manual remarks
are limited to:
a) Type and intensity of precipitation.
b) __Thunderstorms, intensity (if applicable), and
direction.
c) Obstructions to vision when the visibility is less
than 7 miles.
EXAMPLE-
“Remarks...density altitude, two thousand five
hundred...visibility variable between one and two...wind
direction variable between two four zero and three one
zero...observed weather...thunderstorm moderate rain
showers and mist...thunderstorm overhead.”
8.3.5.3 If an automated parameter is “missing” and
no manual input for that parameter is available, the
parameter is announced as “MISSING.” For
example, a report with the dew point “missing,” and
no manual input available, would be announced as
follows:
EXAMPLE-
“Ceiling one thousand overcast, visibility three, precipita-
tion, temperature three zero, dew point missing, wind calm,
altimeter three zero zero one.”
8.3.5.4 “REMARKS” are announced in the follow-
ing order of priority:
a) Automated “REMARKS”:
1) Variable visibility.
2) Density altitude.
b) Manual Input “REMARKS.” As a general rule,
the remarks are announced in the same order as the
parameters appear in the basic text of the observation.
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EXAMPLE-
“Remarks, density altitude, two thousand five hundred,
visibility variable between one and two, wind direction
variable between two four zero and three one zero,
observer ceiling estimated two thousand broken, observer
temperature two, dew point minus five.”
8.4 Automated Surface Observing System
(ASOS)/Automated Weather Sensor System
(AWSS)
8.4.1 The ASOS/AWSS is the primary surface
weather observing system of the U.S. The program to
install and operate these systems throughout the U.S.
is a joint effort of the NWS, the FAA and the
Department of Defense. AWSS is a follow-on
program that provides identical data as ASOS.
ASOS/AWSS is designed to support aviation
operations and weather forecast activities. The
ASOS/AWSS will provide continuous minute-by-
minute observations and perform the basic observing
functions necessary to generate an aviation routine
weather report (METAR) and other aviation weather
information. The information may be transmitted
over a discrete VHF radio frequency or the voice
portion of a local NAVAID. ASOS/AWSS transmis-
sions on a discrete VHF radio frequency are
engineered to be receivable to a maximum of 25 NM
from the ASOS/AWSS site and a maximum altitude
of 10,000 feet AGL. At many locations, ASOS/
AWSS signals may be received on the surface of the
airport, but local conditions may limit the maximum
reception distance and/or altitude. While the
automated system and the human may differ in their
methods of data collection and interpretation, both
produce an observation quite similar in form and
content. For the “objective” elements such as
pressure, ambient temperature, dew point tempera-
ture, wind, and precipitation accumulation, both the
automated system and the observer use a fixed
location and time-averaging technique. The quantita-
tive differences between the observer and the
automated observation of these elements are
negligible. For the “subjective” elements, however,
observers use a fixed time, spatial averaging
technique to describe the visual elements (sky
condition, visibility and present weather), while the
automated systems use a fixed location, time
averaging technique. Although this is a fundamental
change, the manual and automated techniques yield
remarkably similar results within the limits of their
respective capabilities. (See FIG GEN 3.5-25 and
FIG GEN 3.5-26, Key to Decode an ASOS/AWSS
(METAR) Observation.
8.4.2 System Description
8.4.2.1 The ASOS/AWSS at each airport location
consists of four main components:
a) Individual weather sensors.
b) Data collection and processing units.
c) Peripherals and displays.
8.4.2.2 The ASOS/AWSS sensors perform the basic
function of data acquisition. They continuously
sample and measure the ambient environment, derive
raw sensor data and make them available to the
collection and processing units.
8.4.3 Every ASOS/AWSS will contain the follow-
ing basic set of sensors.
8.4.3.1 Cloud height indicator (one or possibly
three).
8.4.3.2 Visibility sensor (one or possibly three).
8.4.3.3 Precipitation identification sensor.
8.4.3.4 Freezing rain sensor.
8.4.3.5 Pressure sensors (two sensors at small
airports; three sensors at large airports).
8.4.3.6 Ambient temperature/dew point temperature
sensor.
8.4.3.7 Anemometer (wind direction and speed
sensor).
8.4.3.8 Rainfall accumulation sensor.
8.4.4 The ASOS/AWSS data outlets include:
8.4.4.1 Those necessary for on-site airport users.
8.4.4.2 National communications networks.
8.4.4.3 Computer-generated voice (available
through FAA radio broadcast to pilots and dial-in
telephone line).
NOTE-
Wind direction broadcast over FAA radios is in reference
to magnetic north.
8.5 A comparison of weather observing programs
and the elements observed by each are in
TBL GEN 3.5-2, Weather Observing Programs.
8.6 Service Standards. During 1995, a govern-
ment/industry team worked to comprehensively
reassess the requirements for surface observations at
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the nation's airports. That work resulted in agreement
on a set of service standards and the FAA and NWS
ASOS sites to which the standards would apply. The
term “Service Standards” refers to the level of detail
in the weather observation. The service standards
consist of four different levels of service (A, B, C, and
D) as described below. Specific observational
elements included in each service level are listed in
TBL GEN 3.5-3, Weather Observation Service
Standards.
8.6.1 Service Level D defines the minimum
acceptable level of service. It is a completely
automated service in which the ASOS observation
will constitute the entire observation; i.e., no
additional weather information is added by a human
observer. This service is referred to as a stand alone
D site.
8.6.2 Service Level C is a service in which the human
observer, usually an air traffic controller, augments or
adds information to the automated observation.
Service Level C also includes backup of ASOS
elements in the event of an ASOS malfunction or an
unrepresentative ASOS report.
8.6.3 In backup, the human observer inserts the
correct or missing value for the automated ASOS
elements. This service is provided by air traffic
controllers under the Limited Aviation Weather
Reporting Station (LAWRS) process, FSS and NWS
observers, and, at selected sites, Non-Federal
Observation Program observers.
Two categories of airports require detail beyond
Service Level C in order to enhance air traffic control
efficiency and increase system capacity. Services at
these airports are typically provided by contract
weather observers, NWS observers, and, at some
locations, FSS observers.
8.6.4 Service Level B is a service in which weather
observations consist of all elements provided under
Service Level C, plus augmentation of additional data
beyond the capability of the ASOS. This category of
airports includes smaller hubs or airports special in
other ways that have worse than average bad weather
operations for thunderstorms and/or freezing/frozen
precipitation, and/or that are remote airports.
8.6.5 Service Level A, the highest and most
demanding category, includes all the data reported in
Service Standard B, plus additional requirements as
specified. Service Level A covers major aviation
hubs and/or high volume traffic airports with average
or worse weather.
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TBL GEN 3.5-2
Weather Observing Programs
Element Reported AWOS-A AWOS-1 AWOS-2 AWOS-3 ASOS MANUAL
Altimeter X X X X X X
Wind X X X X X
Temperature/Dew point X X X X X
Density altitude X X X X
Visibility X X X X
Clouds/Ceiling X X X
Precipitation X X
Remarks X X
TBL GEN 3.5-3
Weather Observation Service Standards
SERVICE LEVEL A
Service Level A consists of all the elements of
Service Levels B, C and D plus the elements
listed to the right, if observed.
10 minute longline RVR at precedented sites or
additional _visibility increments of 1/8, 1/16 and 0
Sector visibility
Variable sky condition
Cloud layers above 12,000 feet and cloud types
Widespread dust, sand and other obscurations
Volcanic eruptions
SERVICE LEVEL B
Service Level B consists of all the elements of
Service Levels C and D plus the elements listed to
the right, if observed.
Longline RVR at precedented sites
_(may be instantaneous readout)
Freezing drizzle versus freezing rain
Ice pellets
Snow depth & snow increasing rapidly remarks
Thunderstorm and lightning location remarks
Observed significant weather not at the station
remarks
SERVICE LEVEL C
Service Level C consists of all the elements of Service
Level D plus augmentation and backup by a human
observer or an air traffic control specialist on location
nearby. Backup consists of inserting the correct value if
the system malfunctions or is unrepresentative.
Augmentation consists of adding the elements listed to
the right, if observed. During hours that the observing
facility is closed, the site reverts to Service Level D.
Thunderstorms
Tornadoes
Hail
Virga
Volcanic ash
Tower visibility
Operationally significant remarks as deemed
appropriate by the observer
SERVICE LEVEL D
This level of service consists of an ASOS continually
measuring the atmosphere at a point near the runway. The
ASOS senses and measures the weather parameters listed to
the right.
Wind
Visibility
Precipitation/Obstruction to vision
Cloud height
Sky cover
Temperature
Dew point
Altimeter
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9. Weather Radar Services
9.1 The National Weather Service operates a
network of radar sites for detecting coverage,
intensity, and movement of precipitation. The
network is supplemented by FAA and DOD radar
sites in the western sections of the country. Local
warning radars augment the network by operating on
an as needed basis to support warning and forecast
programs.
9.2 Scheduled radar observations are taken hourly
and transmitted in alpha-numeric format on weather
telecommunications circuits for flight planning
purposes. Under certain conditions special radar
reports are issued in addition to the hourly
transmittals. Data contained in the reports is also
collected by the National Meteorological Center and
used to prepare hourly national radar summary charts
for dissemination on facsimile circuits.
9.3 All En route Flight Advisory Service facilities
and many Automated Flight Service Stations have
equipment to directly access the radar displays from
the individual weather radar sites. Specialists at these
locations are trained to interpret the display for pilot
briefing and inflight advisory services. The Center
Weather Service Units located in the ARTCCs also
have access to weather radar displays and provide
support to all air traffic facilities within their center's
area.
9.4 A clear radar display (no echoes) does not mean
that there is no significant weather within the
coverage of the radar site. Clouds and fog are not
detected by the radar. However, when echoes are
present, turbulence can be implied by the intensity of
the precipitation, and icing is implied by the presence
of the precipitation at temperatures at or below zero
degrees Celsius. Used in conjunction with other
weather products, radar provides invaluable informa-
tion for weather avoidance and flight planning.
9.5 Additional information on weather radar prod-
ucts and services can be found in FAA Advisory
Circular 00-45, “Aviation Weather Services.”
REFERENCE-
Pilot/Controller Glossary Term- Precipitation Radar Weather
Descriptions.
AIP, Thunderstorms, GEN 3.5, Paragraph 27.
Airport/Facility Directory, Charts, NWS Upper Air Observing Stations
and Weather Network for the location of specific radar sites.
10. ATC Inflight Weather Avoidance
Assistance
10.1 ATC Radar Weather Display
10.1.1 ATC radars are able to display areas of
precipitation by sending out a beam of radio energy
that is reflected back to the radar antenna when it
strikes an object or moisture which may be in the form
of rain drops, hail, or snow. The larger the object is,
or the more dense its reflective surface, the stronger
the return will be presented. Radar weather
processors indicate the intensity of reflective returns
in terms of decibels (dBZ). ATC systems cannot
detect the presence or absence of clouds. The ATC
systems can often determine the intensity of a
precipitation area, but the specific character of that
area (snow, rain, hail, VIRGA, etc.) cannot be
determined. For this reason, ATC refers to all weather
areas displayed on ATC radar scopes as “precipita-
tion.”
10.1.2 All ATC facilities using radar weather
processors with the ability to determine precipitation
intensity, will describe the intensity to pilots as:
10.1.2.1 “LIGHT” (< 30 dBZ)
10.1.2.2 “MODERATE” (30 to 40 dBZ)
10.1.2.3 “HEAVY” (> 40 to 50 dBZ)
10.1.2.4 “EXTREME” (> 50 dBZ)
10.1.3 ATC facilities that, due to equipment
limitations, cannot display the intensity levels of
precipitation, will describe the location of the
precipitation area by geographic position, or position
relative to the aircraft. Since the intensity level is not
available, the controller will state “INTENSITY
UNKNOWN.”
10.1.4 ARTCC facilities normally use a Weather and
Radar Processor (WARP) to display a mosaic of data
obtained from multiple NEXRAD sites. There is a
time delay between actual conditions and those
displayed to the controller. For example, the
precipitation data on the ARTCC controller's display
could be up to 6 minutes old. When the WARP is not
available, a second system, the narrowband Air Route
Surveillance Radar (ARSR) can display two distinct
levels of precipitation intensity that will be described
to pilots as “MODERATE” (30 to 40 dBZ) and
“HEAVY TO EXTREME” ( > 40 dBZ ). The WARP
processor is only used in ARTCC facilities.
10.1.5 ATC radar is not able to detect turbulence.
Generally, turbulence can be expected to occur as the
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rate of rainfall or intensity of precipitation increases.
Turbulence associated with greater rates of rainfall/
precipitation will normally be more severe than any
associated with lesser rates of rainfall/precipitation.
Turbulence should be expected to occur near
convective activity, even in clear air. Thunderstorms
are a form of convective activity that imply severe or
greater turbulence. Operation within 20 miles of
thunderstorms should be approached with great
caution, as the severity of turbulence can be markedly
greater than the precipitation intensity might indicate.
10.2 Weather Avoidance Assistance
10.2.1 To the extent possible, controllers will issue
pertinent information of weather or chaff areas and
assist pilots in avoiding such areas if requested. Pilots
should respond to a weather advisory by either
acknowledging the advisory or by acknowledging the
advisory and requesting an alternative course of
action as follows:
10.2.1.1 Request to deviate off course by stating the
number of miles and the direction of the requested
deviation. In this case, when the requested deviation
is approved the pilot is expected to provide his/her
own navigation, to maintain the altitude assigned by
ATC, and to remain within the specified mileage of
his/her original course.
10.2.1.2 Request a new route to avoid the affected
area.
10.2.1.3 Request a change of altitude.
10.2.1.4 Request radar vectors around the affected
areas.
10.2.2 For obvious reasons of safety, an IFR pilot
must not deviate from the course or altitude/flight
level without a proper ATC clearance. When weather
conditions encountered are so severe that an
immediate deviation is determined to be necessary
and time will not permit approval by ATC, the pilot's
emergency authority may be exercised.
10.2.3 When the pilot requests clearance for a route
deviation or for an ATC radar vector, the controller
must evaluate the air traffic picture in the affected
area and coordinate with other controllers (if ATC
jurisdictional boundaries may be crossed) before
replying to the request.
10.2.4 It should be remembered that the controller's
primary function is to provide safe separation
between aircraft. Any additional service, such as
weather avoidance assistance, can only be provided
to the extent that it does not derogate the primary
function. It is also worth noting that the separation
workload is generally greater than normal when
weather disrupts the usual flow of traffic. ATC radar
limitations and frequency congestion may also be
factors in limiting the controller's capability to
provide additional service.
10.2.5 It is very important that the request for
deviation or radar vector be forwarded to ATC as far
in advance as possible. Delay in submitting it may
delay or even preclude ATC approval or require that
additional restrictions be placed on the clearance.
Insofar as possible, the following information should
be furnished to ATC when requesting clearance to
detour around weather activity:
10.2.5.1 Proposed point where detour will
commence.
10.2.5.2 Proposed route and extent of detour
(direction and distance).
10.2.5.3 Point where original route will be resumed.
10.2.5.4 Flight conditions (IFR or VFR).
10.2.5.5 Any further deviation that may become
necessary as the flight progresses.
10.2.5.6 Advise if the aircraft is equipped with
functioning airborne radar.
10.2.6 To a large degree, the assistance that might be
rendered by ATC will depend upon the weather
information available to controllers. Due to the
extremely transitory nature of severe weather
situations, the controller's weather information may
be of only limited value if based on weather observed
on radar only. Frequent updates by pilots giving
specific information as to the area affected, altitudes,
intensity, and nature of the severe weather can be of
considerable value. Such reports are relayed by radio
or phone to other pilots and controllers, and they also
receive widespread teletypewriter dissemination.
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10.2.7 Obtaining IFR clearance or an ATC radar
vector to circumnavigate severe weather can often be
accommodated more readily in the en route areas
away from terminals because there is usually less
congestion and, therefore, greater freedom of action.
In terminal areas, the problem is more acute because
of traffic density, ATC coordination requirements,
complex departure and arrival routes, and adjacent
airports. As a consequence, controllers are less likely
to be able to accommodate all requests for weather
detours in a terminal area or be in a position to
volunteer such routes to the pilot. Nevertheless,
pilots should not hesitate to advise controllers of any
observed severe weather and should specifically
advise controllers if they desire circumnavigation of
observed weather.
10.3 ATC Severe Weather Avoidance Plans
10.3.1 Air Route Traffic Control Centers and some
Terminal Radar Control facilities utilize plans for
severe weather avoidance within their control areas.
Aviation-oriented meteorologists provide weather
information. Preplanned alternate route packages
developed by the facilities are used in conjunction
with flow restrictions to ensure a more orderly flow
of traffic during periods of severe or adverse weather
conditions.
10.3.2 During these periods, pilots may expect to
receive alternative route clearances. These routes are
predicated upon the forecasts of the meteorologist
and coordination between the Air Traffic Control
System Command Center and the other centers. The
routes are utilized as necessary in order to allow as
many aircraft as possible to operate in any given area,
and frequently they will deviate from the normal
preferred routes. With user cooperation, this plan may
significantly reduce delays.
10.4 Procedures for Weather Deviations and
Other Contingencies in Oceanic Controlled
Airspace
10.4.1 When the pilot initiates communications with
ATC, rapid response may be obtained by stating
“WEATHER DEVIATION REQUIRED” to indicate
priority is desired on the frequency and for ATC
response. _
10.4.2 The pilot still retains the option of initiating
the communications using the urgency call “PAN-
PAN” three times to alert all listening parties of a
special handling condition which will receive ATC
priority for issuance of a clearance or assistance.
10.4.3 ATC will:_
10.4.3.1 Approve the deviation, or
10.4.3.2 Provide vertical separation and then
approve the deviation, or
10.4.3.3 If ATC is unable to establish vertical
separation, ATC shall advise the pilot that standard
separation cannot be applied; provide essential traffic
information for all affected aircraft, to the extent
practicable; and if possible, suggest a course of
action. ATC may suggest that the pilot climb or
descend to a contingency altitude (1,000 feet above or
below that assigned if operating above FL 290;
500_feet above or below that assigned if operating at
or below FL 290).
PHRASEOLOGY-
STANDARD SEPARATION NOT AVAILABLE; DEVIATE
AT PILOT'S DISCRETION; SUGGEST CLIMB (or
descent) TO (appropriate altitude); TRAFFIC (position
and altitude); REPORT DEVIATION COMPLETE.
10.4.4 The pilot will follow the ATC advisory
altitude when approximately 10 NM from track as
well as execute the procedures detailed in para-
graph_10.4.5.
10.4.5 If contact cannot be established or a revised
ATC clearance or advisory is not available and
deviation from track is required, the pilot shall take
the following actions:
10.4.5.1 If possible, deviate away from an organized
track or route system.
10.4.5.2 Broadcast aircraft position and intentions
on the frequency in use, as well as on frequency
121.5_MHz at suitable intervals stating: flight
identification (operator call sign), flight level, track
code or ATS route designator, and extent of deviation
expected.
10.4.5.3 Watch for conflicting traffic both visually
and by reference to the Traffic Alert and Collision
Avoidance System (TCAS), if equipped.
10.4.5.4 Turn on aircraft exterior lights.
10.4.5.5 Deviations of less than 10 NM or operations
within COMPOSITE (NOPAC and CEPAC) Air-
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space, should REMAIN at ASSIGNED altitude.
Otherwise, when the aircraft is approximately 10 NM
from track, initiate an altitude change based upon the
following criteria:
TBL GEN 3.5-4
Route
Centerline/Track
Deviations
>10 NM
Altitude Change
East
000 - 179_M
Left
Right
Descend 300 Feet
Climb 300 Feet
West
180-359_M
Left
Right
Climb 300 Feet
Descend 300 Feet
Pilot Memory Slogan: “East right up,
West right down.”
10.4.5.6 When returning to track, be at the assigned
flight level when the aircraft is within approximately
10 NM of centerline.
10.4.5.7 If contact was not established prior to
deviating, continue to attempt to contact ATC to
obtain a clearance. If contact was established,
continue to keep ATC advised of intentions and
obtain essential traffic information.
11. Notifications Required From Operators
11.1 Preflight briefing and flight documentation
services provided by AFSSs do not require prior
notification.
11.2 Preflight briefing and flight documentation
services provided by a National Weather Service
Office (or contract office) are available upon request
for long-range international flights for which
meteorological data packages are prepared for the
pilot-in-command. Briefing times should be coordi-
nated between the local representative and the local
meteorological office.
11.3 Flight Service Stations do not normally have the
capability to prepare meteorological data packages
for a preflight briefing.
12. Weather Observing Systems and
Operating Procedures
For surface wind readings, most meteorological
reporting stations have a direct reading, 3-cup
anemometer wind system for which a 1-minute mean
wind speed and direction (based on true north) is
taken. Some stations also have a continuous wind
speed recorder which is used in determining the
gustiness of the wind.
13. Runway Visual Range (RVR)
There are currently two configurations of the RVR,
commonly identified as Taskers and New Generation
RVR. The Taskers use transmissometer technology.
The New Generation RVRs use forward scatter
technology and are currently being deployed to
replace the existing Taskers.
13.1 RVR values are measured by transmissometers
mounted on 14-foot towers along the runway. A full
RVR system consists of:
13.1.1 A transmissometer projector and related
items.
13.1.2 A transmissometer receiver (detector) and
related items.
13.1.3 An analogue recorder.
13.1.4 A signal data converter and related items.
13.1.5 A remote digital or remote display program-
mer.
13.2 The transmissometer projector and receiver are
mounted on towers 250 feet apart. A known intensity
of light is emitted from the projector and is measured
by the receiver. Any obscuring matter, such as rain,
snow, dust, fog, haze, or smoke, reduces the light
intensity arriving at the receiver. The resultant
intensity measurement is then converted to an RVR
value by the signal data converter. These values are
displayed by readout equipment in the associated air
traffic facility and updated approximately once every
minute for controller issuance to pilots.
13.3 The signal data converter receives information
on the high-intensity runway edge light setting in use
(step 3, 4, or 5), transmission values from the
transmissometer, and the sensing of day or night
conditions. From the three data sources, the system
will compute appropriate RVR values.
13.4 An RVR transmissometer established on a
250-foot baseline provides digital readouts to a
minimum of 600 feet, which are displayed in
200-foot increments to 3,000 feet, and in 500-foot
increments from 3,000 feet to a maximum value of
6,000 feet.
13.5 RVR values for Category IIIa operations extend
down to 700-foot RVR; however, only 600 and
800_feet are reportable RVR increments. The
800_RVR reportable value covers a range of 701 feet
to 900 feet and is therefore a valid minimum
indication of Category IIIa operations.
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13.6 Approach categories with the corresponding
minimum RVR values are listed in TBL GEN 3.5-5.
TBL GEN 3.5-5
Approach Category/Minimum RVR Table
Category Visibility (RVR)
Nonprecision 2,400 feet
Category I 1,800 feet
Category II 1,200 feet
Category IIIa 700_ feet
Category IIIb 150 feet
Category IIIc _0_feet
13.7 Ten-minute maximum and minimum RVR
values for the designated RVR runway are reported in
the body of the aviation weather report when the
prevailing visibility is less than 1 mile and/or the RVR
is 6,000 feet or less. ATCTs report RVR when the
prevailing visibility is 1 mile or less and/or the RVR
is 6,000 feet or less.
13.8 Details on the requirements for the operational
use of RVR are contained in FAA Advisory
Circular_97-1, “Runway Visual Range (RVR).”
Pilots are responsible for compliance with minimums
prescribed for their class of operations in appropriate
Federal Aviation Regulations and/or operations
specifications.
13.8.1 RVR values are also measured by forward
scatter meters mounted on 14-foot frangible
fiberglass poles. A full RVR system consists of:
13.8.1.1 Forward scatter meter with a transmitter,
receiver and associated items.
13.8.1.2 A runway light intensity monitor (RLIM).
13.8.1.3 An ambient light sensor (ALS).
13.8.1.4 A data processor unit (DPU).
13.8.1.5 A controller display (CD).
13.8.2 The forward scatter meter is mounted on a
14-foot frangible pole. Infrared light is emitted from
the transmitter and received by the receiver. Any
obscuring matter such as rain, snow, dust, fog, haze,
or smoke increases the amount of scattered light
reaching the receiver. The resulting measurement
along with inputs from the runway light intensity
monitor and the ambient light sensor are forwarded to
the DPU which calculates the proper RVR value. The
RVR values are displayed locally and remotely on
controller displays.
13.8.3 The runway light intensity monitors both the
runway edge and centerline light step settings (steps_1
through 5). Centerline light step settings are used for
CAT IIIb operations. Edge light step settings are used
for CAT I, II, and IIIa operations.
13.8.4 New Generation RVRs can measure and
display RVR values down to the lowest limits of
Category IIIb operations (150 foot RVR). RVR
values are displayed in 100-foot increments and are
reported as follows:
13.8.4.1 100-foot increments for products below
800 feet.
13.8.4.2 200-foot increments for products between
800 feet and 3,000 feet.
13.8.4.3 500-foot increments for products between
3,000 feet and 6,500 feet.
13.8.4.4 25-meter increments for products below
150 meters.
13.8.4.5 50-meter increments for products between
150 meters and 800 meters.
13.8.4.6 100-meter increments for products
between 800 meters and 1,200 meters.
13.8.4.7 200-meter increments for products
between 1,200 meters and 2,000 meters.
14. Reporting of Cloud Heights
14.1 Ceiling, by definition in Federal Aviation
Regulations, and as used in Aviation Weather Reports
and Forecasts, is the height above ground (or water)
level of the lowest layer of clouds or obscuring
phenomenon that is reported as “broken,” “overcast,”
or “the vertical visibility into an obscuration.” For
example, an aerodrome forecast which reads
“BKN030” refers to heights above ground level
(AGL). An area forecast which reads “BKN030”
states that the height is above mean sea level (MSL).
See FIG GEN 3.5-23 for the Key to Routine Aviation
Weather Reports and Forecasts for the definition of
“broken,” “overcast,” and “obscuration.”_
14.2 Information on cloud base height is obtained by
use of ceilometers (rotating or fixed beam), ceiling
lights, ceiling balloons, pilot reports, and observer
estimations. The systems in use by most reporting
stations are either the observer estimation or the
rotating beam ceilometer.
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14.3 Pilots usually report height values above mean
sea level, since they determine heights by the
altimeter. This is taken into account when disseminat-
ing and otherwise applying information received
from pilots. (“Ceiling” heights are always above
ground level.) In reports disseminated as pilot
reports, height references are given the same as
received from pilots; that is, above mean sea level.
14.4 In area forecasts or inflight Advisories, ceilings
are denoted by the contraction “CIG” when used with
sky cover symbols as in “LWRG TO CIG OVC005,”
or the contraction “AGL” after the forecast cloud
height value. When the cloud base is given in height
above mean sea level, it is so indicated by the
contraction “MSL” or “ASL” following the height
value. The heights of cloud tops, freezing level, icing,
and turbulence are always given in heights above
mean sea level (ASL or MSL).
15. Reporting Prevailing Visibility
15.1 Surface (horizontal) visibility is reported in
METAR reports in terms of statute miles and
increments thereof; e.g., 1
/16,
1
/8,
3/
16,
1
/4,
5/
16,
3/
8,
1/
2,
5/
8,
3
/4,
7
/8, 1, 1 1
/8, etc. (Visibility reported by an
unaugmented automated site is reported differently
than in a manual report; i.e., ASOS: 0, 1
/16,
1/
8,
1
/4,
1/
2,
3
/4, 1, 1 1
/4, 1 1/
2, 1 3/
4, 2, 2 1/
2, 3, 4, 5, etc., AWOS:
M1 /4,
1
/4,
1/
2,
3
/4, 1, 1 1
/4, 1 1/
2, 1 3/
4, 2, 2 1/
2, 3, 4, 5,
etc.) Visibility is determined through the ability to see
and identify preselected and prominent objects at a
known distance from the usual point of observation.
Visibilities which are determined to be less than
7_miles, identify the obscuring atmospheric condi-
tion; e.g., fog, haze, smoke, etc., or combinations
thereof.
15.2 Prevailing visibility is the greatest visibility
equalled or exceeded throughout at least one-half the
horizon circle, not necessarily contiguous. Segments
of the horizon circle which may have a significantly
different visibility may be reported in the remarks
section of the weather report; i.e., the southeastern
quadrant of the horizon circle may be determined to
be 2 miles in mist while the remaining quadrants are
determined to be 3 miles in mist.
15.3 When the prevailing visibility at the usual point
of observation, or at the tower level, is less than
4_miles, certificated tower personnel will take
visibility observations in addition to those taken at the
usual point of observation. The lower of these
two_values will be used as the prevailing visibility for
aircraft operations.
16. Estimating Intensity of Rain and Ice
Pellets
16.1 Rain
16.1.1 Light. From scattered drops that, regardless
of duration, do not completely wet an exposed surface
up to a condition where individual drops are easily
seen.
16.1.2 Moderate. Individual drops are not clearly
identifiable; spray is observable just above pave-
ments and other hard surfaces.
16.1.3 Heavy. Rain seemingly falls in sheets;
individual drops are not identifiable; heavy spray to
a height of several inches is observed over hard
surfaces.
16.2 Ice Pellets
16.2.1 Light. Scattered pellets that do not com-
pletely cover an exposed surface regardless of
duration. Visibility is not affected.
16.2.2 Moderate. Slow accumulation on the
ground. Visibility is reduced by ice pellets to less than
7 statute miles.
16.2.3 Heavy. Rapid accumulation on the ground.
Visibility is reduced by ice pellets to less than 3 statute
miles.
17. Estimating the Intensity of Snow or
Drizzle (Based on Visibility)
17.1 Light. Visibility more than 1
/2 statute mile.
17.2 Moderate. Visibility from more than 1
/4
statute mile to 1
/2 statute mile.
17.3 Heavy. Visibility 1
/4 statute mile or less.
18. Pilot Weather Reports (PIREPs)
18.1 FAA air traffic facilities are required to solicit
PIREPs when the following conditions are reported
or forecast: ceilings at or below 5,000 feet, visibility
at or below 5 miles (surface or aloft), thunderstorms
and related phenomena, icing of a light degree or
greater, turbulence of a moderate degree or greater,
wind shear, and reported or forecast volcanic ash
clouds.
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18.2 Pilots are urged to cooperate and promptly
volunteer reports of these conditions and other
atmospheric data, such as cloud bases, tops and
layers, flight visibility, precipitation, visibility
restrictions (haze, smoke, and dust), wind at altitude,
and temperature aloft.
18.3 PIREPs should be given to the ground facility
with which communications are established; i.e.,
EFAS, AFSS/FSS, ARTCC, or terminal ATC. Radio
call “FLIGHT WATCH,” which serves as a collection
point for the exchange of PIREPs with en route
aircraft, is one of the primary duties of EFAS
facilities.
18.4 If pilots do not make PIREPs by radio, it is
helpful if, upon landing, they report to the nearest
AFSS/FSS or Weather Forecast Office the inflight
conditions which they encountered. Some of the uses
made of the reports are:
18.4.1 The ATCT uses the reports to expedite the
flow of air traffic in the vicinity of the field and for
hazardous weather avoidance procedures.
18.4.2 The AFSS/FSS uses the reports to brief other
pilots, to provide inflight advisories and weather
avoidance information to en route aircraft.
18.4.3 The ARTCC uses the reports to expedite the
flow of en route traffic, to determine most favorable
altitudes, and to issue hazardous weather information
within the center's area.
18.4.4 The NWS uses the reports to verify or amend
conditions contained in aviation forecasts and
advisories; (In some cases, pilot reports of hazardous
conditions are the triggering mechanism for the
issuance of advisories.)
18.4.5 The NWS, other government organizations,
the military, and private industry groups use PIREPs
for research activities in the study of meteorological
phenomena.
18.4.6 All air traffic facilities and the NWS forward
the reports received from pilots into the weather
distribution system to assure the information is made
available to all pilots and other interested parties.
18.5 The FAA, NWS, and other organizations that
enter PIREPs into the weather reporting system use
the format listed in TBL GEN 3.5-6, PIREP Element
Code Chart. Items 1 through 6 are included in all
transmitted PIREPs along with one or more of items
7 through 13. Although the PIREP should be as
complete and concise as possible, pilots should not be
overly concerned with strict format or phraseology.
The important thing is that the information is relayed
so other pilots may benefit from your observation. If
a portion of the report needs clarification, the ground
station will request the information.
18.6 Completed PIREPs will be transmitted to
weather circuits as in the following examples:
EXAMPLE-
KCMH UA/OV APE 230010/TM 1516/FL085/TP
BE20/SK BKN065/WX FV03SM HZ FU/TA 20/TB LGT.
Translation: one zero miles southwest of Appleton VOR;
time 1516 UTC; altitude eight thousand five hundred;
aircraft type BE20; base of the broken cloud layer is six
thousand five hundred; flight visibility 3 miles with haze
and smoke; air temperature 20 degrees Celsius; light
turbulence.
EXAMPLE-
KCRW UA/OV KBKW 360015-KCRW/TM 1815/
FL120/TP BE99/SK IMC/WX RA-/TA M08/WV
290030/TB LGT-MDT/IC LGT RIME/RM MDT MXD
ICG DURC KROA NWBND FL080-100 1750Z.
Translation: from 15 miles north of Beckley VOR to
Charleston_VOR; time 1815 UTC; altitude 12,000 feet;
type aircraft, BE-99; in clouds; rain; temperature
minus_8_Celsius; wind 290 degrees magnetic at 30 knots;
light to moderate turbulence; light rime icing during climb
northwestbound from Roanoke, VA, between 8,000 and
10,000 feet at 1750_UTC.
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Federal Aviation Administration Nineteenth Edition
TBL GEN 3.5-6
PIREP Element Code Chart
PIREP ELEMENT PIREP CODE CONTENTS
1. 3-letter station identifier XXX Nearest weather reporting location to the reported phenomenon
2. Report type UA or UUA Routine or urgent PIREP
3. Location /OV In relation to a VOR
4. Time /TM Coordinated Universal Time
5. Altitude /FL Essential for turbulence and icing reports
6. Type aircraft /TP Essential for turbulence and icing reports
7. Sky cover /SK Cloud height and coverage (sky clear, few, scattered, broken, or
overcast)
8. Weather /WX Flight visibility, precipitation, restrictions to visibility, etc.
9. Temperature /TA Degrees Celsius
10. Wind /WV Direction in degrees magnetic north and speed in knots
11. Turbulence /TB See paragraph 22
12. Icing /IC See paragraph 20
13. Remarks /RM For reporting elements not included or to clarify previously
reported items
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19. Mandatory MET Points
19.1 Within the ICAO CAR/SAM Regions and
within the U.S. area of responsibility, several
mandatory MET reporting points have been
established. These points are located within the
Houston, Miami, and San Juan Flight Information
Regions (FIR). These points have been established
for flights between the South American and
Caribbean Regions and Europe, Canada and the U.S.
19.2 Mandatory MET Reporting Points Within the Houston FIR
Point For Flights Between
ABBOT Acapulco and Montreal, New York, Toronto, Mexico City and New Orleans.
ALARD New Orleans and Belize, Guatemala, San Pedro Sula, Mexico City and Miami, Tampa.
ARGUS Toronto and Guadalajara, Mexico City, New Orleans and Mexico City.
SWORD Dallas-Fort Worth, New Orleans, Chicago and Cancun, Cozumel, and Central America.
19.3 Mandatory MET Reporting Points Within the Miami FIR
Point For Flights Between
Grand
Turk
New York and Aruba, Curacao, Kingston, Miami and Belem, St. Thomas, Rio de Janeiro, San_Paulo,
St._Croix, Kingston and Bermuda.
GRATX Madrid and Miami, Havana.
MAPYL New York and Guayaquil, Montego Bay, Panama, Lima, Atlanta and San Juan.
RESIN New Orleans and San Juan.
SLAPP New York and Aruba, Curacao, Kingston, Port-au-Prince. Bermuda and Freeport, Nassau. New York
and Barranquilla, Bogota, Santo Domingo, Washington and Santo Domingo, Atlanta and San_Juan.
19.4 Mandatory MET Reporting Points Within the San Juan FIR
Point For Flights Between
GRANN Toronto and Barbados, New York and Fort de France. At intersection of routes A321, A523, G432.
KRAFT San Juan and Buenos Aires, Caracas, St. Thomas, St. Croix, St. Maarten, San Juan, Kingston and
Bermuda.
PISAX New York and Barbados, Fort de France, Bermuda and Antigua, Barbados.
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TBL GEN 3.5-7
Intensity Ice Accumulation
Trace Ice becomes perceptible. Rate of accumulation slightly greater than rate of sublimation.
Deicing/anti-icing equipment is not utilized unless encountered for an extended period of time (over
1 hour).
Light The rate of accumulation may create a problem if flight is prolonged in this environment (over
1_hour). Occasional use of deicing/anti-icing equipment removes/prevents accumulation. It does not
present a problem if the deicing/anti-icing equipment is used.
Moderate The rate of accumulation is such that even short encounters become potentially hazardous and use of
deicing/anti-icing equipment or diversion is necessary.
Severe The rate of accumulation is such that deicing/anti-icing equipment fails to reduce or control the
hazard. Immediate diversion is necessary.
Pilot Report: Aircraft Identification, Location, Time (UTC), Intensity of Type1 , Altitude/FL, Aircraft Type, Indicated
Air Speed (IAS), and Outside Air Temperature (OAT)2
.
1
Rime or Clear Ice: Rime ice is a rough, milky, opaque ice formed by the instantaneous freezing of small supercooled
water droplets. Clear ice is a glossy, clear, or translucent ice formed by the relatively slow freezing of large
supercooled water droplets.
2
The Outside Air Temperature (OAT) should be requested by the AFSS/FSS or ATC if not included in the PIREP.
20. PIREPs Relating to Airframe Icing
20.1 The effects of ice accretion on aircraft are:
cumulative-thrust is reduced, drag increases, lift
lessens, weight increases. The results are an increase
in stall speed and a deterioration of aircraft
performance. In extreme cases, 2 to 3 inches of ice
can form on the leading edge of the airfoil in less than
5 minutes. It takes but 1
/2 inch of ice to reduce the
lifting power of some aircraft by 50 percent and to
increase the frictional drag by an equal percentage.
20.2 A pilot can expect icing when flying in visible
precipitation, such as rain or cloud droplets, and the
temperature is between +02 and -10 degrees Celsius.
When icing is detected, a pilot should do one of two
things (particularly if the aircraft is not equipped with
deicing equipment). The pilot should get out of the
area of precipitation or go to an altitude where the
temperature is above freezing. This “warmer”
altitude may not always be a lower altitude. Proper
preflight action includes obtaining information on the
freezing level and the above-freezing levels in
precipitation areas. Report the icing to an ATC or FSS
facility, and if operating IFR, request new routing or
altitude if icing will be a hazard. Be sure to give the
type of aircraft to ATC when reporting icing.
TBL GEN 3.5-7, describes how to report icing
conditions.
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21. Definitions of Inflight Icing Terms
See TBL GEN 3.5-8, Icing Types, and TBL GEN 3.5-9, Icing Conditions.
TBL GEN 3.5-8
Icing Types
Clear Ice See Glaze Ice.
Glaze Ice Ice, sometimes clear and smooth, but usually containing some air pockets, which results in a
lumpy translucent appearance. Glaze ice results from supercooled drops/droplets striking a
surface but not freezing rapidly on contact. Glaze ice is denser, harder, and sometimes more
transparent than rime ice. Factors, which favor glaze formation, are those that favor slow
dissipation of the heat of fusion (i.e., slight supercooling and rapid accretion). With larger
accretions, the ice shape typically includes “horns” protruding from unprotected leading edge
surfaces. It is the ice shape, rather than the clarity or color of the ice, which is most likely to
be accurately assessed from the cockpit. The terms “clear” and “glaze” have been used for
essentially the same type of ice accretion, although some reserve “clear” for thinner accretions
which lack horns and conform to the airfoil.
Intercycle Ice Ice which accumulates on a protected surface between actuation cycles of a deicing system.
Known or Observed or
Detected Ice Accretion
Actual ice observed visually to be on the aircraft by the flight crew or identified by on-board
sensors.
Mixed Ice Simultaneous appearance or a combination of rime and glaze ice characteristics. Since the
clarity, color, and shape of the ice will be a mixture of rime and glaze characteristics, accurate
identification of mixed ice from the cockpit may be difficult.
Residual Ice Ice which remains on a protected surface immediately after the actuation of a deicing system.
Rime Ice A rough, milky, opaque ice formed by the rapid freezing of supercooled drops/droplets after
they strike the aircraft. The rapid freezing results in air being trapped, giving the ice its opaque
appearance and making it porous and brittle. Rime ice typically accretes along the stagnation
line of an airfoil and is more regular in shape and conformal to the airfoil than glaze ice. It is
the ice shape, rather than the clarity or color of the ice, which is most likely to be accurately
assessed from the cockpit.
Runback Ice Ice which forms from the freezing or refreezing of water leaving protected surfaces and
running back to unprotected surfaces.
Note-
Ice types are difficult for the pilot to discern and have uncertain effects on an airplane in flight. Ice type definitions will
be included in the AIP for use in the “Remarks” section of the PIREP and for use in forecasting.
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GEN 3.5-40
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Federal Aviation Administration Nineteenth Edition
TBL GEN 3.5-9
Icing Conditions
Appendix C Icing Conditions Appendix C (14 CFR, Part 25 and 29) is the certification icing condition standard
for approving ice protection provisions on aircraft. The conditions are specified in
terms of altitude, temperature, liquid water content (LWC), representative droplet
size (mean effective drop diameter ), and cloud horizontal extent.
Forecast Icing Conditions Environmental conditions expected by a National Weather Service or an
FAA-approved weather provider to be conducive to the formation of inflight icing
on aircraft.
Freezing Drizzle (FZDZ) Drizzle is precipitation at ground level or aloft in the form of liquid water drops
which have diameters less than 0.5 mm and greater than 0.05 mm. Freezing drizzle
is drizzle that exists at air temperatures less than 0_C (supercooled), remains in
liquid form, and freezes upon contact with objects on the surface or airborne.
Freezing Precipitation Freezing precipitation is freezing rain or freezing drizzle falling through or outside
of visible cloud.
Freezing Rain (FZRA) Rain is precipitation at ground level or aloft in the form of liquid water drops which
have diameters greater than 0.5 mm. Freezing rain is rain that exists at air
temperatures less than 0_C (supercooled), remains in liquid form, and freezes upon
contact with objects on the ground or in the air.
Icing in Cloud Icing occurring within visible cloud. Cloud droplets (diameter < 0.05 mm) will be
present; freezing drizzle and/or freezing rain may or may not be present.
Icing in Precipitation Icing occurring from an encounter with freezing precipitation, that is, supercooled
drops with diameters exceeding 0.05 mm, within or outside of visible cloud.
Known Icing Conditions Atmospheric conditions in which the formation of ice is observed or detected in
flight.
Note-
Because of the variability in space and time of atmospheric conditions, the existence
of a report of observed icing does not assure the presence or intensity of icing
conditions at a later time, nor can a report of no icing assure the absence of icing
conditions at a later time.
Potential Icing Conditions Atmospheric icing conditions that are typically defined by airframe manufacturers
relative to temperature and visible moisture that may result in aircraft ice accretion
on the ground or in flight. The potential icing conditions are typically defined in the
Airplane Flight Manual or in the Airplane Operation Manual.
Supercooled Drizzle Drops
(SCDD)
Synonymous with freezing drizzle aloft.
Supercooled Drops or /Droplets Water drops/droplets which remain unfrozen at temperatures below 0 _C.
Supercooled drops are found in clouds, freezing drizzle, and freezing rain in the
atmosphere. These drops may impinge and freeze after contact on aircraft surfaces.
Supercooled Large Drops (SLD) Liquid droplets with diameters greater than 0.05 mm at temperatures less than
0_C, i.e., freezing rain or freezing drizzle.
