The most common method used by flight crews to
obtain specific inflight weather information is to use a
source that broadcasts weather for the specific airport.
Information about ceilings, visibility, wind, temperature,
barometric pressure, and field conditions can be
obtained from most types of broadcast weather
services. Broadcast weather can be transmitted to
the aircraft in radio voice format or digital format,
if it is available, via an ACARS system.
AUTOMATIC TERMINAL INFORMATION SERVICE
The weather broadcast system found most often at
airports with air traffic control towers in the National
Airspace System (NAS) is the automatic terminal
information service (ATIS). The AIM defines ATIS as the continuous broadcast of recorded non-control
information in selected high activity terminal areas.
The main purpose of ATIS is the reduction of frequency
congestion and controller workload. It is
broadcast over very high frequency (VHF) radio
frequencies, and is designed to be receivable up to
60 NM from the transmitter at altitudes up to 25,000
feet above ground level(AGL). ATIS is typically
derived from an automated weather observation
system or a human weather observer’s report.
AUTOMATED WEATHER OBSERVING PROGRAMS
Automated surface observation systems can provide
pilots with weather information over discrete VHF frequencies
or over the voice portion of local NAVAIDs.The automated weather observing system (AWOS) and
automated surface observing system (ASOS) provide
real-time weather information that can be used by flight
crews to make approach decisions, and by the NWS to
generate aviation routine weather reports (METARs).
Flight crews planning approaches to airports where
ATIS is not available may be able to obtain current
airport conditions from an AWOS/ASOS facility.
FAA-owned and operated AWOS-2 and AWOS-3
systems are approved sources of weather for Part 121
and 135 operations. Also, NWS-operated ASOSs are
approved sources of weather for Part 121 and 135
operations. An AWOS/ASOS cannot be used as an
authorized weather source for Part 121 or 135 instrument
flight rules (IFR) operations if the visibility or
altimeter setting is reported missing from the report.
Refer to the AIM for the most current information
on automated weather observation systems.
In the event that an airport has weather observation capability,
but lacks the appropriate equipment to transmit
that information over a radio frequency, air route traffic
control centers (ARTCCs) can provide flight crews with
hourly METAR or non-routine (special) aviation
weather report (SPECI) information for those airports.
For example, as an aircraft approaches an airport, the
center controller can voluntarily or upon request provide
the pilot with the most recent weather observation.
Terminal Radar Approach Control (TRACON) facilities
also provide weather observation information on a workload-
permitting basis. Another option to obtain a current
METAR or SPECI is to contact an En Route Flight
Advisory Service facility (Flight Watch).
There are many practical reasons for reviewing weather
information prior to initiating an instrument approach.
Pilots must familiarize themselves with the condition
of individual airports and runways so that they may
make informed decisions regarding fuel management,
diversions, and alternate planning. Because this information
is critical, CFRs require pilots to comply with
specific weather minimums for planning and execution
of instrument flights and approaches.
PART 91 OPERATORS
According to Part 91.103, the pilot in command must
become familiar with all available information concerning
a flight prior to departure. Included in this
directive is the fundamental basis for pilots to review
NOTAMs and pertinent weather reports and forecasts
for the intended route of flight. This review should
include current weather reports and terminal forecasts
for all intended points of landing and alternate airports.
In addition, a thorough review of an airport’s current
weather conditions should always be conducted prior
to initiating an instrument approach. Pilots should also consider weather information as a planning tool for fuel
For flight planning purposes, weather information
must be reviewed in order to determine the necessity
and suitability of alternate airports. For Part 91 operations,
the 600-2 and 800-2 rule applies to airports with
precision and nonprecision approaches, respectively.
Approaches with vertical guidance (APV) are considered
semi-precision and nonprecision since they do not
meet the International Civil Aviation Organization
(ICAO) Annex 10 standards for a precision approach.
(See Final Approach Segment section later in this
chapter for more information regarding APV
approaches.) Exceptions to the 600-2 and 800-2 alternate
minimums are listed in the front of the National
Aeronautical Charting Office (NACO) U.S. Terminal
Procedures Publication (TPP) and are indicated by an
“ ” symbol on the approach charts for the airport.
This does not preclude flight crews from initiating
instrument approaches at alternate airports when the
weather conditions are below these minimums. The
600-2 and 800-2 rules, or any exceptions, only apply to
flight planning purposes, while published landing minimums
apply to the actual approach at the alternate.