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day as the area is heated by the sun. These rising vertical
air currents are commonly known as thermals. Generally,
stable air has mild thermals and therefore less turbulence
than unstable air. Unstable air rises faster, creating greater
turbulence. Highly unstable air rises rapidly and, with enough
moisture, can build into thunderstorms.
Air stability is easily determined by the rate at which the
temperature drops with increased altitude. A standard
atmosphere is where the temperature drops 2 °C for every
1,000 foot increase. If the temperature drops less than 2 °C
per thousand feet, the air is more stable with less vertical wind
(thermals) developed during the day. If the temperature drops
more than 2 °C per thousand feet, the air is more unstable
with more powerful vertical air currents developed during
the day, creating greater turbulence.
In addition to air stability, barometric pressure has a large
effect on weather. Low pressure in the area, below the
standard atmosphere of 29.92 "Hg, is generally rising air
with dynamic and unsettled weather. High pressure above
the standard atmosphere in the area is generally sinking air
resulting in good weather for fl ying.
Many airports have automated weather systems in which
pilots can call the automated weather sensor platforms that
collect weather data at airports and listen to this information
via radio and/or land line. Radio frequencies are on the
sectional chart and the A/FD has the telephone numbers
for these stations. The systems currently available are the
Automated Surface Observing System (ASOS), Automated
Weather Sensor System (AWSS), and Automated Weather
Observation System (AWOS).
Local conditions of wind, moisture, stability, and barometric
pressure are factors that should be researched before fl ight to
make a competent decision of go or no go to fl y. High winds
and moist unstable air with a low barometric pressure indicate
undesirable fl ying conditions. Light winds and dry stable air
with high pressure indicate favorable fl ying conditions.
Pilots should research and document these local conditions
before fl ight to predict the fl ying conditions and compare
the actual fl ying conditions to the predictions to learn and
develop knowledge from the information resources available
for fl ight.
In addition to weather, the National Airspace needs to be
checked to ensure there are no temporary fl ight restrictions
(TFR) for the locations planned to fl y. TFRs may be found
at www.tfr.faa.gov/. For a complete prefl ight briefi ng of
weather and TFRs, call 1-800-WX-BRIEF.
Clouds visually tell what the air is doing, which provides
valuable information for any flight. To understand the
different cloud formations and the ground/air effects
produced, refer to weather theory in the Pilot’s Handbook of
Aeronautical Knowledge. [Figure 5-6] Cloud clearance and
visibility should be maintained for the operations intended
to be conducted. The chapter covering the National Airspace
System (NAS) provides cloud clearance requirements in
each class of airspace. A pilot should not fl y when ground
and fl ight visibility are below minimums for his or her pilot
certifi cate and the class of airspace where operating.
Knowledge of mechanical turbulence and how to determine
where it can occur is also important. The lee side of objects
can feel turbulence from the wind up to ten times the height
of the object. The stronger the wind is, the stronger the
turbulence is. [Figures 5-7 and 5-8]
In addition to adhering to the regulations and manufacturer
recommendations for weather conditions, it is important to
develop a set of personal minimums such as wind limitations,
time of day, and temperature-dew point spread. These
minimums will evolve as a pilot gains experience and are
also dependent on recency and currency in the make/model
of aircraft being fl own.
Weight and Loading
Weight and loading must be considered before each fl ight.
Do not exceed the maximum gross weight as specifi ed in the
pilot’s operating handbook (POH). The balance of the pilot,
passenger, fuel, and baggage is usually not an issue, but must
be reviewed in the POH for the specifi c make/model since
some may have balance limitations. The fore and aft carriage
attachment to the wing hang point must be within the limits as
specifi ed in the POH for weight and loading of the carriage.
Always follow the POH performance limitations.
5-6
I N D W
Figure 5-7. Turbulence created by manmade items.
Figure 5-6. Cloud diagram.
20,000 AGL
　
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