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Instrument Flying Handbook Menu>Aerodynamic
Factors>Atmosphere>Standard
Atmosphere
The International
Civil Aviation Organization (ICAO) established the ICAO Standard
Atmosphere as a way of creating an international standard for
reference and computations. Instrument indications and aircraft
performance specifications are derived using this standard as
a reference. Because the standard atmosphere is a derived set
of conditions that rarely exist in reality, pilots need to understand
how deviations from the standard affect both instrument indications
and aircraft performance.
In the standard atmosphere, sea level pressure
is 29.92" Hg and the temperature is 15 °C (59 °F).
The standard lapse rate for pressure is approximately a 1"
Hg decrease per 1,000 feet increase in altitude. The standard
lapse rate for temperature is a 2 °C (3.6 °F) decrease
per 1,000 feet increase, up to the tropopause. Since all aircraft
performance is compared and evaluated in the environment of
the standard atmosphere, all aircraft performance instrumentation
is calibrated for the standard atmosphere. Because the actual
operating conditions rarely, if ever, fit the standard atmosphere,
certain corrections must apply to the instrumentation and aircraft
performance.
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