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Newton’s Second Law of Motion is the Law of Momentum,
which states that a body will accelerate in the same direction
as the force acting upon that body, and the acceleration
will be directly proportional to the net force and inversely
proportional to the mass of the body. Acceleration refers
either to an increase or decrease in velocity, although
deceleration is commonly used to indicate a decrease. This
law governs the aircraft’s ability to change flight path and
speed, which are controlled by attitude (both pitch and bank)
and thrust inputs. Speeding up, slowing down, entering
climbs or descents, and turning are examples of accelerations
that the pilot controls in everyday flight. [Figure 2-5]
Newton’s Third Law, the Law of Reaction
Newton’s Third Law of Motion is the Law of Reaction,
which states that for every action there is an equal and
opposite reaction. As shown in Figure 2-6, the action of
the jet engine’s thrust or the pull of the propeller lead to the
reaction of the aircraft’s forward motion. This law is also
responsible for a portion of the lift that is produced by a wing,
from the downward deflection of the airflow around it. This
downward force of the relative wind results in an equal but
opposite (upward) lifting force created by the airflow over
the wing. [Figure 2-6]
Atmosphere
The atmosphere is the envelope of air which surrounds the
Earth. A given volume of dry air contains about 78 percent
nitrogen, 21 percent oxygen, and about 1 percent other gases
such as argon, carbon dioxide, and others to a lesser degree.
Although seemingly light, air does have weight and a one
square inch column of the atmosphere at sea level weighs
approximately 14.7 pounds. About one-half of the air by
weight is within the first 18,000 feet. The remainder of the air
is spread over a vertical distance in excess of 1,000 miles.
Air density is a result of the relationship between temperature
and pressure. Air density is inversely related to temperature
and directly related to pressure. For a constant pressure to be
2-5
Figure 2-5. Newton’s Second Law of Motion: the Law of Figure 2-6. Newton’s Third Law of Motion: the Law of Reaction.
Momentum.
maintained as temperature increases, density must decrease,
and vice versa. For a constant temperature to be maintained
as pressure increases, density must increase, and vice versa.
These relationships provide a basis for understanding
instrument indications and aircraft performance.
Layers of the Atmosphere
There are several layers to the atmosphere with the
troposphere being closest to the Earth’s surface extending to
about 60,000 feet at the equator. Following is the stratosphere,
mesosphere, ionosphere, thermosphere, and finally the
exosphere. The tropopause is the thin layer between the
troposphere and the stratosphere. It varies in both thickness
and altitude but is generally defined where the standard
lapse (generally accepted at 2° C per 1,000 feet) decreases
significantly (usually down to 1° C or less).
International Standard Atmosphere (ISA)
The International Civil Aviation Organization (ICAO)
established the ICAO Standard Atmosphere as a way
of creating an international standard for reference and
performance 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" inches
of mercury (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 top of the stratosphere. 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. For instance, at
10,000 ISA predicts that the air pressure should be 19.92" Hg
(29.92" - 10" Hg = 19.92") and the outside temperature at -5°C
(15° C - 20° C). If the temperature or the pressure is different
　
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