aircraft in flying through the air, and the airfoils
accuracy of this method is limited because you
produce more lift. Pressure, temperature, and
must assume a standard lapse rate; that is, for a
moisture content all affect air density. Measure-
given number of feet of altitude, an exact change
ments of air density are in weight per unit volume
in pressure occurs. This seldom happens, which
(for example, pounds per cubic foot). However,
limits the accuracy of the altimeter. Above 18,000
a more convenient measurement of air density for
feet, all altimeters are set to 29.92 (pressure
the pilot is density altitude. This is the altitude
altitude). Although the altimeter is not accurate,
assigned to a given density in the standard
as long as all aircraft have the same barometric
atmosphere.
pressure setting, aircraft vertical separation is
Density altitude does not show on an instru-
controlled.
ment. It is usually taken from a table or computed
by comparing pressure, altitude, and temperature.
DENSITY ALTITUDE. --A very important
Although moisture content affects air density, its
factor in determining the performance of an
effect is negligible.
aircraft or engine is the density of the air. The
Figure 6-7 shows the different types of
denser the air, the more horsepower the engine
altitude. At Navy Memphis, the field elevation is
can produce. Also, there is more resistance to the
Figure 6-7.-Different types of altitude.
6-7
