The standard atmosphere is theoretical. It was
derived by averaging the readings taken over a period
of years. The list of altitudes and their corresponding
values of temperature and pressure given in figure 2-7
were determined by these averages.
INDICATED ALTITUDE.— The term indicated
altitude means the value of altitude that is displayed
on the pressure altimeter.
CALIBRATED ALTITUDE.— Calibrated altitude
is indicated altitude corrected for installation/
positional error.
PRESSURE ALTITUDE.— The height of the
aircraft above the standard datum plane is called
pressure altitude.
DENSITY ALTITUDE.— Density is mass per
unit volume. The density of the air varies with
temperature and with height. Warm air expands, and
is less dense than cold air. Normally, the higher the
pressure altitude, the less dense the air becomes. The
density of the air can be expressed in terms of the
standard atmosphere. Density altitude is the pressure
altitude corrected for temperature. This calculation
converts the density of the air to the standard
atmospheric altitude having the same density.
Density altitude is used in performance data and true
airspeed calculations.
TRUE ALTITUDE.— True altitude is the actual
vertical distance above mean sea level, measured in
feet. It can be determined by two methods: (1) Set the
local altimeter setting on the barometric scale of the
pressure altimeter to obtain the indicated true altitude.
The indicated true altitude can then be resolve to the
true altitude by use of a DR computer. (2) Measure
altitude over water with an absolute altimeter.
ABSOLUTE ALTITUDE.— The height above
the terrain is called absolute altitude. It is computed
by subtracting terrain elevation from true altitude, or
it can be read directly from an absolute altimeter.
Pressure Altimeter
As stated earlier, every naval aircraft has a
pressure altimeter. The altitude indicated is indicated
altitude, not absolute altitude.
PRINCIPLES OF OPERATION.— The pres-
sure altimeter is an aneroid barometer calibrated to
indicate feet of altitude instead of pressure. The
pointers are connected by a mechanical linkage to a
set of aneroid cells. These aneroid cells expand or
contract with changes in barometric pressure. The
cells assume a particular thickness at a given pressure
level, and thereby position the altitude pointers
accordingly.
On the face of the indicator is a
barometric scale that indicates the barometric
pressure (in.Hg) of the point or plane from which the
instrument is measuring altitude. If you turn the
barometric pressure set knob on the altimeter, it
manually changes the setting on the scale. It also
results in simultaneous movement of the pointers to
the corresponding altitude reading.
Like all measurements, an altitude reading is
meaningless if the reference point is unknown. The
pressure altimeter face supplies both values. The
position of the pointers indicate the altitude in feet,
and the barometric scale indicates the pressure of the
reference plane.
TYPES OF PRESSURE ALTIMETERS.—
There are two different types of altimeters that you
will be concerned with. They are the counter-pointer
altimeter and the counter-drum-pointer altimeter.
Counter-Pointer Altimeter.—Ths altimeter has
a two-digit counter display unit located in the
9 o’clock position of the dial. The counter indicates
altitude in 1,000-foot increments from 0 to 80,000
feet (fig. 2-8). A single conventional pointer indicates
hundreds of feet on the fixed circular scale. The
pointer makes 1 revolution per 1,000 feet of altitude,
and as it passes through the 900- to 1,000-foot area of
the dial, the 1,000-foot counter is actuated. The shaft
of the 1,000-foot counter actuates the 10,000-foot
counter at each 10,000 feet of altitude change. To
determine the indicated altitude, you read the
Figure 2-8.-Counter-pointer altimeter.
2-8
