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other utilizes the mechanical makeup of the altimeter setting
system.
When the glider altitude is changed, the changing
pressure within the altimeter case expands or contracts
the aneroid barometer that through linkage rotates the
pointers. A decrease in pressure causes the altimeter to
indicate an increase in altitude, and an increase in pressure
causes the altimeter to indicate a decrease in altitude.
It is obvious then that if the glider is flown from a pressure
level of 28.75 in. Hg. to a pressure level of 29.75 in.
Hg., the altimeter would show a decrease of approximately
1,000 feet in altitude. [Figure 4-6]
The other method of moving the pointers does not rely
on changing air pressure, but the mechanical construction
of the altimeter. When the knob on the altimeter is
rotated, the altimeter setting pressure scale moves
Figure 4-6. Flying from an area of high pressure to an area of lower pressure, without resetting your altimeter,
results in your glider’s true altitude being lower than indicated.
4-7
simultaneously with the altimeter pointers. This may
be confusing because the numerical values of pressure
indicated in the window increase while the altimeter
indicates an increase in altitude; or decrease while the
altimeter indicates a decrease in altitude. This is
contrary to the reaction on the pointers when air
pressure changes, and is based solely on the mechanical
makeup of the altimeter. To further explain this point,
assume that the correct altimeter setting is 29.50 or a
.50 difference. This would cause a 500-foot error in
altitude. In this case if the altimeter setting is adjusted
from 30.00 to 29.50, the numerical value decreases and
the altimeter indicates a decrease of 500 feet in altitude.
Before this correction was made, the glider was
flying at an altitude of 500 feet lower than was shown
on the altimeter.
TYPES OF ALTITUDE
Knowing the glider’s altitude is vitally important to the
pilot for several reasons. The pilot must be sure that the
glider is flying high enough to clear the highest terrain
or obstruction along the intended route; this is especially
important when visibility is restricted. To keep
above mountain peaks, the pilot must be aware of the
glider’s altitude and elevation of the surrounding terrain
at all times. Knowledge of the altitude is necessary to
calculate true airspeeds.
Altitude is vertical distance above some point or level
used as a reference. There may be as many kinds of altitude
as there are reference levels from which altitude is
measured and each may be used for specific reasons.
The following are the four types of altitude that affect
glider pilots. [Figure 4-7]
Indicated Altitude—That altitude read directly from
the altimeter (uncorrected) after it is set to the current
altimeter setting.
True Altitude—The true vertical distance of the glider
above sea level—the actual altitude. (Often expressed in this
manner: 10,900 feet MSL.) Airport, terrain, and obstacle
elevations found on aeronautical charts are true altitudes.
Absolute Altitude—The vertical distance above the
terrain.
Pressure Altitude—The altitude indicated when the
altimeter setting window (barometric scale) is adjusted
to 29.92. This is the standard datum plane, a theoretical
plane where air pressure (corrected to 15° C or 59° F)
is equal to 29.92 in. Hg. Pressure altitude is used for
computer solutions to determine density altitude, true
altitude, true airspeed, etc.
Density Altitude—This altitude is pressure altitude
corrected for nonstandard temperature variations.
When conditions are standard, pressure altitude and density
altitude are the same. Consequently, if the temperature
is above standard, the density altitude will be higher
than pressure altitude. If the temperature is below standard,
the density altitude will be lower than pressure altitude.
This is an important altitude because it is directly
related to the glider’s takeoff and climb performance.
VARIOMETER
The variometer gives the glider pilot information on
performance of the glider while flying through the
atmosphere. The variometer operates on the same principle
as the altimeter, however, it indicates rate of
climb or descent instead of vertical distance. The variometer
depends upon the pressure lapse rate in the
atmosphere to derive information about rate of climb
or rate of descent. Most non-electrical variometers use
a separate insulated tank, such as a Thermos or capacity
flask, as a reference chamber. The tubing is
　
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