measure the altitude. In standard day conditions,
The range of the instrument is 80 to 650 knots
pressure altitude and true altitude are the same.
indicated airspeed and from 0.5 to 2.0 Mach
number. Its calibrated operating limit is 50,000
ABSOLUTE ALTITUDE --Absolute altitude
feet of altitude. A stationary airspeed dial masks
is the distance between the aircraft and the terrain
the upper range of the movable Mach dial at low
over which it is flying. It is referred to as the
altitudes. The stationary airspeed dial is graduated
altitude above ground level (AGL). AGL is
in knots. The instrument incorporates a landing
usually useless information because of variations
speed index and a Mach number setting index. You
in the terrain. However, it is useful when flying
can adjust both indexes by a knob on the lower
near the ground; for example, in a takeoff or
left-hand corner of the instrument. You can adjust
landing pattern. You find AGL by subtracting the
the landing speed index over a range of 80 to 150
elevation of the terrain beneath the aircraft from
knots. The index operates with the knob in its
the altitude read on the altimeter (MSL). A radar
normal position. You may adjust the Mach number
altimeter indicates actual altitude above the
index over the entire Mach range. The index
terrain; you call this indication radar altitude.
adjusts by depressing the knob and turning it.
PRESSURE ALTITUDE. --It is not possible
Altimeter
to have a ruler extending from an aircraft and
reaching to sea level to measure altitude. To
An altimeter is an instrument that measures
measure altitude, instruments sense air pressure
static pressure. Before you can understand how
and compare it to known values of standard air
the altimeter works, you need to understand
pressure at specific, measured altitudes. The
altitude. Remember, even though the altimeter
altitude you read from a properly calibrated
reads in feet, it is actually measuring pressure.
altimeter referenced to 29.92 inches of mercury
The word altitude is vague, so it needs further
(Hg) is the pressure altitude.
defining. The term altitude includes altitude above
Refer back to figure 6-2. If a pressure altimeter
mean sea level (MSL) and altitude above ground
senses 6.75 pounds per square inch pressure with the
level (AGL). It also, includes pressure altitude,
altimeter set to sea level and barometric pressure
indicated altitude, density altitude, and elevation.
29.92 inches of mercury, the altimeter indicates
20,000 feet. This does not mean that the aircraft is
MEAN SEA LEVEL. --Since about 80
exactly 20,000 feet above mean sea level. It means
percent of the earth's surface is water, it is natural
the aircraft is in an air mass exerting a pressure
to use sea level as an altitude reference point. The
equivalent to 20,000 feet on a standard day. You
pull of gravity isn't the same at sea level all over
can see that pressure altitude is not true altitude.
the world because the earth isn't perfectly round
INDICATED AND CALIBRATED ALTI-
and because of tides. To adjust for this, an
TUDE. --Unfortunately, standard atmospheric
average (or mean) value is set; this is the mean
conditions very seldom exist. Atmospheric
sea level. Mean sea level is the point where gravity
conditions and barometric pressure can vary
acting on the atmosphere produces a pressure of
considerably. A pressure change of one-hundredth
14.70 pounds per square inch. This pressure
(0.01) of an inch of mercury represents a 9-foot
supports a column of mercury in a barometer to
change in altitude at sea level. Barometric
a height of 29.92 inches. This is the reference point
pressure changes between 29.50 and 30.50 are not
from which you measure all other altitudes. See
uncommon (a pressure change of about 923 feet).
figures 6-1 and 6-2. The altitude you read from
Indicated altitude is the uncorrected reading of
an altimeter refers to MSL.
a barometric altimeter. Calibrated altitude is the
indicated altitude corrected for inherent and
ELEVATION AND TRUE ALTITUDE. --
installation errors of the altimeter instrument. On
Elevation is the height of a land mass above mean
an altimeter without such errors, indicated altitude
sea level. Elevation is measured with precision
and calibrated altitude are identical. Assume that
instruments that are far more accurate than the
this is the case for the rest of this discussion.
standard aircraft altimeter. You can find elevation
When flying below 18,000 feet, the aircraft
information on charts or, for a particular spot,
altimeter must be set to the altimeter setting
painted on a hangar near an aircraft ramp or taxi
(barometric pressure corrected to sea level) of a
area.
selected ground station within 100 miles of the
aircraft. Altitude read from an altimeter set to
True altitude is the actual number of feet
local barometric pressure is indicated altitude. The
above MSL. A ruler or yardstick is used to
