Cessna 172N - Page 137

CESSNA
MODEL
172N
SECTION
7
AIRPLANE &
SYSTEMS DESCRIPTIONS
the
alternate
static
source
valve shorrld
be
prlled
on.
Pressures
within the
cabin
will
vary
with
open
cabin
ventilators
and
windows.
Refer to Sections 3
and
5
for the effect
of
'uarying
cabin
pres-
sures
on airspeed and altimeter readings.
AIRSPEED INDICATOR
The airspeed
indicator
is calibrated in
knots
and miles
per
horr.
Limitation
and range
markings include the white arc
(41
to
85
knots),
green
arc
(47
to
128 knots),
yellow
arc
(128
to 160 knots),
and a red
line
(160
knots).
If
a
true
airspeed
indicator
is installed, it
is equipped with
a
rotatable
ring
which
works in conjunction with
the airspeed
indicator dial
in
a man-
ner similar to
the
operation of
a
flight computer. To operate the
indicator,
first
rotate
the
ring
until
pressure
altitude
is aligned with
outside air tem-
perature
in degrees Fahrenheit. Pressure
altitude
should not
be cordused
with
indicated altitude. To
obtain
pressure
altitude, momentarily
set the
barometric
scale on
the
altimeter
to
29.92
and
read
pressure
altitude on
the
altimeter.
Be
sure
to return the altimeter
barometric scale
to the orig-
inal
barometric
setting
after
pressure
altitude
has
been obtained. Having
set
the ring to correct for altitude
and
temperature,
then
read
the airspeed
shown
on the rotatable ring by the
indicator
pointer.
For
best
accuracy,
this indication
should
be corrected
to
calibrated airspeed by referring to
the
Airspeed
Calibration
chart in Section 5. Knowing
the
calibrated air-
speed,
read true airspeed
on
the ring
opposite the calibrated
airspeed.
R,AIE-O F-CII'YT
B
I
N D
ICATO R
The rate-of-climb
indicator depicts airplane rate
of climb or
descent
in feet
per
minute.
The
pointer
is
actuated by atmospheric
pressure
:::1"-":.
resulting
from changes of altitude as
supplied by the
static
ALII'YIEIER
Airplane
altitude
is depicted by
a
barometric
type
altimeter. A knob
near the
lower
left
portion
of
the indicator
provides
adjustment
of
the in-
strument's barometric scale to the
current
altimeter
setting.
VACUUM SYSTEM AND
INSTRUMENTS
An
engine-driven vacuum system
(see
figure 7-9)
provides
the
suction
necessary
to operate
the attitude
indicator and
directional indicator. The
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