Cessna 172P Pilot's Operating Handbook And Flight Manual page 156

SECTION
7
AIRPLANE & SYSTEMS
DESCR,IPTIONS
CESSNA
MODEL
172P
.
-
The
heated
pitot
system
(if installed)
consists
of
a
heating element
in
the
pitot
tube,
a
rocker switch
labeted
prror
HT,
a
6-amp
cirõuit breaker,
and
associated
wiring,
The switch
and
circuit
breaker are located
on the
left
side
of the switch
and control
pa,nel.
when the pitot heat switch is
turned
on, the
element in
the
pitot
tube is
heated
eleclrically
to
maintain
proper operation
in
possible
icing conditions. pitot
heat should
be used
only
as
required.
A static pressure alternate
source valve
may
be
installed
on the
switch
and
control panel
below the throttle,
and can
be used
if
the
externat static
source is malfunctioning, This valve supplies static
pressure
from inside
the cabin instead of
the
external static port.
If
erroneous
instru
pected
due
to
water or
ice
in
the
pressure
line
going
al static
pressure source.
the
alternate static
source
d on.
Pressures
within
the cabin
wilt
vary with
open heater/vents
and
windows. Refer
to section
5
for the effect of varyiñg cabin pressures
on
airspeed readings.
AIRSPEED INDICATOR
dicator
is
calibrated in
knots
and
miles per
hour.
e
markings (in KIAS) include the white arc
(BB
to
85
to
127
knots), yellow
arc (IZ7
to
159
knots).
and
a
red
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
manner
similar to the
oporation
of
a
flight
computer. To operate the
indicator,
first
rotate the
ring until
pressure altiüude
is
aligned with
outside
air
temperature
in
degrees
Fahrenheit. Pressure altitude should
not be
confused
with
indicated altitude.
To obtain pressure altitude,
momentarily set the barometric scale on the altimeter to
pg.g2
and read
pressure
altitude
on the
altimeter.
Be
sure
to
return
the
altimeter baromet-
ric
scale
to
the
obtained.
Havi
the
true airspe
best accuracy,
airspeed
by referring to the
Airspeed Calibration chart
in
Section
5.
Knowing the calibrated
airspeed, read
true
airspeed
on
the
ring
opposite
the
calibrated
airspeed.
7-32
30
May
1980