Cessna 172P - 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