Textron Cessna 172R - Page 236

SECTION
7
AIRPLANE
&
SYSTEMS
DESCRIPTION
CESSNA
MODEL
172R
Oil
pressure
signals
are
generated
from
an
oil
pressure
line/transducer
combination.
An
oil
pressure
line is routed
from
the
upper
front
of
the
engine
case
to
the
rear
engine
baffle.
At
the
baffle,
the
oil
pressure
line is connected
to
a
transducer.
This
I
transducer
produces
an
electrical
signal
which is
translated
into
a
pressure
reading
by
the oil
pressure
gage
located
on
the
LH
instrument
panel.
In
addition,
a
separate
low
oil
pressure
indication is
provided
through
the
panel
annunciator.
This
annunciator is
wired
to
a
pressure
switch
located
on
the
rear
of
the
engine
accessory
case.
When
oil
pressure
is below
20
PSI,
the
switch
grounds
and
completes
the annunciator
circuit,
illuminating
the red
OIL
PRESS
light.
When
pressure
exceeds
20
PSI,
the
ground
is removed
and
the
OIL
PRESS
annunciator
goes
out.
NOTE
The
low oil
pressure
switch
is also
connected
to
the
hour
(Hobbs)
meter.
When
pressure
exceeds
20
PSI,
a
ground
is
supplied
to
the hour
meter,
completing
the hour
meter
circuit.
Oil
temperature
signals
are
generated
from
a
resistance-type
probe
located
in
the
engine
accessory
case.
As
oil
temperature
changes,
the
probe
resistance
changes.
This resistance
is
translated into
oil
temperature
readings
on
the
cockpit
indicator.
The
engine
driven
mechanical tachometer
is located
on
the
instrument
panel
to
the
right
of
the
pilot's
control
wheel.
The
instrument
is
calibrated
in
increments
of
100
RPM
and
indicates
both
engine
and
propeller
speed.
An
hour
meter
in
the
lower
section
of
the dial
records
elapsed
engine
time
in
hours
and tenths.
Instrument
markings
include the
normal
operating
range
(green
arc)
from
1
900
to
2400
RPM.
The
exhaust
gas
temperature
(EGT)
indicator
is located
on
the
LH
instrument
panel
as
part
of
the
EGT/Fuel
Flow
indicator.
Since
exhaust
gas
temperature
varies
with
fuel-air
ration
(mixture),
density
altitude,
throttle
position
and
RPM,
the
instrument is
a
useful
aid
in
adjusting
the
mixture
for
best
economy
or
performance.
The
EGT
indicator allows
the
pilot
to
lean
(reduce
the
proportion
of fuel
in
the
fuel-air
mixture)
to
a
known value
using
the
maximum
or
"peak"
exhaust
gas
temperature
as
a
reference.
An
index
pointer
which
can
be
positioned
manually
is
provided
for
the
pilot
to
mark the
location
of
the
peak.
Never
lean
using
EGT
when
operating
at
more
than
80%
power.
7-20
Revision
7
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