SECTION 7
AIRPLANE & SYSTEMS DESCRIPTION
CESSNA
MODEL 172S
I
Oil
pressure
signals
are
generated
from
an
oil
J
pressur~
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, thegroundis'r~moved
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
.change.s,
the
probe
resistance
changes .. This
resistance
is
translated into oil temperaturereadingson
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 1900 to 2400 RPM.
'The exhaust gas temperature
(EGT) indicator is located on the
LHinstrument
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 75% power.
7-20
Revision 4
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