Cessna 172R, Operating Handbook

Page: 236 / 500

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