Revision 4 - Cessna 172SPHUSR05 Pilot Operating Handbook

SECTION 7
&
AI RPLAN E SYSTEMS D ESC R I PTION
I
Oil pressu re signals are generated fro m an oil pressu re
l i ne/transdu cer combination . An oil pressu re line is routed fro m the
upper front of the engine case to the rear engine baffle. At the
baffle, the oil pressu re line is connected to a transducer. This
I
t ransdu cer produces an electrical signal wh ich is translated i nto a
pressu re read i ng by the oil pressure gage located on the LH
,
i nstru ment panel .
I n addition, a separate low oil pressu re i ndication is provided
through the panel annu nciator. This ann u nciator is wired to a
pressu re switch located on the rear of the engine accessory case.
When oil pressure is below 20 PS I , the switch grou nds and
co mpletes the annunciator circuit, i l l u m i nating the red O I L P RESS
lig ht. When pressu re exceeds 20 PSI , the gro u nd is removed and
the OIL P RESS annunciator goes out.
The low oil pressure switch is also con nected to the
hour (Hobbs) meter.
PSI, a gro u nd is supplied to the hour meter,
completing the hour meter circuit.
Oil temperature sig nals are generated fro m a resistance-type
probe located in the engine accessory case. As oil temperatu re
changes , the
translated into oil temperatu re read i ngs on the cockpit i ndicator.
The engi ne driven mechanical tachometer is located on the
i nstru ment panel to the right of the pilot's control wheel . The
i nstru ment is cal ibrated i n i ncrements of 1 00 RPM and i ndicates
both eng i ne and propeller speed . An hour meter in the lower section
of the d ial records elapsed engine ti me i n hours and tenths.
I nstru ment markings i ncl ude the normal operating range (g reen arc)
fro m 1 900 to 2400 RPM.
The exhaust gas temperatu re (EGT) i ndicator is located on the
LH i nstru ment panel as part of the EGT/Fuel Flow indicator. Si nce
exhaust gas temperatu re varies with fuel-air ration ( m ixtu re) , density
altitude, th rottle position and RPM, the instru ment is a useful aid i n
adj usti ng t h e mixture for best economy or performance. T h e EGT
i ndicator allows the pi lot to lean (reduce the proportion of fuel in the
fuel-air m ixtu re) to a known value using the maxim u m or "peak"
exhaust gas temperatu re as a reference .
can be positioned manual ly is provided for the pilot to mark the
location of the peak.
more than 75% power.
7-20
NOTE
When pressure exceeds 20
probe resistance
Never lean usi ng EGT when operati ng at
changes. This
An i ndex poi nter which
C ESSNA
MODEL 1 72S
resistance is

Revision 4