SECTION 7
CESSNA
AIRPLANE & SYSTEMS DESCRIPTIONS
MODEL 172RG
Figure 7-5. Fuel System
7-24
1 July 1979
CESSNA
SECTION 7
MODEL 172RG
AIRPLANE & SYSTEMS DESCRIPTIONS
Fuel flows by gravity from the two integral wing tanks to a four-
position selector valve, labeled BOTH, RIGHT, LEFT, and OFF. With the
selector valve in either the BOTH, RIGHT, or LEFT position, fuel flows
through a strainer to the engine-driven fuel pump, and from the pump to
the carburetor. When the auxiliary fuel pump is operating, it draws fuel
from a tee located between the strainer and the engine-driven fuel pump,
and delivers it to the carburetor. From the carburetor, mixed fuel and air
flows to the cylinders through intake manifold tubes. The manual primer
draws its fuel from the fuel strainer and injects it into the engine intake
ports.
The airplane may be serviced to a reduced fuel capacity to permit
heavier cabin loadings by filling each fuel tank to the bottom edge of the
fuel filler collar, thus giving a reduced fuel load of 24.0 gallons in each tank
(22 gallons usable in all flight conditions).
Fuel system venting is essential to system operation. Blockage of the
system will result in decreasing fuel flow and eventual engine stoppage.
Venting is accomplished by an interconnecting line from the right fuel
tank to the left tank. The left fuel tank is vented overboard through a vent
line, equipped with a check valve, which protrudes from the bottom
surface of the left wing near the wing strut. The right fuel tank filler cap is
also vented.
Fuel quantity is measured by two float-type fuel quantity transmitters
(one in each tank) and indicated by two electrically-operated fuel quantity
indicators on the left side of the instrument panel. The fuel quantity
indicators are calibrated in gallons (lower scale) and pounds (upper scale).
An empty tank is indicated by a red line and the letter E. When an indicator
shows an empty tank, approximately 2 gallons remain in a tank as
unusable fuel. The indicators cannot be relied upon for accurate readings
during skids, slips, or unusual attitudes. If both indicator pointers should
rapidly move to a zero reading, check the cylinder head temperature gage
for operation. If these gages are not indicating, an electrical malfunction
has occurred.
The auxiliary fuel pump switch is located on the left side of the switch
and control panel and is a rocker-type switch. It is labeled AUX FUEL
PUMP. When the pump is operating, it will maintain fuel pressure to the
carburetor. It should be used whenever the indicated fuel pressure falls
below 0.5 PSI, but is not required when gravity flow and/ or the engine-
driven fuel pump can maintain indicated pressures above 0.5 PSI.
The fuel selector valve should be in the BOTH position for takeoff,
climb, descent, landing, and maneuvers that involve prolonged slips or
skids. Operation from either LEFT or RIGHT tank is reserved for level
cruising flight only.
1 July 1979
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