Textron Cessna C172S PIM, Pilot Operating Handbook

Page: 253 / 322

CESSNA SECTION 7
MODEL 172S NAV III AIRPLANE AND SYSTEM DESCRIPTION
GFC 700 AFCS
U.S.
FUEL SYSTEM
(Continued)
FUEL DISTRIBUTION
Fuel flows by gravity from the two wing tanks to a three-position fuel
selector valve, labeled BOTH, RIGHT and LEFT, and on to the fuel
reservoir tank. From the fuel reservoir tank, fuel flows through the
electrically-driven auxiliary fuel pump, through the fuel shutoff valve,
the fuel strainer, and to the engine-driven fuel pump. From the engine-
driven fuel pump, fuel is delivered to the fuel/air control unit on the
bottom of the engine. The fuel/air control unit (fuel servo) meters fuel
flow in proportion to induction air flow. After passing through the control
unit, metered fuel goes to a fuel distribution valve (flow divider) located
on the top of the engine. From the fuel distribution valve, individual fuel
lines are routed to air bleed type injector nozzles located in the intake
chamber of each cylinder.
FUEL INDICATING SYSTEM
Fuel quantity is measured by two fuel quantity sensors, one in each fuel
tank, and is displayed on the EIS pages. The indicators are marked in
gallons of fuel (GAL). An empty tank is displayed on the fuel quantity
indicator (FUEL QTY GAL) as a red line on the far left of the indicator
scale, and the number 0. When an indicator shows an empty tank,
approximately 1.5 gallons of unusable fuel remain in the tank. The
indicators should not be relied upon for accurate readings during skids,
slips or unusual attitudes.
The fuel quantity indicator shows the fuel available in the tank up to the
limit of the sensor measurement range. At this level, additional fuel may
be added to completely fill the tank, but no additional movement of the
indicator will result. The limit for sensor measurement range is
approximately 24 gallons and is indicated by the maximum limit of the
green band. When the fuel level decreases below the maximum limit of
the fuel sensor, the fuel quantity indicator will display fuel quantity
measured in each tank. A visual check of each wing tank fuel level
must be performed prior to each flight. Compare the visual fuel level
and indicated fuel quantity to accurately estimate usable fuel.
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172SPHBUS-00 7-39