Fuel Return System; Fuel Venting; Reduced Tank Capacity - Cessna SkyHawk SP 172S Operating Handbook

SECTION 7
AIRPLANE AND SYSTEM DESCRIPTION
FUEL SYSTEM
(Continued)
FUEL RETURN SYSTEM
CESSNA
MODEL 172S NAV Ill
GFC 700AFCS
A fuel return system was incorporated to improve engine operation
during extended idle operation in hot weather environments. The major
components of the system include an orifice fitting located in the top of
the fuel-air control unit (fuel servo), fuel return line, with check valve,
and a fuel reservoir tank. The fuel return system is designed to return a
metered amount of fuel/vapor back to the fuel reservoir tank. The
increased fuel flow, due to the fuel return system, results in lower fuel
operating temperatures at the engine inlet, which minimizes the amount
of fuel vapor generated in the fuel lines during hot weather operations.
Refer to Section 4 for Hot Weather operating information.
FUEL VENTING
Fuel system venting is essential to system operation. Complete
blockage of the fuel venting system will result in decreasing fuel flow
and eventual engine stoppage. The fuel venting system consists of an
interconnecting vent line between the fuel tanks and a check valve
equipped overboard vent in the left fuel tank assembly. The overboard
vent protrudes from the bottom surface of the left wing, just inboard of
the wing strut upper attachment point. The fuel filler caps are vacuum
vented; the fuel filler cap vents will open and allow air to enter the fuel
tanks in case the overboard vents become blocked.
REDUCED TANK CAPACITY
The airplane may be serviced to a reduced capacity to permit heavier
cabin loadings. This is accomplished by filling each tank to the bottom
edge of the fuel filler indicator tab, thus giving a reduced fuel load of
17.5 gallons usable in each tank.
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U.S.
172SPHBUS-OO