Fuel Additives - Cessna 172S NAV III Pilot Operating Handbook

SECTION 8 CESSNA
HANDLING, SERVICE AND MAINTENANCE MODEL 172S NAV III
FUEL
(Continued)
FUEL ADDITIVES
Strict adherence to recommended preflight draining instructions as
called for in Section
4
will eliminate any free water accumulations
from the tank sumps. While small amounts of water may still remain
in solution in the gaSOline, it will normally be consumed and go
unnoticed in the operation of the engine.
One exception to this can be encountered when operating under the
combined effect of: (1) use of certain fuels, with (2) high humidity
conditions on the ground (3) followed by flight at high altitude and
low temperature. Under these unusual conditions, small amounts of
water in solution can precipitate from the fuel stream and freeze in
sufficient quantities to induce partial icing of the engine fuel system.
While these conditions are quite rare and will not normally pose a
problem to owners and operators, they do exist in certain areas of
the world and consequently must be dealt with, when encountered.
To help decrease the possibility of fuel icing occurring under these
unusual conditions, it is permissible to add isopropyl alcohol or
diethylene glycol monomethyl ether (DiEGME) compound to the fuel
supply.
Alcohol or DiEGME compound added to the fuel absorbs the
dissolved water from the gasoline. Alcohol has a freezing
temperature depressant effect.
NOTE
When using fuel additives, it must be remembered
that the final goal is to obtain a correct fuel-to-
additive ratio in the tank, and not just with fuel
coming out of the refueling nozzle. For example,
adding 15 gallons of correctly proportioned fuel to a
tank which contains 20 gallons of untreated fuel will
result in a lower-than-acceptable concentration level
to the 35 gallons of fuel which now reside in the
tank.
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8-18 U.S.
172SPHAUS-00