Cessna 172R Pilot Operating Handbook page 226

SECTION 8
CESSNA
HANDLING, SERVICE & MAINTENANCE
MODEL 172R
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.
I1
Therefore, to help alleviate 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.
The introduction of alcohol or DiEGME compound into the fuel
provides two distinct effects: (1) it absorbs the dissolved water from
the gasoline and (2) 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.
Alcohol, if used, is to be blended with the fuel in a concentration
of 1% by volume. Concentrations greater than 1% are
not
recommended since they can be detrimental to fuel tank materials.
The manner in which the alcohol is added to the fuel
is
significant because alcohol is most effective when it is completely
dissolved in the fuel. To ensure proper mixing, the following
is
recommended:
1.
For best results, the alcohol should be added during the
fueling operation by pouring the alcohol directly on the
fuel stream issuing from the fueling nozzle.
8-16
Feb 28/97