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Fuel
Aviation gasoline, or AVGAS, is identified by an octane
or performance number (grade), which designates
the antiknock value or knock resistance of the fuel
mixture in the engine cylinder. The higher the grade
of gasoline, the more pressure the fuel can withstand
without detonating. Lower grades of fuel are used in
lower-compression engines because these fuels ignite
at a lower temperature. Higher grades are used
in higher-compression engines, because they must
ignite at higher temperatures, but not prematurely. If
the proper grade of fuel is not available, use the next
higher grade as a substitute. Never use a lower grade.
This can cause the cylinder head temperature to exceed
its normal operating range, which may result in
detonation.
Unfortunately, aviation gasoline or AVGAS 100LL is
not recommended by at least one of the major twostroke
engine manufacturers. Even though the “LL”
stands for “Low Lead,” 100LL contains more lead
than the old premium leaded gas dispensed at automotive
filling stations. The lead in the fuel leaves deposits
in the piston ring grooves, freezing the rings in
position and reducing engine performance.
Spark plugs are also very susceptible to lead fouling.
This is especially true in two-stroke engines that
use cooler ignition temperatures than standard aircraft
engines.
AVGAS does have some advantages. It degrades
slower than regular gas, maintaining its efficiency for
a full 3 months. AVGAS 100LL has no seasonal or
regional variations and is manufactured according to
a standardized “recipe” worldwide.
If the airport has only 100LL available, it is permissible,
absent any limitations of the engine manufacturer,
to mix 100LL and 89 octane gasoline, for use
in two-stroke engines. A 50-50 ratio will boost the
octane rating and limit the amount of lead available
for fouling. Generally speaking, this is a reasonable
compromise when 89 octane is not available.
Two-stroke engine manufacturers, and four-storke
engines used on powered parachutes, typically recommend
the use of 89 octane minimum auto fuel for
their engines. Additives are put into auto gas primarily
to reduce harmful emissions rather than boost performance.
The additives are supposed to be listed at
the pump, but the accuracy of this posting should be
questioned.
Methanol alcohol has corrosive properties and can
damage engines. Engine manufacturers do not recommend
more that 3 percent methanol in fuel. Consult
the POH for specifics on your engine.
Ethanol alcohol is less corrosive than methanol. However,
it attracts water and is not as economical as gasoline.
Ethanol does not get very good fuel economy.
Avoid fuels with any more than 10 percent of ethanol
in it. Consult your POH for specifics on your engine.
Manufacturers provide specific recommendations for
the percentage of alcohol in fuel. The posting on the
4-12
pump may not be accurate and alcohol content can
vary greatly between fuel brands and stations. Additionally,
higher percentages of alcohol will be added
to auto gas in the future. A simple test can be conducted
to measure the fuel’s alcohol content to ensure
the fuel you use stays within the manufacturer’s recommendations.
Use a general aviation sump collector, which includes
graduation marks. Add water to a specific mark. Then
add fuel to fill the collector up to the line for gas. Cover
the top and shake it vigorously. After it settles, the
water and alcohol will combine and it will look like
there is now more water in the sump collector. The
difference between the initial amount of water you
first put into the collector and the new level of combined
water and alcohol equals the amount of alcohol
in the fuel. Compare this amount of alcohol and the
amount of fuel to determine the percentage of alcohol
content in the fuel.
Methyl Tertiary Butyl Ether (MTBE) does not have
the corrosive or water attractive properties of the
previously mentioned additives and is added to fuel
to improve air quality. It has been banned in several
states because it is carcinogenic and has been found
in groundwater. It does not attract water, but it is expensive,
so you will find it only in some of the better
grade fuels.
Fuel Contamination
Clean fuel is imperative for the safe operation of a
PPC. Of the accidents attributed to powerplant failure
from fuel contamination, most have been traced to:
• Failure to remove contamination from the fuel
system during preflight.
　
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