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Fuel Gauges
The fuel quantity gauge indicates the amount of fuel measured
by a sensing unit in each fuel tank and is displayed in gallons.
Do not depend solely on the accuracy of the fuel quantity
gauge. Always visually check the fuel level in the tank
during the prefl ight inspection, and then compare it with the
corresponding fuel quantity indication. It is also important
to track infl ight fuel consumption. Be sure to consult the
POH and know the approximate consumption rate to ensure
suffi cient fuel for fl ight. If an auxiliary electric fuel pump is
installed in the fuel system, a fuel pressure gauge is sometimes
included. This gauge indicates the pressure in the fuel lines.
The normal operating pressure can be found in the POH.
Fuel Filter
After leaving the fuel tank, the fuel passes through a fi lter
before it enters the fuel pump or carburetor. This fi lter
removes sediments that might be in the fuel. [Figure 4-18]
Fuel
Aviation gasoline (AVGAS) is identifi ed 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,
AVGAS 100 Low Lead (LL) may not be recommended by
two-stroke engine manufacturers and may not be preferred by
the four-stroke manufactures. Even though the “LL” stands for
low lead, 100LL contains more lead than the old leaded gas
dispensed at automotive fi lling 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 auto gas, maintaining its effi ciency 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 auto 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
4-15
reasonable compromise when the proper auto gas octane is
not available.
Manufacturers of two-stroke engines and four-stroke engines
used on WSC aircraft 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 than
fi ve percent methanol in fuel. Consult the POH for specifi cs
on an 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.
Consult the POH for specifi cs on an engine. Manufacturers
provide specifi c recommendations for the percentage of
alcohol in fuel. The posting on the 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 used stays within the
manufacturer’s recommendations. Use a general aviation
sump collector which includes graduation marks. Add water
to a specifi c mark. Then add fuel to fi ll 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 fi rst put
　
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