Fuel tank
PRIMER
Fuel pickup with screen
Carburetor fuel bowl
Optional primer plunger
Fuel filter
Optional primer bulb
used to pump fuel
into the carburetor
fuel bowls for starting
(an alternative electric
fuel pump can also
be used)
Fuel fill with
integral vent
Throttle Control
Engine driven
fuel pump
Figure 4-17. Typical Carburetor Fuel System.
corrected, can quickly lead to failure of the piston, cylinder,
or valves. In less severe cases, detonation causes engine
overheating, roughness, or loss of power.
Detonation is characterized by high cylinder head temperatures
and is most likely to occur when operating at high power
settings. Some common operational causes of detonation
include:
• Using a lower fuel grade than that specifi ed by the
aircraft manufacturer or operating the engine after
it has been sitting for an extended period; after 3
weeks or as indicated by the POH, drain old fuel and
replenish with fresh fuel.
• Operating the engine at high power settings with an
excessively lean mixture.
• Extended ground operations.
Detonation may be avoided by following these basic
guidelines during the various phases of ground and fl ight
operations:
• Make sure the proper grade of fuel is being used. Drain
and refuel if the fuel is old.
• Develop a habit of monitoring the engine instruments
to verify proper operation according to procedures
established by the manufacturer.
Preignition occurs when the fuel/air mixture ignites prior
to the engine’s normal ignition event. Premature burning
is usually caused by a residual hot spot in the combustion
chamber, often created by a small carbon deposit on a spark
plug, a cracked spark plug insulator, or other damage in the
cylinder that causes a part to heat suffi ciently to ignite the
fuel/air charge. Preignition causes the engine to lose power
and produces high operating temperature. As with detonation,
preignition may also cause severe engine damage because
the expanding gases exert excessive pressure on the piston
while still on its compression stroke.
Detonation and preignition often occur simultaneously and
one may cause the other. Since either condition causes high
engine temperature accompanied by a decrease in engine
performance, it is often diffi cult to distinguish between the
two. Using the recommended grade of fuel and operating the
engine within its proper temperature and RPM ranges reduce
the chance of detonation or preignition.
Fuel Systems
The fuel system is designed to provide an uninterrupted fl ow
of clean fuel from the fuel tank to the engine. See Chapter
3, Components and Systems, for more information on fuel
tanks. See earlier section in this chapter for specifi cs on fuel
injection systems. The fuel must be available to the engine
under all conditions of engine power, altitude, attitude, and
during all approved fl ight maneuvers. [Figure 4-17]
Fuel Pumps
WSC aircraft with carburetors have engine-driven fuel pump
systems. A diaphragm pump is the primary pump in the fuel
system for two-stroke engines. Air pulses in the crankcase
actuate a diaphragm and provide fuel under pressure to the
carburetor. Four-stroke engines have a mechanical pump
driven directly off the engine.
Sometimes an electric auxiliary pump is provided for use in
engine starting and in the event the engine pump fails. The
auxiliary pump, also known as a boost pump, provides added
reliability to the fuel system. The electric auxiliary pump is
controlled by a switch in the fl ight deck.
Carburetor Fuel Bowls
FUEL FLOWFUEL FLOWFUEL FLOW
Fuel Filter
Fuel Pump
Figure 4-18. Fuel system showing fuel filter to fuel pump to
carburetor float bowls.
Fuel Plunger Primer
The optional fuel plunger primer is used to draw fuel from
the tanks to supply it directly into the engine prior to starting.
This is particularly helpful during cold weather when engines
are hard to start because there is not enough heat available to
vaporize the fuel in the carburetor. For some aircraft, it is the
only way to deliver fuel to the engine when fi rst starting. After
the engine starts and is running, the fuel pump pushes fuel
to the carburetors and begins normal fuel delivery. To avoid
overpriming, read the priming instructions in the POH.
Choke
A choke or fuel enriching system is an alternate method to
provide additional fuel to the engine for initial cold starting.
Actuating the choke control allows more fuel to fl ow into
the carburetor.
Fuel Bulb Primer
The fuel bulb primer is manually actuated by squeezing the
bulb to draw fuel from the fuel tanks. This charges the fuel
lines and carburetor fl oat bowls before starting the engine
the fi rst time on a given day. After the engine starts, the fuel
pump is able to deliver the fuel to the fuel bowls. An electric
auxiliary fuel pump can also be used to charge the fuel lines
and carburetor fuel bowls before starting. This auxiliary fuel
pump is also used as a backup pump of the engine driven
fuel pump fails.
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
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
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 ground water. It does not
attract water, but it is expensive, and found only in some of
the better grade fuels.
Fuel Contamination
Clean fuel is imperative for the safe operation of a WSC
aircraft. Of the accidents attributed to powerplant failure from
fuel contamination, most have been traced to:
• Failure to remove contamination from the fuel system
during prefl ight.
• Servicing aircraft with improperly fi ltered fuel from
small tanks or drums.
• Storing aircraft with partially fi lled fuel tanks.
• Lack of proper maintenance.
Rust is common in metal fuel containers and is a common
fuel contaminant. Metal fuel tanks should be fi lled after each
fl ight, or at least after the last fl ight of the day to prevent
moisture condensation within the tank.
Another way to prevent fuel contamination is to avoid
refueling from cans and drums. Use a water fi ltering funnel
or a funnel with a chamois skin when refueling from cans or
drums. However, the use of a chamois will not always ensure
decontaminated fuel. Worn out chamois will not fi lter water;
neither will a new, clean chamois that is already water-wet or
damp. Most imitation chamois skins will not fi lter water.
Bad Gasoline
Letting fuel sit for weeks without using it will cause it to go
bad. Even if gas does not go bad, it will often lose octane
with time. For premixed gasoline and two-stroke oil, there
is another set of problems. Fuel and oil are normally mixed
at a 50:1 ratio. If premixed gas sits in a plastic container for
a while, the gas will evaporate leaving a richer oil mixture
in the container. In any case, fresh gas should be used when
possible.
Refueling Procedures
Never mix oil and fuel in an enclosed area. Not only are the
fumes irritating, but with the right fuel/air mixture can cause
an explosion. Do all oil and gas mixing outside. Refueling
from fuel cans should also be done outside. Never smoke
while refueling. Be careful when refueling an aircraft that has
just landed. There is danger of spilling fuel on a hot engine
component, particularly an exhaust system component.
Refueling should be done using only safety-approved fuel
containers marked with the type of fuel stored in them.
Confusing premixed fuel and fuel that has no oil in it can
be disastrous.
Figure 4-19. With these translucent containers, it can be noted that
the left hand container is just auto fuel and the right hand container
shows the auto fuel is premixed with oil for a two-stroke engine.
Metal Versus Plastic Fuel Containers
There are advantages to using both metal and plastic
containers. Metal cans will not allow the sun’s ultraviolet
rays in to harm the fuel. It also will not develop static charges
that a plastic container develops. However, a metal can is
more prone to sweating when going from cool to warm
temperatures on humid days. Metal cans and gas tanks are
best kept either empty or full of fuel to leave no room for
moist air.
Plastic fuel containers are easy to handle, inexpensive,
available at discount stores, and do not scratch the fi nish on
airframes. Plastic cans also do not sweat, and do not need to
be stored topped off. However, fuel does deteriorate a little
faster in plastic. Also, plastic containers can get charged
with static electricity while sliding around in the bed of a
pickup truck, especially if the truck has a plastic bed liner.
[Figure 4-19]
Many states now have laws prohibiting people from fi lling
plastic containers unless fi rst placed on the ground. Static
electricity can also be formed by the friction of air passing
over the surfaces of a WSC aircraft in fl ight and by the
fl ow of fuel through the hose and nozzle during refueling,
if fueling at a pump. Nylon, Dacron, and wool clothing are
especially prone to accumulate and discharge static electricity
from the person to the funnel or nozzle. To guard against the
possibility of static electricity igniting fuel fumes, a ground
wire should be attached to the aircraft before the fuel cap is
removed from the tank. The refueling nozzle should then
be grounded to the aircraft before refueling is begun and
should remain grounded throughout the refueling process.
The passage of fuel through a chamois increases the charge
of static electricity and the danger of sparks.
The aircraft must be properly grounded and the nozzle,
chamois fi lter, and funnel bonded to the aircraft. If a can is
used, it should be connected to either the grounding post or
the funnel. Cell phones should not be used while refueling
due to possible fi re risks.
Mixing Two-Stroke Oil and Fuel
Two-stroke engines require special two-stroke oil to be mixed
into the fuel before entering the engine to provide lubrication.
In some engines, an oil injection pump is used to deliver the
exact amount of oil into the intake of the engine depending
on the throttle setting. An advantage of an oil injection
system is that pilots do not need to premix any oil into the
fuel. However, an important prefl ight check is to ensure the
two-stroke oil reservoir is properly fi lled.
If a two-stroke engine does not have an oil injection system,
it is critical to mix the oil with the fuel before it is put into
the tank. Just pouring oil into the fuel tank does not allow
the oil to mix with the gas, and makes it diffi cult to measure
the proper amount of oil for mixing.
To mix two-stroke oil:
• Find a clean, approved container. Pour some gas into
it to help pre-dilute the two-stroke oil.
• Pour in a known amount of two-stroke oil into the
container. Oil should be approved for air-cooled
engines at 50:1 mixing ratio (check the engine
manufacturer for proper fuel to oil ratio for the WSC
aircraft). Use a measuring cup if necessary. Shake the
oil-gas mixture to dilute the oil with gasoline.
• Add gasoline until the 50:1 ratio is reached. If using a
water separating funnel, ensure the funnel is grounded
or at least in contact with the fuel container.
• Put the cap on the fuel can and shake the gasoline and
oil mixture thoroughly.
Starting System
Most small aircraft use a direct-cranking electric starter
system. This system consists of a source of electricity, wiring,
switches, and solenoids to operate the starter and a starter
motor. The starter engages the aircraft fl ywheel or gearbox,
rotating the engine at a speed that allows the engine to start
and maintain operation.
Electrical power for starting is usually supplied by an on-board
battery. When the battery switch is turned ON, electricity is
Figure 4-20. Cylinder head temperature probe (yellow wire) is
under spark plug.
Water Cooler
Oil Cooler
Figure 4-21. Cooling radiators—oil cooler is on top and water
cooler is on bottom.
supplied to the main power bus through the battery solenoid.
Both the starter and the starter switch draw current from the
main bus, but the starter will not operate until the starting
solenoid is energized by the starter switch being turned to the
“start” position. When the starter switch is released from the
“start” position, the solenoid removes power from the starter
motor. The starter motor is protected from being driven by
the engine through a clutch in the starter drive that allows
the engine to run faster than the starter motor.
Oil Systems
In a four-stroke engine, the engine oil system performs
several important functions, including:
• Lubricating the engine’s moving parts.
• Cooling the engine by reducing friction.
• Removing heat from the cylinders.
• Providing a seal between the cylinder walls and
pistons.
• Carrying away contaminants.
Four-stroke engines use either a wet sump or dry sump
oil system. Refer to the Pilot’s Handbook of Aeronautical
Knowledge for more information on four-stroke oil
systems.
Engine Cooling Systems
The burning fuel within the cylinders produces intense heat,
most of which is expelled through the exhaust system. Much
of the remaining heat, however, must be removed, or at least
dissipated, to prevent the engine from overheating.
While the oil system in a four-stroke engine and the fuel-oil
mix in a two-stroke engine is vital to the internal cooling of
the engine, an additional method of cooling is necessary for
the engine’s external surface. WSC engines operate with
either air-cooled or liquid-cooled systems.
Many WSC aircraft are equipped with a cylinder head
temperature (CHT) gauge. This instrument indicates a direct
and immediate cylinder temperature change. This instrument
is calibrated in degrees Celsius or Fahrenheit. Proper CHT
ranges can be found in the POH/AFM/AOI for that machine.
[Figure 4-20]
Air cooling is accomplished by air being pulled into the
engine shroud by a cooling fan. Baffl es route this air over
fi ns attached to the engine cylinders where the air absorbs the
engine heat. Expulsion of the hot air takes place through one
or more openings in the shroud. If cylinder head temperatures
rise too much in an air-cooled engine, it is because of
lubrication problems, cooling fan drive belt damage or wear,
or air blockage in the cooling fi ns by a bird or insect nest.
[Figure 4-1]
Liquid cooling systems pump coolant through jackets in
the cylinders and head. The heated liquid is then routed to
a radiator where the heat is radiated to the atmosphere. The
cooled liquid is then returned to the engine. If the radiator is
mounted close to the propeller, the propeller can constantly
move air across the radiator and keep the engine cool even
when the WSC is not moving. [Figure 4-21] Radiators
mounted away from the propeller make it more diffi cult for
the radiator to cool the engine unless the WSC is moving.
[Figure 4-22]
Breaking in an engine through ground runs on a hot day
is when radiator placement is most critical. Liquid-cooled
engines can overheat for a number of reasons, such as coolant
Water Cooler Radiator
Figure 4-22. Side mounted water cooler radiators integral with
cowl.
not at proper levels, a leak, failed water pump, or a blockage
of the radiator.
Operating an engine above its maximum design temperature
can cause a loss of power and detonation. It will also lead
to serious permanent damage, such as scoring the cylinder
walls and damaging the pistons and rings. Monitor the engine
temperature instruments to avoid high operating temperature.
Operating the engine lower than its designed temperature
range can cause piston seizure and scarring on the cylinder
walls. This happens most often in liquid-cooled WSC aircraft
in cold weather where large radiators designed for summer
fl ying may need to be partially blocked off.
Chapter Summary
Powerplants are generally classifi ed by:
1. Number of piston strokes needed to complete a
cycle—two strokes or four strokes.
2. Method of cooling—liquid or air.
Exhaust systems route the exhaust gases from the cylinders
out to the atmosphere. Two-stroke engines require tuned
exhaust systems matched to the specifi c engine for proper
operation.
Engines must be warmed up properly or engine damage
and seizure can result. Gearboxes reduce the engine rpm to
a usable propeller rpm. Induction systems mix gas and air
for cylinders and must be properly adjusted for different
altitudes.
Typical aircraft ignition systems are separate from the
electrical systems and typically have two separate ignition
systems. Aircraft ignition systems are composed of a
magneto/generator, control box, high voltage wires, spark
plugs, and ignition switches. Automotive engines typically
run the ignition system off the battery.
Proper combustion is a result of proper mixture and good
fuel. Good fuel management and proper engine cooling are
important considerations for reliable engine operation.