The first indication of carburetor icing in a powered
parachute is a decrease in engine RPM, which may be
followed by engine roughness. Although carburetor
ice can occur during any phase of flight, it is particu -
larly dangerous when using reduced power during a
descent. Under certain conditions, carburetor ice could
build unnoticed until you try to add power. To combat
the effects of carburetor ice, some engines have a carb
heat option. Some of the newer four-stroke engines
have carburetor heat turned on all the time to combat
icing. Two-stroke engines are typically less suscep -
tible to icing but specific installations dictate how sus-
ceptible the carburetor is to icing. Consult the aircraft
POH for the probability of carb ice for the specific
installation you have and for carb ice procedures.
Fuel Injection Systems
In a fuel injection system, the fuel is injected either
directly into the cylinders, or just ahead of the intake
valve. A fuel injection system usually incorporates
these basic components: an engine-driven fuel pump,
a fuel-air control unit, fuel manifold (fuel distributor),
discharge nozzles, an auxiliary fuel pump, and fuel
pressure/flow indicators. [Figure 4-11]
The auxiliary fuel pump provides fuel under pressure
to the fuel-air control unit for engine starting and/or
emergency use. After starting, the engine-driven fuel
pump provides fuel under pressure from the fuel tank
to the fuel-air control unit. This control unit, which
essentially replaces the carburetor, meters the fuel and
sends it to the fuel manifold valve at a rate controlled
by the throttle. After reaching the fuel manifold valve,
the fuel is distributed to the individual fuel discharge
nozzles. The discharge nozzles, which are located in
each cylinder head, inject the fuel-air mixture directly
into each cylinder intake port.
Operating an aircraft at a lower altitude airport with
the jets set for higher altitudes will create too lean of
a mixture, heat up the engine, and cause the engine
to seize. The pilot must be aware of the jetting for
the machine to adjust the mixture. Consult your POH
for specific procedures for setting jets at different al -
titudes.
Four-Stroke Mixture Settings
Four-stroke engines typically have automatic mixture
control for higher altitudes or a mixture control that
can be operated by the pilot.
Carburetor Icing
One disadvantage of the carburetor system versus the
fuel injected system is its icing tendency. Carbure -
tor ice occurs due to the effect of fuel vaporization
and the decrease in air pressure in the venturi, which
causes a sharp temperature drop in the carburetor. If
water vapor in the air condenses when the carbure -
tor temperature is at or below freezing, ice may form
on internal surfaces of the carburetor, including the
throttle valve.
Ice generally forms in the vicinity of the venturi throat.
This restricts the flow of the fuel-air mixture and re -
duces power. If enough ice builds up, the engine may
cease to operate. Carburetor ice is most likely to oc -
cur when temperatures are below 70°F (21°C) and the
relative humidity is above 80 percent. However, due
to the sudden cooling that takes place in the carbure -
tor, icing can occur even with temperatures as high
as 100°F (38°C) and humidity as low as 50 percent.
This temperature drop can be as much as 60 to 70°F.
Therefore, at an outside air temperature of 100°F, a
temperature drop of 70°F results in an air temperature
in the carburetor of 30°F. [Figure 4-10]
Figure 4-10. Although carburetor ice is most likely to
form when the temperature and humidity are in ranges
indicated by this chart, carburetor ice is also possible
under conditions not depicted.
Figure 4-11. Fuel injection system.
Some of the advantages of fuel injection are:
• No carburetor icing.
• Better fuel flow.
• Faster throttle response.
• Precise control of mixture.
• Better fuel distribution.
• Easier cold weather starts.
Disadvantages include:
• Difficulty in starting a hot engine.
• Vapor locks during ground operations on hot
days.
• Problems associated with restarting an engine
that quits because of fuel starvation.
Ignition System
The ignition system provides the spark that ignites the
fuel-air mixture in the cylinders. Components include
a magneto generator, an electronic control box that
replaces mechanical points, spark plugs, high-voltage
leads and the ignition switch(es). Individual manu -
facturer designs will vary and pilots must be familiar
with the aircraft operating procedures for the PPC be-
ing flown.
A magneto uses a permanent magnet to generate an
electrical current independent of the aircraft’s elec -
trical system which might include a battery. The air -
craft electrical system can fail—the battery can go
dead—however, this has no effect on the ignition sys-
tem which uses a separate generator in the magneto.
The electricity from the separate ignition coil on the
magneto generator goes into the ignition control box
where the correct voltage is produced and timed to
fire the spark plugs at the proper time. The magneto
also sends a signal to the electric control box to pro -
vide the timing signal to fire the spark plugs.
Most modern PPCs use an electronic timing system
instead of the mechanical points inside the old magne-
tos which also housed the points. Capacitor discharge
ignition (CDI) systems are a common example of an
electronic ignition system. Electronic ignition sys -
tems operate without any moving parts to increase
reliability and efficiency. A CDI system begins to fire
when the starter is engaged and the crankshaft begins
to turn. It continues to operate whenever the crank -
shaft is rotating.
Most powered parachutes incorporate a dual ignition
system with two individual coil systems in the mag -
neto, two individual electronic ignition timing sys -
tems (electric box), two separate sets of wires, and
two spark plugs per cylinder. Dual ignition systems
increase overall reliability of the engine. Each igni -
tion system operates independently to fire one of the
two spark plugs. If one ignition system fails, the other
is unaffected. The engine will continue to operate nor-
mally, although you can expect a slight decrease in
engine power.
The operation of the ignition system is controlled in
the cockpit by the ignition switch(es). Since there are
two individual ignition systems, there are normally
two separate ignition toggle switches.
You can identify a malfunctioning ignition system
during the pretakeoff check by observing the decrease
in RPM that occurs when you first turn off one igni -
tion switch, turn it back on, and then turn off the other.
A noticeable decrease in engine RPM is normal dur -
ing this check. If the engine stops running when you
switch to one ignition system or if the RPM drop ex -
ceeds the allowable limit, do not fly the powered para-
chute until the problem is corrected. The cause could
be fouled plugs, broken or shorted wires between the
magneto and the plugs, or improperly timed firing of
the plugs because of the control box.
It should be noted that “no drop” in RPM is not nor -
mal, and in that instance, the powered parachute
should not be flown. Following engine shutdown,
keep the ignition switches in the OFF position. Even
with the battery and master switches OFF, the engine
can fire and turn over if you leave an ignition switch
ON and the propeller is moved because the ignition
system requires no outside source of electrical power.
The potential for serious injury in this situation is ob-
vious.
Combustion
During normal combustion, the fuel-air mixture burns
in a very controlled and predictable manner. Although
the process occurs in a fraction of a second, the mix -
ture actually begins to burn at the point where it is
ignited by the spark plugs, then burns away from the
plugs until it is consumed completely. This type of
combustion causes a smooth buildup of temperature
and pressure and ensures that the expanding gases de-
liver the maximum force to the piston at exactly the
right time in the power stroke.
Detonation is an uncontrolled, explosive ignition of
the fuel-air mixture within the cylinder’s combustion
chamber. It causes excessive temperatures and pres -
sures which, if not corrected, can quickly lead to fail-
ure of the piston, cylinder, or valves. In less severe
cases, detonation causes engine overheating, rough -
ness, or loss of power.
Detonation is characterized by high cylinder head
temperatures, and is most likely to occur when operat-
ing at high power settings. Some common operational
causes of detonation include:
• Using a lower fuel grade than that specified
by the aircraft manufacturer or operating the
engine after it has been sitting for an extended
period; after 3 weeks or as indicated by
your POH you should drain old fuel out and
replenish with fresh fuel.
• Operating the engine at high power settings
with an excessively lean mixture.
• Detonation also can be caused by extended
ground operations.
Detonation may be avoided by following these basic
guidelines during the various phases of ground and
flight 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 insula -
tor, or other damage in the cylinder that causes a part
to heat sufficiently to ignite the fuel-air charge. Preig-
nition causes the engine to lose power, and produces
high operating temperature. As with detonation, pre -
ignition may also cause severe engine damage, be -
cause the expanding gases exert excessive pressure on
the piston while still on its compression stroke.
Detonation and preignition often occur simultaneous-
ly and one may cause the other. Since either condition
causes high engine temperature accompanied by a
decrease in engine performance, it is often difficult to
distinguish between the two. Using the recommended
grade of fuel and operating the engine within its prop-
er temperature and RPM ranges reduce the chance of
detonation or preignition.
Fuel Systems
The fuel system is designed to provide an uninterrupt-
ed flow of clean fuel from the fuel tank to the engine.
See Chapter 3 for more information on fuel tanks. The
fuel must be available to the engine under all condi -
tions of engine power, altitude, attitude, and during all
approved flight maneuvers. [Figure 4-12]
Fuel Pumps
Powered parachutes have fuel pump systems. The
main pump system is engine-driven and sometimes an
electrically-driven 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 electrically-driven auxiliary pump is controlled
by a switch in the cockpit.
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.
Fuel Plunger Primer
The fuel plunger primer is used to draw fuel from the
tanks to supply it directly into the cylinders prior to
starting the engine. 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 powered parachutes, it is
the only way to deliver fuel to the engine when first
starting. After the engine starts and is running, the
fuel pump pushes fuel to the carburetors and begins
Figure 4-12. Fuel pump system.
normal fuel delivery. To avoid overpriming, read the
priming instructions in your POH for your powered
parachute.
Choke
A choke or fuel enrichening system is an alternate
method to provide additional fuel to the engine for
initial cold starting. Actuating the choke control al -
lows more fuel to flow into the carburetor.
Fuel Bulb Primer
The fuel bulb primer is manually actuated by squeez-
ing the bulb to draw fuel from the tanks. This charges
the fuel lines and carburetor float bowls before start -
ing the engine the first time on a given day. After the
engine starts, the fuel pump is able to deliver the fuel
to the fuel bowls.
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 ac-
curacy of the fuel quantity gauge. Always visually
check the fuel level in the tank during the preflight in-
spection, and then compare it with the corresponding
fuel quantity indication. It is also important to track
your inflight fuel consumption. Be sure to consult the
POH for your powered parachute and know the ap -
proximate consumption rate to ensure sufficient fuel
for your flight.
If an auxiliary electric fuel pump is installed in the
fuel system, a fuel pressure gauge is sometimes in -
cluded. 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
filter before it enters the fuel pump or carburetor. This
filter removes sediments that might be in the fuel.
Fuel
Aviation gasoline, or A VGAS, is identified by an oc-
tane 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 ig -
nite 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 ex -
ceed its normal operating range, which may result in
detonation.
Unfortunately, aviation gasoline or A VGAS 100LL is
not recommended by at least one of the major two-
stroke engine manufacturers. Even though the “LL”
stands for “Low Lead,” 100LL contains more lead
than the old premium leaded gas dispensed at auto -
motive filling stations. The lead in the fuel leaves de-
posits in the piston ring grooves, freezing the rings in
position and reducing engine performance.
Spark plugs are also very susceptible to lead foul -
ing. This is especially true in two-stroke engines that
use cooler ignition temperatures than standard aircraft
engines.
A VGAS does have some advantages. It degrades
slower than regular gas, maintaining its efficiency for
a full 3 months. A VGAS 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 permis -
sible, absent any limitations of the engine manufac -
turer, 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 rec -
ommend the use of 89 octane minimum auto fuel for
their engines. Additives are put into auto gas primar-
ily to reduce harmful emissions rather than boost per-
formance. 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 recom-
mend more that 3 percent methanol in fuel. Consult
the POH for specifics on your engine.
Ethanol alcohol is less corrosive than methanol. How-
ever, it attracts water and is not as economical as gas-
oline.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
pump may not be accurate and alcohol content can
vary greatly between fuel brands and stations. Addi -
tionally, higher percentages of alcohol will be added
to auto gas in the future. A simple test can be con -
ducted to measure the fuel’s alcohol content to ensure
the fuel you use stays within the manufacturer’s rec -
ommendations.
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. Cov-
er 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 com -
bined 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 ex -
pensive, 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.
• Servicing aircraft with improperly filtered fuel
from small tanks or drums.
• Storing aircraft with partially filled fuel tanks.
• Lack of proper maintenance.
Rust is common in metal fuel containers and is a com-
mon fuel contaminant. Metal fuel tanks should be
filled after each flight, or at least after the last flight 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
filtering 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 filter water; neither will a
new, clean chamois that is already water-wet or damp.
Most imitation chamois skins will not filter 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 its octane with time. For those that premix gaso-
line and two-stroke oil, there is another set of prob -
lems. 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 out leaving a richer oil mixture
in the container. In any case, fresh gas should be used
as much as 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 you 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 refueling an aircraft that has just landed.
There is the danger of spilling fuel on a hot engine com-
ponent, particularly an exhaust system component.
Refueling should be done using only safety-approved
fuel containers. The fuel containers should be marked
with the type of fuel stored in them. Confusing pre -
mixed fuel and fuel that has no oil in it can be disas -
trous.
There are advantages to both metal and plastic con -
tainers. Metal cans won’t allow the sun’s ultraviolet
rays in to harm the fuel. It also won’t develop static
charges like a plastic container may. However, a metal
can will be more prone to sweating when going from
cool to warm temperatures on humid days. Metal cans
and metal 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, inexpen -
sive, available at discount stores, and do not scratch
the finish on airframes. Plastic cans also do not sweat,
so they don’t need to be stored topped off. However,
fuel does deteriorate a little faster in plastic. Also,
plastic containers can get charged with static electric-
ity while sliding around in the bed of a pickup truck,
especially if the truck has a plastic bed liner. Many
states now have laws prohibiting people from filling
plastic containers unless first placed on the ground.
Static electricity can also be formed by the friction of
air passing over the surfaces of a powered parachute
in flight and by the flow 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 be -
fore 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 noz -
zle, chamois filter, 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 as they could pose a fire risk.
Mixing Two-Stroke Oil and Fuel
Two-stroke engines require special two-stroke oil to
be mixed into the fuel before reaching the cylinder of
the engine. 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 ad-
vantage of an oil injection system is pilots don’t have
to premix any oil into the fuel. However, an important
preflight check is to make sure the two-stroke oil res-
ervoir is properly filled.
If a two-stroke engine doesn’t have an oil injection
system, it is critical to mix oil into fuel before it is
put into the tank. Just pouring oil into the fuel tank
doesn’t give it the proper chance to mix with the gas
and makes it difficult to measure the proper amount of
oil for mixing. To mix two-stroke oil you should:
• Find a clean, approved container. Pour a little
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 your PPC). Use a measuring cup if
necessary. Shake the oil-gas mixture around a
little to dilute the oil with gasoline.
• Add gasoline until the 50:1 ratio is reached. If
you choose to use a water separating funnel,
make sure 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 flywheel or the 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 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 per -
forms several important functions, including:
• Lubrication of the engine’s moving parts.
• Cooling of 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 Chapter 5 of the Pilot’s
Handbook of Aeronautical Knowledge for more in -
formation on four-stroke oil systems.
Engine Cooling Systems
The burning fuel within the cylinders produces in -
tense heat, most of which is expelled through the ex -
haust 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 in -
ternal cooling of the engine, an additional method of
cooling is necessary for the engine’s external surface.
Powered parachute engines operate with either air-
cooled or liquid-cooled systems.
Many powered parachutes are equipped with a cylin-
der head temperature (CHT) gauge. This instrument
indicates a direct and immediate cylinder temperature
change. This instrument is calibrated in degrees Cel -
sius or Fahrenheit. Proper CHT ranges can be found
in the pilot’s operating handbook for that machine.
Air cooling is accomplished by air being pulled into
the engine shroud by a cooling fan. Baffles route this
air over fins 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 prob-
lems: cooling fan drive belt damage or wear, or air
blockage in the cooling fins by a bird or insect nest.
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 low and close to the
propeller, the propeller can constantly move air across
the radiator and keep the engine cool even when the
powered parachute is not moving. Radiators mounted
high and away from the propeller raise the center of
gravity and make it more difficult for the radiator to
cool the engine unless the powered parachute is mov-
ing. 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 not at proper levels, a leak, a
failed water pump, or a blockage of the radiator. Op -
erating an engine above its maximum design tempera-
ture can cause a loss of power and detonation. It will
also lead to serious permanent damage, such as scor -
ing 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 tempera-
ture range can cause piston seizure and scarring on
the cylinder walls. This happens most often in liquid-
cooled powered parachutes in cold weather where
large radiators designed for summer flying may need
to be partially blocked off.