Relative humidity
Outside air temperature
100%
50%
80%
60%
70%
90%
20°F/-7°C 32°F/0°C 70°F/21°C 100°F/38°C
High carburetor
icing potential
Carburetor icing possible
Figure 7-12. Although carburetor ice is most likely to form when
the temperature and humidity are in ranges indicated by this chart,
carburetor icing is possible under conditions not depicted.
presence. If detected, full carburetor heat should be applied
immediately, and it should be left in the ON position until
the pilot is certain that all the ice has been removed. If ice
is present, applying partial heat or leaving heat on for an
insufficient time might aggravate the situation. In extreme
cases of carburetor icing, even after the ice has been removed,
full carburetor heat should be used to prevent further ice
formation. If installed, a carburetor temperature gauge is
useful in determining when to use carburetor heat.
Whenever the throttle is closed during flight, the engine cools
rapidly and vaporization of the fuel is less complete than if
the engine is warm. Also, in this condition, the engine is more
susceptible to carburetor icing. If carburetor icing conditions
are suspected and closed-throttle operation anticipated, adjust
the carburetor heat to the full ON position before closing the
throttle and leave it on during the closed-throttle operation.
The heat aids in vaporizing the fuel and helps prevent the
formation of carburetor ice. Periodically, open the throttle
smoothly for a few seconds to keep the engine warm;
otherwise, the carburetor heater may not provide enough
heat to prevent icing.
The use of carburetor heat causes a decrease in engine
power, sometimes up to 15 percent, because the heated
air is less dense than the outside air that had been entering
the engine. This enriches the mixture. When ice is present
in an aircraft with a fixed-pitch propeller and carburetor
heat is being used, there is a decrease in rpm, followed by
a gradual increase in rpm as the ice melts. The engine also
should run more smoothly after the ice has been removed.
If ice is not present, the rpm decreases and then remains
constant. When carburetor heat is used on an aircraft with a
constant-speed propeller and ice is present, a decrease in the
manifold pressure is noticed, followed by a gradual increase.
If carburetor icing is not present, the gradual increase in
manifold pressure is not apparent until the carburetor heat
is turned off.
It is imperative for a pilot to recognize carburetor ice when it
forms during flight to prevent a loss in power, altitude, and/or
airspeed. These symptoms may sometimes be accompanied
by vibration or engine roughness. Once a power loss is
noticed, immediate action should be taken to eliminate ice
already formed in the carburetor and to prevent further ice
formation. This is accomplished by applying full carburetor
heat, which will further reduce power and may cause engine
roughness as melted ice goes through the engine. These
symptoms may last from 30 seconds to several minutes,
depending on the severity of the icing. During this period, the
pilot must resist the temptation to decrease the carburetor heat
usage. Carburetor heat must remain in the full-hot position
until normal power returns.
Celsius degrees) (Remember there are 180 Fahrenheit degrees
from freezing to boiling versus 100 degrees for the Celsius
scale.) Therefore, an outside air temperature of 100 F (38 C),
a temperature drop of an absolute 70 F degrees (38.89 Celsius
degrees) results in an air temperature in the carburetor of 30
F (-1 C). [Figure 7-12]
The first indication of carburetor icing in an aircraft with
a fixed-pitch propeller is a decrease in engine rpm, which
may be followed by engine roughness. In an aircraft with a
constant-speed propeller, carburetor icing is usually indicated
by a decrease in manifold pressure, but no reduction in rpm.
Propeller pitch is automatically adjusted to compensate for
loss of power. Thus, a constant rpm is maintained. Although
carburetor ice can occur during any phase of flight, it is
particularly dangerous when using reduced power during a
descent. Under certain conditions, carburetor ice could build
unnoticed until power is added. To combat the effects of
carburetor ice, engines with float-type carburetors employ a
carburetor heat system.
Carburetor Heat
Carburetor heat is an anti-icing system that preheats the air
before it reaches the carburetor and is intended to keep the
fuel-air mixture above freezing to prevent the formation of
carburetor ice. Carburetor heat can be used to melt ice that has
already formed in the carburetor if the accumulation is not too
great, but using carburetor heat as a preventative measure is
the better option. Additionally, carburetor heat may be used
as an alternate air source if the intake filter clogs, such as in
sudden or unexpected airframe icing conditions. The carburetor
heat should be checked during the engine runup. When using
carburetor heat, follow the manufacturer’s recommendations.
When conditions are conducive to carburetor icing during
flight, periodic checks should be made to detect its
Fuel tank
Auxiliary fuel pump
Engine-driven fuel pump
Fuel-air control unit
Fuel manifold valveFuel lines
Figure 7-13. Fuel injection system.
Since the use of carburetor heat tends to reduce the output
of the engine and to increase the operating temperature,
carburetor heat should not be used when full power is required
(as during takeoff) or during normal engine operation, except
to check for the presence of, or to remove, carburetor ice.
Carburetor Air Temperature Gauge
Some aircraft are equipped with a carburetor air temperature
gauge, which is useful in detecting potential icing conditions.
Usually, the face of the gauge is calibrated in degrees Celsius
with a yellow arc indicating the carburetor air temperatures
where icing may occur. This yellow arc typically ranges
between –15 °C and +5 °C (5 °F and 41 °F). If the air
temperature and moisture content of the air are such that
carburetor icing is improbable, the engine can be operated
with the indicator in the yellow range with no adverse effects.
If the atmospheric conditions are conducive to carburetor
icing, the indicator must be kept outside the yellow arc by
application of carburetor heat.
Certain carburetor air temperature gauges have a red radial
that indicates the maximum permissible carburetor inlet air
temperature recommended by the engine manufacturer. If
present, a green arc indicates the normal operating range.
Outside Air Temperature Gauge
Most aircraft are also equipped with an outside air
temperature (OAT) gauge calibrated in both degrees
Celsius and Fahrenheit. It provides the outside or ambient
air temperature for calculating true airspeed and is useful in
detecting potential icing conditions.
Fuel Injection Systems
In a fuel injection system, the fuel is injected directly into
the cylinders, or just ahead of the intake valve. The air
intake for the fuel injection system is similar to that used
in a carburetor system, with an alternate air source located
within the engine cowling. This source is used if the external
air source is obstructed. The alternate air source is usually
operated automatically, with a backup manual system that
can be used if the automatic feature malfunctions.
A fuel injection system usually incorporates six basic
components: an engine-driven fuel pump, a fuel-air control
unit, a fuel manifold (fuel distributor), discharge nozzles,
an auxiliary fuel pump, and fuel pressure/flow indicators.
[Figure 7-13]
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 fuel based on the mixture control setting 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.
A fuel injection system is considered to be less susceptible
to icing than a carburetor system, but impact icing on the air
intake is a possibility in either system. Impact icing occurs
when ice forms on the exterior of the aircraft and blocks
openings, such as the air intake for the injection system.
The following are advantages of using fuel injection:
• Reduction in evaporative icing
• Better fuel flow
• Faster throttle response
• Precise control of mixture
• Better fuel distribution
• Easier cold weather starts
The following are disadvantages of using fuel injection:
• 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
Superchargers and Turbosuperchargers
To increase an engine’s horsepower, manufacturers have
developed forced induction systems called supercharger
and turbosupercharger systems. They both compress the
intake air to increase its density. The key difference lies in
the power supply. A supercharger relies on an engine-driven
air pump or compressor, while a turbocharger gets its power
from the exhaust stream that runs through a turbine, which in
turn spins the compressor. Aircraft with these systems have
a manifold pressure gauge, which displays MAP within the
engine’s intake manifold.
On a standard day at sea level with the engine shut
down, the manifold pressure gauge indicates the ambient
absolute air pressure of 29.92 "Hg. Because atmospheric
pressure decreases approximately 1 "Hg per 1,000 feet of
altitude increase, the manifold pressure gauge indicates
approximately 24.92 "Hg at an airport that is 5,000 feet above
sea level with standard day conditions.
As a normally aspirated aircraft climbs, it eventually reaches
an altitude where the MAP is insufficient for a normal climb.
This altitude limit is known as the aircraft’s service ceiling,
and it is directly affected by the engine’s ability to produce
power. If the induction air entering the engine is pressurized,
or boosted, by either a supercharger or a turbosupercharger,
the aircraft’s service ceiling can be increased. With these
systems, an aircraft can fly at higher altitudes with the
advantage of higher true airspeeds and the increased ability
to circumnavigate adverse weather.
Superchargers
A supercharger is an engine-driven air pump or compressor
that provides compressed air to the engine to provide
additional pressure to the induction air so that the engine can
produce additional power. It increases manifold pressure and
forces the fuel-air mixture into the cylinders. Higher manifold
pressure increases the density of the fuel-air mixture and
increases the power an engine can produce. With a normally
aspirated engine, it is not possible to have manifold pressure
higher than the existing atmospheric pressure. A supercharger
is capable of boosting manifold pressure above 30 "Hg.
For example, at 8,000 feet, a typical engine may be able to
produce 75 percent of the power it could produce at mean
sea level (MSL) because the air is less dense at the higher
altitude. The supercharger compresses the air to a higher
density allowing a supercharged engine to produce the same
manifold pressure at higher altitudes as it could produce
at sea level. Thus, an engine at 8,000 feet MSL could still
produce 25 "Hg of manifold pressure whereas, without a
supercharger, it could only produce 22 "Hg. Superchargers
are especially valuable at high altitudes (such as 18,000 feet)
where the air density is 50 percent that of sea level. The use
of a supercharger in many cases will supply air to the engine
at the same density it did at sea level.
The components in a supercharged induction system are similar
to those in a normally aspirated system, with the addition of
a supercharger between the fuel metering device and intake
manifold. A supercharger is driven by the engine through a
gear train at one speed, two speeds, or variable speeds. In
addition, superchargers can have one or more stages. Each
stage also provides an increase in pressure and superchargers
may be classified as single stage, two stage, or multistage,
depending on the number of times compression occurs.
An early version of a single-stage, single-speed supercharger
may be referred to as a sea-level supercharger. An engine
equipped with this type of supercharger is called a sea-level
engine. With this type of supercharger, a single gear-driven
impeller is used to increase the power produced by an engine
at all altitudes. The drawback with this type of supercharger is
a decrease in engine power output with an increase in altitude.
Single-stage, single-speed superchargers are found on many
high-powered radial engines and use an air intake that faces
forward so the induction system can take full advantage of
Figure 7-14. Power output of normally aspirated engine compared
to a single-stage, two-speed supercharged engine.
Brake horsepower
Sea level Density altitude
Two-speed supercharged engine
High blowerLow blower
Normally aspirated engine
the ram air. Intake air passes through ducts to a carburetor,
where fuel is metered in proportion to the airflow. The
fuel-air charge is then ducted to the supercharger, or blower
impeller, which accelerates the fuel-air mixture outward.
Once accelerated, the fuel-air mixture passes through a
diffuser, where air velocity is traded for pressure energy.
After compression, the resulting high pressure fuel-air
mixture is directed to the cylinders.
Some of the large radial engines developed during World
War II have a single-stage, two-speed supercharger. With
this type of supercharger, a single impeller may be operated
at two speeds. The low impeller speed is often referred to
as the low blower setting, while the high impeller speed is
called the high blower setting. On engines equipped with a
two-speed supercharger, a lever or switch in the flight deck
activates an oil-operated clutch that switches from one speed
to the other.
Under normal operations, takeoff is made with the
supercharger in the low blower position. In this mode, the
engine performs as a ground-boosted engine, and the power
output decreases as the aircraft gains altitude. However, once
the aircraft reaches a specified altitude, a power reduction is
made, and the supercharger control is switched to the high
blower position. The throttle is then reset to the desired
manifold pressure. An engine equipped with this type of
supercharger is called an altitude engine. [Figure 7-14]
Turbosuperchargers
The most efficient method of increasing horsepower in an
engine is by using a turbosupercharger or turbocharger.
Installed on an engine, this booster uses the engine’s exhaust
gases to drive an air compressor to increase the pressure of
the air going into the engine through the carburetor or fuel
injection system to boost power at higher altitude.
The major disadvantage of the gear-driven supercharger––use
of a large amount of the engine’s power output for the amount
of power increase produced––is avoided with a turbocharger
because turbochargers are powered by an engine’s exhaust
gases. This means a turbocharger recovers energy from hot
exhaust gases that would otherwise be lost.
A second advantage of turbochargers over superchargers is
the ability to maintain control over an engine’s rated sea-
level horsepower from sea level up to the engine’s critical
altitude. Critical altitude is the maximum altitude at which
a turbocharged engine can produce its rated horsepower.
Above the critical altitude, power output begins to decrease
like it does for a normally aspirated engine.
Turbochargers increase the pressure of the engine’s induction
air, which allows the engine to develop sea level or greater
horsepower at higher altitudes. A turbocharger is comprised
of two main elements: a compressor and turbine. The
compressor section houses an impeller that turns at a high rate
of speed. As induction air is drawn across the impeller blades,
the impeller accelerates the air, allowing a large volume of
air to be drawn into the compressor housing. The impeller’s
action subsequently produces high-pressure, high-density
air that is delivered to the engine. To turn the impeller, the
engine’s exhaust gases are used to drive a turbine wheel
that is mounted on the opposite end of the impeller’s drive
shaft. By directing different amounts of exhaust gases to flow
over the turbine, more energy can be extracted, causing the
impeller to deliver more compressed air to the engine. The
waste gate, essentially an adjustable butterfly valve installed
in the exhaust system, is used to vary the mass of exhaust gas
flowing into the turbine. When closed, most of the exhaust
gases from the engine are forced to flow through the turbine.
When open, the exhaust gases are allowed to bypass the
turbine by flowing directly out through the engine’s exhaust
pipe. [Figure 7-15]
Since the temperature of a gas rises when it is compressed,
turbocharging causes the temperature of the induction air to
increase. To reduce this temperature and lower the risk of
detonation, many turbocharged engines use an intercooler.
This small heat exchanger uses outside air to cool the hot
compressed air before it enters the fuel metering device.
Exhaust gas discharge
Pressurized air from the
turbocharger is supplied
to the cylinders.
Intake manifold
Intake air is ducted to the
turbocharger where it is
compressed.
Air intakeThis controls the amount
of exhaust through the
turbine. Waste gate
position is actuated by
engine oil pressure.
Waste gas
The turbocharger in-
corporates a turbine,
which is driven by ex-
haust gases and a com-
pressor that pressurizes
the incoming air.
Turbocharger
This regulates airflow
to the engine.
Throttle body
Exhaust gas is ducted
through the exhaust man-
ifold and is used to turn
the turbine which drives
the compressor.
Exhaust manifold
Figure 7-15. Turbocharger components.
System Operation
On most modern turbocharged engines, the position of
the waste gate is governed by a pressure-sensing control
mechanism coupled to an actuator. Engine oil directed into
or away from this actuator moves the waste gate position.
On these systems, the actuator is automatically positioned to
produce the desired MAP simply by changing the position
of the throttle control.
Other turbocharging system designs use a separate manual
control to position the waste gate. With manual control,
the manifold pressure gauge must be closely monitored to
determine when the desired MAP has been achieved. Manual
systems are often found on aircraft that have been modified
with aftermarket turbocharging systems. These systems
require special operating considerations. For example, if the
waste gate is left closed after descending from a high altitude,
it is possible to produce a manifold pressure that exceeds the
engine’s limitations. This condition, called an overboost,
may produce severe detonation because of the leaning effect
resulting from increased air density during descent.
Although an automatic waste gate system is less likely to
experience an overboost condition, it can still occur. If takeoff
power is applied while the engine oil temperature is below its
normal operating range, the cold oil may not flow out of the
waste gate actuator quickly enough to prevent an overboost.
To help prevent overboosting, advance the throttle cautiously
to prevent exceeding the maximum manifold pressure limits.
A pilot flying an aircraft with a turbocharger should be aware
of system limitations. For example, a turbocharger turbine
and impeller can operate at rotational speeds in excess of
80,000 rpm while at extremely high temperatures. To achieve
high rotational speed, the bearings within the system must be
constantly supplied with engine oil to reduce the frictional
forces and high temperature. To obtain adequate lubrication,
the oil temperature should be in the normal operating range
before high throttle settings are applied. In addition, allow
the turbocharger to cool and the turbine to slow down before
shutting the engine down. Otherwise, the oil remaining in
the bearing housing will boil, causing hard carbon deposits
to form on the bearings and shaft. These deposits rapidly
deteriorate the turbocharger’s efficiency and service life. For
further limitations, refer to the AFM/POH.
High Altitude Performance
As an aircraft equipped with a turbocharging system climbs,
the waste gate is gradually closed to maintain the maximum
allowable manifold pressure. At some point, the waste gate
is fully closed and further increases in altitude cause the
manifold pressure to decrease. This is the critical altitude,
Upper magneto wires
Lower magneto wires
4
2
3
1
Upper spark plugs Lower spark plugs
Right magnetoLeft magneto
Figure 7-16. Ignition system components.
which is established by the aircraft or engine manufacturer.
When evaluating the performance of the turbocharging
system, be aware that if the manifold pressure begins
decreasing before the specified critical altitude, the engine
and turbocharging system should be inspected by a qualified
aviation maintenance technician (AMT) to verify that the
system is operating properly.
Ignition System
In a spark ignition engine, the ignition system provides a
spark that ignites the fuel-air mixture in the cylinders and is
made up of magnetos, spark plugs, high-tension leads, and
an ignition switch. [Figure 7-16]
A magneto uses a permanent magnet to generate an electrical
current completely independent of the aircraft’s electrical
system. The magneto generates sufficiently high voltage
to jump a spark across the spark plug gap in each cylinder.
The system begins to fire when the starter is engaged and the
crankshaft begins to turn. It continues to operate whenever
the crankshaft is rotating.
Most standard certificated aircraft incorporate a dual ignition
system with two individual magnetos, separate sets of wires,
and spark plugs to increase reliability of the ignition system.
Each magneto operates independently to fire one of the two
spark plugs in each cylinder. The firing of two spark plugs
improves combustion of the fuel-air mixture and results in a
slightly higher power output. If one of the magnetos fails, the
other is unaffected. The engine continues to operate normally,
although a slight decrease in engine power can be expected.
The same is true if one of the two spark plugs in a cylinder fails.
The operation of the magneto is controlled in the flight deck
by the ignition switch. The switch has five positions:
1. OFF
2. R (right)
3. L (left)
4. BOTH
5. START
With RIGHT or LEFT selected, only the associated magneto
is activated. The system operates on both magnetos when
BOTH is selected.
A malfunctioning ignition system can be identified during
the pretakeoff check by observing the decrease in rpm that
occurs when the ignition switch is first moved from BOTH
to RIGHT and then from BOTH to LEFT. A small decrease
in engine rpm is normal during this check. The permissible
decrease is listed in the AFM or POH. If the engine stops
running when switched to one magneto or if the rpm drop
exceeds the allowable limit, do not fly the aircraft until
the problem is corrected. The cause could be fouled plugs,
Figure 7-17. Wet-sump oil system.
Engine
and
Accessory
Bearings
200
245
I50
I00
75
°F P
S
I
II5
60
I00
20
0
OIL
P
R
E
S
S
T
E
M
P
Oil cooler and filter
Oil pressure relief valve
Oil filler cap and dipstick
High pressure oil screen
Oil pump
Low pressure oil screen
Oil sump
Pressure oil from
oil pump
Sump oil and return
oil from relief valve
broken or shorted wires between the magneto and the plugs,
or improperly timed firing of the plugs. It should be noted
that “no drop” in rpm is not normal, and in that instance, the
aircraft should not be flown.
Following engine shutdown, turn the ignition switch to the
OFF position. Even with the battery and master switches
OFF, the engine can fire and turn over if the ignition switch
is left ON and the propeller is moved because the magneto
requires no outside source of electrical power. Be aware of
the potential for serious injury in this situation.
Even with the ignition switch in the OFF position, if
the ground wire between the magneto and the ignition
switch becomes disconnected or broken, the engine could
accidentally start if the propeller is moved with residual fuel
in the cylinder. If this occurs, the only way to stop the engine
is to move the mixture lever to the idle cutoff position, then
have the system checked by a qualified AMT.
Oil Systems
The engine oil system performs several important functions:
• 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
Reciprocating engines use either a wet-sump or a dry-sump
oil system. In a wet-sump system, the oil is located in a sump
that is an integral part of the engine. In a dry-sump system,
the oil is contained in a separate tank and circulated through
the engine by pumps. [Figure 7-17]
The main component of a wet-sump system is the oil pump,
which draws oil from the sump and routes it to the engine. After
the oil passes through the engine, it returns to the sump. In
some engines, additional lubrication is supplied by the rotating
crankshaft, which splashes oil onto portions of the engine.
An oil pump also supplies oil pressure in a dry-sump
system, but the source of the oil is located external to the
engine in a separate oil tank. After oil is routed through
the engine, it is pumped from the various locations in the
engine back to the oil tank by scavenge pumps. Dry-sump
systems allow for a greater volume of oil to be supplied to
the engine, which makes them more suitable for very large
reciprocating engines.
The oil pressure gauge provides a direct indication of the oil
system operation. It ensures the pressure in pounds per square
inch (psi) of the oil supplied to the engine. Green indicates
the normal operating range, while red indicates the minimum
and maximum pressures. There should be an indication of
oil pressure during engine start. Refer to the AFM/POH for
manufacturer limitations.
Figure 7-19. Outside air aids in cooling the engine.
Fixed cowl opening
Baffle
Baffle
Cylinders
Air inlet
Figure 7-18. Always check the engine oil level during the preflight
inspection.
The oil temperature gauge measures the temperature of oil.
A green area shows the normal operating range, and the red
line indicates the maximum allowable temperature. Unlike
oil pressure, changes in oil temperature occur more slowly.
This is particularly noticeable after starting a cold engine,
when it may take several minutes or longer for the gauge to
show any increase in oil temperature.
Check oil temperature periodically during flight especially
when operating in high or low ambient air temperature.
High oil temperature indications may signal a plugged oil
line, a low oil quantity, a blocked oil cooler, or a defective
temperature gauge. Low oil temperature indications may
signal improper oil viscosity during cold weather operations.
The oil filler cap and dipstick (for measuring the oil quantity)
are usually accessible through a panel in the engine cowling. If
the quantity does not meet the manufacturer’s recommended
operating levels, oil should be added. The AFM/POH or
placards near the access panel provide information about
the correct oil type and weight, as well as the minimum and
maximum oil quantity. [Figure 7-18]
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.
Otherwise, the extremely high engine temperatures can lead
to loss of power, excessive oil consumption, detonation, and
serious engine damage.
While the oil system is vital to the internal cooling of the
engine, an additional method of cooling is necessary for the
engine’s external surface. Most small aircraft are air cooled,
although some are liquid cooled.
Air cooling is accomplished by air flowing into the engine
compartment through openings in front of the engine
cowling. Baffles route this air over fins attached to the engine
cylinders, and other parts of the engine, where the air absorbs
the engine heat. Expulsion of the hot air takes place through
one or more openings in the lower, aft portion of the engine
cowling. [Figure 7-19]
The outside air enters the engine compartment through an
inlet behind the propeller hub. Baffles direct it to the hottest
parts of the engine, primarily the cylinders, which have fins
that increase the area exposed to the airflow.
The air cooling system is less effective during ground
operations, takeoffs, go-arounds, and other periods of high-
power, low-airspeed operation. Conversely, high-speed
descents provide excess air and can shock cool the engine,
subjecting it to abrupt temperature fluctuations.
Operating the engine at higher than its designed temperature
can cause loss of power, excessive oil consumption, and
detonation. It will also lead to serious permanent damage,
such as scoring the cylinder walls, damaging the pistons and
rings, and burning and warping the valves. Monitoring the
flight deck engine temperature instruments aids in avoiding
high operating temperature.
Under normal operating conditions in aircraft not equipped
with cowl flaps, the engine temperature can be controlled
by changing the airspeed or the power output of the engine.
High engine temperatures can be decreased by increasing the
airspeed and/or reducing the power.
The oil temperature gauge gives an indirect and delayed
indication of rising engine temperature, but can be used
for determining engine temperature if this is the only
means available.
Most aircraft are equipped with a cylinder-head temperature
gauge that indicates a direct and immediate cylinder
temperature change. This instrument is calibrated in degrees
Celsius or Fahrenheit and is usually color coded with a green
arc to indicate the normal operating range. A red line on
the instrument indicates maximum allowable cylinder head
temperature.
To avoid excessive cylinder head temperatures, increase
airspeed, enrich the fuel-air mixture, and/or reduce
power. Any of these procedures help to reduce the engine
temperature. On aircraft equipped with cowl flaps, use the
cowl flap positions to control the temperature. Cowl flaps
are hinged covers that fit over the opening through which the
hot air is expelled. If the engine temperature is low, the cowl
flaps can be closed, thereby restricting the flow of expelled
hot air and increasing engine temperature. If the engine
temperature is high, the cowl flaps can be opened to permit
a greater flow of air through the system, thereby decreasing
the engine temperature.
Exhaust Systems
Engine exhaust systems vent the burned combustion gases
overboard, provide heat for the cabin, and defrost the
windscreen. An exhaust system has exhaust piping attached
to the cylinders, as well as a muffler and a muffler shroud.
The exhaust gases are pushed out of the cylinder through
the exhaust valve and then through the exhaust pipe system
to the atmosphere.
For cabin heat, outside air is drawn into the air inlet and is
ducted through a shroud around the muffler. The muffler is
heated by the exiting exhaust gases and, in turn, heats the
air around the muffler. This heated air is then ducted to the
cabin for heat and defrost applications. The heat and defrost
are controlled in the flight deck and can be adjusted to the
desired level.
Exhaust gases contain large amounts of carbon monoxide,
which is odorless and colorless. Carbon monoxide is deadly,
and its presence is virtually impossible to detect. To ensure
that exhaust gases are properly expelled, the exhaust system
must be in good condition and free of cracks.
Some exhaust systems have an EGT probe. This probe
transmits the EGT to an instrument in the flight deck. The
EGT gauge measures the temperature of the gases at the
exhaust manifold. This temperature varies with the ratio of
fuel to air entering the cylinders and can be used as a basis
for regulating the fuel-air mixture. The EGT gauge is highly
accurate in indicating the correct fuel-air mixture setting.
When using the EGT to aid in leaning the fuel-air mixture,
fuel consumption can be reduced. For specific procedures,
refer to the manufacturer’s recommendations for leaning the
fuel-air mixture.
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. Most aircraft have starters that automatically engage
and disengage when operated, but some older aircraft have
starters that are mechanically engaged by a lever actuated by
the pilot. The starter engages the aircraft flywheel, rotating
the engine at a speed that allows the engine to start and
maintain operation.
Electrical power for starting is usually supplied by an onboard
battery, but can also be supplied by external power through
an external power receptacle. When the battery switch is
turned on, electricity is supplied to the main power bus bar
through the battery solenoid. Both the starter and the starter
switch draw current from the main bus bar, 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. [Figure 7-20]
When starting an engine, the rules of safety and courtesy
should be strictly observed. One of the most important safety
rules is to ensure there is no one near the propeller prior to
starting the engine. In addition, the wheels should be chocked
and the brakes set to avoid hazards caused by unintentional
movement. To avoid damage to the propeller and property,
the aircraft should be in an area where the propeller will not
stir up gravel or dust.
Combustion
During normal combustion, the fuel-air mixture burns in a
very controlled and predictable manner. In a spark ignition
engine, the process occurs in a fraction of a second. The
mixture actually begins to burn at the point where it is ignited