Aircraft Systems
Chapter 7
Introduction
This chapter covers the primary systems found on most
aircraft. These include the engine, propeller, induction,
ignition, as well as the fuel, lubrication, cooling, electrical,
landing gear, and environmental control systems.
Powerplant
An aircraft engine, or powerplant, produces thrust to propel
an aircraft. Reciprocating engines and turboprop engines
work in combination with a propeller to produce thrust.
Turbojet and turbofan engines produce thrust by increasing
the velocity of air flowing through the engine. All of these
powerplants also drive the various systems that support the
operation of an aircraft.
Figure 7-1. Radial engine.
Opposed cylinders
Figure 7-2. Horizontally opposed engine.
Reciprocating Engines
Most small aircraft are designed with reciprocating
engines. The name is derived from the back-and-forth, or
reciprocating, movement of the pistons that produces the
mechanical energy necessary to accomplish work.
Driven by a revitalization of the general aviation (GA)
industry and advances in both material and engine design,
reciprocating engine technology has improved dramatically
over the past two decades. The integration of computerized
engine management systems has improved fuel efficiency,
decreased emissions, and reduced pilot workload.
Reciprocating engines operate on the basic principle of
converting chemical energy (fuel) into mechanical energy.
This conversion occurs within the cylinders of the engine
through the process of combustion. The two primary
reciprocating engine designs are the spark ignition and the
compression ignition. The spark ignition reciprocating engine
has served as the powerplant of choice for many years. In
an effort to reduce operating costs, simplify design, and
improve reliability, several engine manufacturers are turning
to compression ignition as a viable alternative. Often referred
to as jet fuel piston engines, compression ignition engines
have the added advantage of utilizing readily available and
lower cost diesel or jet fuel.
The main mechanical components of the spark ignition and
the compression ignition engine are essentially the same.
Both use cylindrical combustion chambers and pistons that
travel the length of the cylinders to convert linear motion
into the rotary motion of the crankshaft. The main difference
between spark ignition and compression ignition is the
process of igniting the fuel. Spark ignition engines use a
spark plug to ignite a pre-mixed fuel-air mixture. (Fuel-air
mixture is the ratio of the “weight” of fuel to the “weight”
of air in the mixture to be burned.) A compression ignition
engine first compresses the air in the cylinder, raising its
temperature to a degree necessary for automatic ignition
when fuel is injected into the cylinder.
These two engine designs can be further classified as:
1. Cylinder arrangement with respect to the crankshaft—
radial, in-line, v-type, or opposed
2. Operating cycle—two or four
3. Method of cooling—liquid or air
Radial engines were widely used during World War II and
many are still in service today. With these engines, a row or
rows of cylinders are arranged in a circular pattern around
the crankcase. The main advantage of a radial engine is the
favorable power-to-weight ratio. [Figure 7-1]
In-line engines have a comparatively small frontal area, but
their power-to-weight ratios are relatively low. In addition,
the rearmost cylinders of an air-cooled, in-line engine
receive very little cooling air, so these engines are normally
limited to four or six cylinders. V-type engines provide
more horsepower than in-line engines and still retain a small
frontal area.
Continued improvements in engine design led to the
development of the horizontally-opposed engine, which
remains the most popular reciprocating engines used on
smaller aircraft. These engines always have an even number
of cylinders, since a cylinder on one side of the crankcase
“opposes” a cylinder on the other side. [Figure 7-2] The
majority of these engines are air cooled and usually are
mounted in a horizontal position when installed on fixed-wing
airplanes. Opposed-type engines have high power-to-weight
ratios because they have a comparatively small, lightweight
crankcase. In addition, the compact cylinder arrangement
reduces the engine’s frontal area and allows a streamlined
installation that minimizes aerodynamic drag.
Exhaust valve Fuel injector
Piston
Forced air
2. Power stroke
1. Intake/compression
and exhaust
Figure 7-3. Two-stroke compression ignition.
Exhaust valve
Intake valve
Spark plug
Piston
CrankshaftConnecting rod
Crankcase
Cylinder
Figure 7-4. Main components of a spark ignition reciprocating
engine.
Depending on the engine manufacturer, all of these
arrangements can be designed to utilize spark or compression
ignition and operate on either a two- or four-stroke cycle.
In a two-stroke engine, the conversion of chemical energy
into mechanical energy occurs over a two-stroke operating
cycle. The intake, compression, power, and exhaust processes
occur in only two strokes of the piston rather than the more
common four strokes. Because a two-stroke engine has
a power stroke upon each revolution of the crankshaft, it
typically has higher power-to-weight ratio than a comparable
four-stroke engine. Due to the inherent inefficiency and
disproportionate emissions of the earliest designs, use of the
two-stroke engine has been limited in aviation.
Recent advances in material and engine design have
reduced many of the negative characteristics associated
with two-stroke engines. Modern two-stroke engines often
use conventional oil sumps, oil pumps, and full pressure
fed lubrication systems. The use of direct fuel injection
and pressurized air, characteristic of advanced compression
ignition engines, make two-stroke compression ignition
engines a viable alternative to the more common four-stroke
spark ignition designs. [Figure 7-3]
Spark ignition four-stroke engines remain the most common
design used in GA today. [Figure 7-4] The main parts of a
spark ignition reciprocating engine include the cylinders,
crankcase, and accessory housing. The intake/exhaust valves,
spark plugs, and pistons are located in the cylinders. The
crankshaft and connecting rods are located in the crankcase.
The magnetos are normally located on the engine accessory
housing. In a four-stroke engine, the conversion of chemical energy into
mechanical energy occurs over a four-stroke operating cycle.
The intake, compression, power, and exhaust processes occur
in four separate strokes of the piston in the following order.
1. The intake stroke begins as the piston starts its
downward travel. When this happens, the intake
valve opens and the fuel-air mixture is drawn into the
cylinder.
2. The compression stroke begins when the intake valve
closes, and the piston starts moving back to the top of
the cylinder. This phase of the cycle is used to obtain
a much greater power output from the fuel-air mixture
once it is ignited.
3. The power stroke begins when the fuel-air mixture is
ignited. This causes a tremendous pressure increase
in the cylinder and forces the piston downward away
from the cylinder head, creating the power that turns
the crankshaft.
1. Intake 2. Compression
3. Power 4. Exhaust
Exhaust valveIntake valve
Spark plugPiston
Crankshaft Connecting rod
Figure 7-5. The arrows in this illustration indicate the direction of
motion of the crankshaft and piston during the four-stroke cycle.
4. The exhaust stroke is used to purge the cylinder of
burned gases. It begins when the exhaust valve opens,
and the piston starts to move toward the cylinder head
once again.
Even when the engine is operated at a fairly low speed,
the four-stroke cycle takes place several hundred times
each minute. [Figure 7-5] In a four-cylinder engine, each
cylinder operates on a different stroke. Continuous rotation
of a crankshaft is maintained by the precise timing of the
power strokes in each cylinder. Continuous operation of the
engine depends on the simultaneous function of auxiliary
systems, including the induction, ignition, fuel, oil, cooling,
and exhaust systems.
The latest advance in aircraft reciprocating engines was
pioneered in the mid-1960s by Frank Thielert, who looked
to the automotive industry for answers on how to integrate
diesel technology into an aircraft engine. The advantage
of a diesel-fueled reciprocating engine lies in the physical
similarity of diesel and kerosene. Aircraft equipped with a
diesel piston engine runs on standard aviation fuel kerosene,
which provides more independence, higher reliability, lower
consumption, and operational cost saving.
In 1999, Thielert formed Thielert Aircraft Engines (TAE)
to design, develop, certify, and manufacture a brand-new
Jet-A-burning diesel cycle engine (also known as jet-fueled
piston engine) for the GA industry. By March 2001, the first
prototype engine became the first certified diesel engine
since World War II. TAE continues to design and develop
diesel cycle engines and other engine manufacturers, such as
Société de Motorisations Aéronautiques (SMA), now offer
jet-fueled piston engines as well. TAE engines can be found
on the Diamond DA40 single and the DA42 Twin Star; the
first diesel engine to be part of the type certificate of a new
original equipment manufacturer (OEM) aircraft.
These engines have also gained a toehold in the retrofit
market with a supplemental type certificate (STC) to re-
engine the Cessna 172 models and the Piper PA-28 family.
The jet-fueled piston engine’s technology has continued to
progress and a full authority digital engine control (FADEC,
discussed more fully later in the chapter) is standard on such
equipped aircraft, which minimizes complication of engine
control. By 2007, various jet-fueled piston aircraft had logged
well over 600,000 hours of service.
Propeller
The propeller is a rotating airfoil, subject to induced drag,
stalls, and other aerodynamic principles that apply to any
airfoil. It provides the necessary thrust to pull, or in some
cases push, the aircraft through the air. The engine power is
used to rotate the propeller, which in turn generates thrust
very similar to the manner in which a wing produces lift.
The amount of thrust produced depends on the shape of the
airfoil, the angle of attack (AOA) of the propeller blade, and
the revolutions per minute (rpm) of the engine. The propeller
itself is twisted so the blade angle changes from hub to tip.
The greatest angle of incidence, or the highest pitch, is at the
hub while the smallest angle of incidence or smallest pitch
is at the tip. [Figure 7-6]
The reason for the twist is to produce uniform lift from the
hub to the tip. As the blade rotates, there is a difference in
the actual speed of the various portions of the blade. The tip
of the blade travels faster than the part near the hub, because
the tip travels a greater distance than the hub in the same
length of time. [Figure 7-7] Changing the angle of incidence
(pitch) from the hub to the tip to correspond with the speed
produces uniform lift throughout the length of the blade. A
propeller blade designed with the same angle of incidence
60 in.
40 in.
20 in.
Short travel distance— slow speed
—
129 knots
Moderate travel distance— moderate speed—259 knots
Greater travel distance— very high speed— 389 knots
2,500 rpm
2,500 rpm
2,500 rpm
Figure 7-6. Changes in propeller blade angle from hub to tip.
Figure 7-7. Relationship of travel distance and speed of various
portions of propeller blade. Figure 7-8. Engine rpm is indicated on the tachometer.
RPM
HUNDREDS
25
3
5
I5
30
20
I0
35
25
3
5
I5
30
20
I0
35
HOURS
AVOID
CONTINUOUS
OPERATION
BETWEEN 2250
AND 2350 RPM
I
I0
throughout its entire length would be inefficient because as
airspeed increases in flight, the portion near the hub would
have a negative AOA while the blade tip would be stalled.
Small aircraft are equipped with either one of two types of
propellers: fixed-pitch or adjustable-pitch.
Fixed-Pitch Propeller
A propeller with fixed blade angles is a fixed-pitch propeller.
The pitch of this propeller is set by the manufacturer and
cannot be changed. Since a fixed-pitch propeller achieves
the best efficiency only at a given combination of airspeed
and rpm, the pitch setting is ideal for neither cruise nor
climb. Thus, the aircraft suffers a bit in each performance
category. The fixed-pitch propeller is used when low weight,
simplicity, and low cost are needed.
There are two types of fixed-pitch propellers: climb and
cruise. Whether the airplane has a climb or cruise propeller
installed depends upon its intended use. The climb propeller
has a lower pitch, therefore less drag. Less drag results in
higher rpm and more horsepower capability, which increases
performance during takeoffs and climbs but decreases
performance during cruising flight.
The cruise propeller has a higher pitch, therefore more
drag. More drag results in lower rpm and less horsepower
capability, which decreases performance during takeoffs and
climbs but increases efficiency during cruising flight.
The propeller is usually mounted on a shaft, which may be
an extension of the engine crankshaft. In this case, the rpm
of the propeller would be the same as the crankshaft rpm. On
some engines, the propeller is mounted on a shaft geared to
the engine crankshaft. In this type, the rpm of the propeller
is different than that of the engine.
In a fixed-pitch propeller, the tachometer is the indicator of
engine power. [Figure 7-8] A tachometer is calibrated in
hundreds of rpm and gives a direct indication of the engine
and propeller rpm. The instrument is color coded with a green
arc denoting the maximum continuous operating rpm. Some
tachometers have additional markings to reflect engine and/or
propeller limitations. The manufacturer’s recommendations
should be used as a reference to clarify any misunderstanding
of tachometer markings.
The rpm is regulated by the throttle, which controls the fuel-
air flow to the engine. At a given altitude, the higher the
tachometer reading, the higher the power output of the engine.
When operating altitude increases, the tachometer may not
show correct power output of the engine. For example, 2,300
rpm at 5,000 feet produces less horsepower than 2,300 rpm
at sea level because power output depends on air density. Air
density decreases as altitude increases and a decrease in air
IN Hg
ALg.
MANIFOLD
PRESS
35
5010
15
25
40
30
20
45
35
5010
15
25
40
30
20
45
Figure 7-9. Engine power output is indicated on the manifold
pressure gauge.
density (higher density altitude) decreases the power output
of the engine. As altitude changes, the position of the throttle
must be changed to maintain the same rpm. As altitude is
increased, the throttle must be opened further to indicate the
same rpm as at a lower altitude.
Adjustable-Pitch Propeller
The adjustable-pitch propeller was the forerunner of the
constant-speed propeller. It is a propeller with blades whose
pitch can be adjusted on the ground with the engine not
running, but which cannot be adjusted in flight. It is also
referred to as a ground adjustable propeller. By the 1930s,
pioneer aviation inventors were laying the ground work for
automatic pitch-change mechanisms, which is why the term
sometimes refers to modern constant-speed propellers that
are adjustable in flight.
The first adjustable-pitch propeller systems provided only two
pitch settings: low and high. Today, most adjustable-pitch
propeller systems are capable of a range of pitch settings.
A constant-speed propeller is a controllable-pitch propeller
whose pitch is automatically varied in flight by a governor
maintaining constant rpm despite varying air loads. It is the
most common type of adjustable-pitch propeller. The main
advantage of a constant-speed propeller is that it converts
a high percentage of brake horsepower (BHP) into thrust
horsepower (THP) over a wide range of rpm and airspeed
combinations. A constant-speed propeller is more efficient
than other propellers because it allows selection of the most
efficient engine rpm for the given conditions.
An aircraft with a constant-speed propeller has two controls:
the throttle and the propeller control. The throttle controls
power output, and the propeller control regulates engine
rpm. This regulates propeller rpm, which is registered on
the tachometer.
Once a specific rpm is selected, a governor automatically
adjusts the propeller blade angle as necessary to maintain
the selected rpm. For example, after setting the desired rpm
during cruising flight, an increase in airspeed or decrease in
propeller load causes the propeller blade angle to increase
as necessary to maintain the selected rpm. A reduction in
airspeed or increase in propeller load causes the propeller
blade angle to decrease.
The propeller’s constant-speed range, defined by the high
and low pitch stops, is the range of possible blade angles for
a constant-speed propeller. As long as the propeller blade
angle is within the constant-speed range and not against
either pitch stop, a constant engine rpm is maintained. If
the propeller blades contact a pitch stop, the engine rpm
will increase or decrease as appropriate, with changes in
airspeed and propeller load. For example, once a specific
rpm has been selected, if aircraft speed decreases enough to
rotate the propeller blades until they contact the low pitch
stop, any further decrease in airspeed will cause engine rpm
to decrease the same way as if a fixed-pitch propeller were
installed. The same holds true when an aircraft equipped with
a constant-speed propeller accelerates to a faster airspeed. As
the aircraft accelerates, the propeller blade angle increases to
maintain the selected rpm until the high pitch stop is reached.
Once this occurs, the blade angle cannot increase any further
and engine rpm increases.
On aircraft equipped with a constant-speed propeller, power
output is controlled by the throttle and indicated by a manifold
pressure gauge. The gauge measures the absolute pressure of
the fuel-air mixture inside the intake manifold and is more
correctly a measure of manifold absolute pressure (MAP). At
a constant rpm and altitude, the amount of power produced
is directly related to the fuel-air mixture being delivered to
the combustion chamber. As the throttle setting is increased,
more fuel and air flows to the engine and MAP increases.
When the engine is not running, the manifold pressure gauge
indicates ambient air pressure (i.e., 29.92 inches mercury
(29.92 "Hg)). When the engine is started, the manifold
pressure indication decreases to a value less than ambient
pressure (i.e., idle at 12 "Hg). Engine failure or power loss
is indicated on the manifold gauge as an increase in manifold
pressure to a value corresponding to the ambient air pressure
at the altitude where the failure occurred. [Figure 7-9]
The manifold pressure gauge is color coded to indicate the
engine’s operating range. The face of the manifold pressure
gauge contains a green arc to show the normal operating
range and a red radial line to indicate the upper limit of
manifold pressure.
For any given rpm, there is a manifold pressure that should
not be exceeded. If manifold pressure is excessive for a given
rpm, the pressure within the cylinders could be exceeded,
placing undue stress on the cylinders. If repeated too
frequently, this stress can weaken the cylinder components
and eventually cause engine failure.
A pilot can avoid conditions that overstress the cylinders
by being constantly aware of the rpm, especially when
increasing the manifold pressure. Consult the manufacturer’s
recommendations for power settings of a particular engine to
maintain the proper relationship between manifold pressure
and rpm.
When both manifold pressure and rpm need to be changed,
avoid engine overstress by making power adjustments in
the proper order:
• When power settings are being decreased, reduce
manifold pressure before reducing rpm. If rpm is
reduced before manifold pressure, manifold pressure
automatically increases, possibly exceeding the
manufacturer’s tolerances.
• When power settings are being increased, reverse the
order—increase rpm first, then manifold pressure.
• To prevent damage to radial engines, minimize
operating time at maximum rpm and manifold
pressure, and avoid operation at maximum rpm and
low manifold pressure.
The engine and/or airframe manufacturer’s recommendations
should be followed to prevent severe wear, fatigue, and
damage to high-performance reciprocating engines.
Propeller Overspeed in Piston Engine Aircraft
On March 17, 2010, the Federal Aviation Administration
(FAA) issued Special Airworthiness Information Bulletin
(SAIB) CE-10-21. The subject was Propellers/Propulsers;
Propeller Overspeed in Piston Engine Aircraft to alert
operators, pilots, and aircraft manufacturers of concerns
for an optimum response to a propeller overspeed in piston
engine aircraft with variable pitch propellers. Although a
SAIB is not regulatory in nature, the FAA recommends that
the information be read and taken into consideration for the
safety of flight.
The document explains that a single-engine aircraft
experienced a propeller overspeed during cruise flight at
7,000 feet. The pilot reported that the application of throttle
resulted in a propeller overspeed with no appreciable thrust.
The pilot attempted to glide to a nearby airport and established
the “best glide” speed of 110 knots, as published in the Pilot’s
Operating Handbook (POH), but was unable to reach the
airport and was forced to conduct an off-field landing.
It was further explained that a determination was made that the
propeller experienced a failure causing the blade pitch change
mechanism to move to the low pitch stop position. This caused
the propeller to operate as a fixed-pitch propeller such that it
changes rpm with changes in power and airspeed. The low
pitch setting allows for maximum power during takeoff but
can result in a propeller overspeed at a higher airspeed.
A performance evaluation of the flight condition was
performed for the particular aircraft model involved in this
incident. This evaluation indicated that an airspeed lower
than the best glide speed would have resulted in increased
thrust enabling the pilot to maintain level flight. There are
numerous variables in aircraft, engines, and propellers
that affect aircraft performance. For some aircraft models,
the published best glide speed may not be low enough to
generate adequate thrust for a given propeller installation in
this situation (propeller blades at low pitch stop position).
The operators of aircraft with variable pitch propellers should
be aware that in certain instances of propeller overspeed, the
airspeed necessary to maintain level flight may be different
than the speed associated with engine-out best glide speed.
The appropriate emergency procedures should be followed to
mitigate the emergency situation in the event of a propeller
overspeed; however, pilots should be aware that some
reduction in airspeed may result in the ability for continued
safe flight and landing. The determination of an airspeed that
is more suitable than engine-out best glide speed should only
be conducted at a safe altitude when the pilot has time to
determine an alternative course of action other than landing
immediately.
Induction Systems
The induction system brings in air from the outside, mixes
it with fuel, and delivers the fuel-air mixture to the cylinder
where combustion occurs. Outside air enters the induction
system through an intake port on the front of the engine
cowling. This port normally contains an air filter that inhibits
the entry of dust and other foreign objects. Since the filter
may occasionally become clogged, an alternate source of
air must be available. Usually, the alternate air comes from
inside the engine cowling, where it bypasses a clogged air
filter. Some alternate air sources function automatically,
while others operate manually.
Fuel
Fuel level is maintained
by a float-type device.
Float chamber
Fuel is received into
the carburetor through
the fuel inlet.
Fuel inlet
The mixture needle
controls fuel to the
discharge nozzle.
Mixture needle position
can be adjusted using
the mixture control.
Mixture needle
Air enters the carburetor
through the air inlet.
Air inlet
The blend of fuel and
air is routed to the
combustion chambers
to be burned.
Fuel-air mixture
The flow of the fuel-air
mixture is controlled by
the throttle valve. The
throttle valve is adjusted
from the flight deck by
the throttle.
Throttle valve
Fuel is forced through
the discharge nozzle
into the venturi by
greater atmospheric
pressure in the float
chamber.
Discharge nozzle
The shape of the venturi
creates an area of low
pressure.
Venturi
The air bleed allows air to be mixed
with fuel being drawn out of the
discharge nozzle to decrease fuel
density and promote fuel vaporization.
Air bleed
Figure 7-10. Float-type carburetor.
Two types of induction systems are commonly used in small
aircraft engines:
1. The carburetor system mixes the fuel and air in
the carburetor before this mixture enters the intake
manifold.
2. The fuel injection system mixes the fuel and air
immediately before entry into each cylinder or injects
fuel directly into each cylinder.
Carburetor Systems
Aircraft carburetors are separated into two categories: float-
type carburetors and pressure-type carburetors. Float-type
carburetors, complete with idling, accelerating, mixture
control, idle cutoff, and power enrichment systems, are the
most common of the two carburetor types. Pressure-type
carburetors are usually not found on small aircraft. The basic
difference between a float-type and a pressure-type carburetor
is the delivery of fuel. The pressure-type carburetor delivers
fuel under pressure by a fuel pump.
In the operation of the float-type carburetor system, the
outside air first flows through an air filter, usually located
at an air intake in the front part of the engine cowling. This
filtered air flows into the carburetor and through a venturi, a
narrow throat in the carburetor. When the air flows through
the venturi, a low-pressure area is created that forces the
fuel to flow through a main fuel jet located at the throat. The
fuel then flows into the airstream where it is mixed with the
flowing air. [Figure 7-10]
The fuel-air mixture is then drawn through the intake
manifold and into the combustion chambers where it is
ignited. The float-type carburetor acquires its name from a
float that rests on fuel within the float chamber. A needle
attached to the float opens and closes an opening at the
bottom of the carburetor bowl. This meters the amount of
fuel entering into the carburetor, depending upon the position
of the float, which is controlled by the level of fuel in the
float chamber. When the level of the fuel forces the float
to rise, the needle valve closes the fuel opening and shuts
off the fuel flow to the carburetor. The needle valve opens
again when the engine requires additional fuel. The flow of
the fuel-air mixture to the combustion chambers is regulated
by the throttle valve, which is controlled by the throttle in
the flight deck.
The float-type carburetor has several distinct disadvantages.
First, they do not function well during abrupt maneuvers.
Secondly, the discharge of fuel at low pressure leads to
incomplete vaporization and difficulty in discharging
fuel into some types of supercharged systems. The chief
disadvantage of the float-type carburetor, however, is its
icing tendency. Since the float-type carburetor must discharge
Incoming air
Venturi
Ice
To engine Fuel-air mixture
Ice
Ice
Figure 7-11. The formation of carburetor ice may reduce or block
fuel-air flow to the engine.
Carburetor Icing
As mentioned earlier, one disadvantage of the float-type
carburetor is its icing tendency. Carburetor 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 carburetor temperature is at or below freezing, ice
may form on internal surfaces of the carburetor, including
the throttle valve. [Figure 7-11]
The reduced air pressure, as well as the vaporization of fuel,
contributes to the temperature decrease in the carburetor. Ice
generally forms in the vicinity of the throttle valve and in the
venturi throat. This restricts the flow of the fuel-air mixture
and reduces power. If enough ice builds up, the engine may
cease to operate. Carburetor ice is most likely to occur when
temperatures are below 70 degrees Fahrenheit (°F) or 21
degrees Celsius (°C) and the relative humidity is above 80
percent. Due to the sudden cooling that takes place in the
carburetor, icing can occur even in outside air 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 absolute
(versus relative) Fahrenheit degrees (70 x 100/180 = 38.89
fuel at a point of low pressure, the discharge nozzle must be
located at the venturi throat, and the throttle valve must be
on the engine side of the discharge nozzle. This means that
the drop in temperature due to fuel vaporization takes place
within the venturi. As a result, ice readily forms in the venturi
and on the throttle valve.
A pressure-type carburetor discharges fuel into the airstream
at a pressure well above atmospheric pressure. This results
in better vaporization and permits the discharge of fuel into
the airstream on the engine side of the throttle valve. With the
discharge nozzle in this position fuel vaporization takes place
after the air has passed through the throttle valve and at a point
where the drop in temperature is offset by heat from the engine.
Thus, the danger of fuel vaporization icing is practically
eliminated. The effects of rapid maneuvers and rough air on
the pressure-type carburetors are negligible, since their fuel
chambers remain filled under all operating conditions.
Mixture Control
Carburetors are normally calibrated at sea-level air pressure
where the correct fuel-air mixture ratio is established with the
mixture control set in the FULL RICH position. However, as
altitude increases, the density of air entering the carburetor
decreases, while the density of the fuel remains the same. This
creates a progressively richer mixture that can result in engine
roughness and an appreciable loss of power. The roughness
normally is due to spark plug fouling from excessive carbon
buildup on the plugs. Carbon buildup occurs because the
rich mixture lowers the temperature inside the cylinder,
inhibiting complete combustion of the fuel. This condition
may occur during the runup prior to takeoff at high-elevation
airports and during climbs or cruise flight at high altitudes.
To maintain the correct fuel-air mixture, the mixture must
be leaned using the mixture control. Leaning the mixture
decreases fuel flow, which compensates for the decreased
air density at high altitude.
During a descent from high altitude, the fuel-air mixture
must be enriched, or it may become too lean. An overly lean
mixture causes detonation, which may result in rough engine
operation, overheating, and/or a loss of power. The best way
to maintain the proper fuel-air mixture is to monitor the
engine temperature and enrich the mixture as needed. Proper
mixture control and better fuel economy for fuel-injected
engines can be achieved by using an exhaust gas temperature
(EGT) gauge. Since the process of adjusting the mixture can
vary from one aircraft to another, it is important to refer to
the airplane flight manual (AFM) or the POH to determine
the specific procedures for a given aircraft.