Figure 7-21. Normal combustion and explosive combustion.
ExplosionNormal combustion
Figure 7-20. Typical starting circuit.
M
A
I
N
B
U
S
Starter
Battery
A
L
T
B
A
T
Battery
contactor
(solenoid)
Starter
contactor
External
power
relay
+
+
OFF
L
R B
S
Ignition switch
External
power plug
by the spark plugs. It then burns away from the plugs until it
is completely consumed. This type of combustion causes a
smooth build-up of temperature and pressure and ensures that
the expanding gases deliver the maximum force to the piston
at exactly the right time in the power stroke. [Figure 7-21]
Detonation is an uncontrolled, explosive ignition of the
fuel-air mixture within the cylinder’s combustion chamber.
It causes excessive temperatures and pressures which, if not
corrected, can quickly lead to failure of the piston, cylinder,
or valves. In less severe cases, detonation causes engine
overheating, roughness, or loss of power.
Detonation is characterized by high cylinder head temperatures
and is most likely to occur when operating at high power
settings. Common operational causes of detonation are:
• Use of a lower fuel grade than that specified by the
aircraft manufacturer
• Operation of the engine with extremely high manifold
pressures in conjunction with low rpm
• Operation of the engine at high power settings with
an excessively lean mixture
• Maintaining extended ground operations or steep
climbs in which cylinder cooling is reduced
Detonation may be avoided by following these basic
guidelines during the various phases of ground and flight
operations:
• Ensure that the proper grade of fuel is used.
• Keep the cowl flaps (if available) in the full-open
position while on the ground to provide the maximum
airflow through the cowling.
• Use an enriched fuel mixture, as well as a shallow
climb angle, to increase cylinder cooling during
takeoff and initial climb.
• Avoid extended, high power, steep climbs.
• Develop the habit of monitoring the engine instruments
to verify proper operation according to procedures
established by the manufacturer.
Preignition occurs when the fuel-air mixture ignites prior
to the engine’s normal ignition event. Premature burning
is usually caused by a residual hot spot in the combustion
chamber, often created by a small carbon deposit on a spark
plug, a cracked spark plug insulator, or other damage in the
cylinder that causes a part to heat sufficiently to ignite the
fuel-air charge. Preignition causes the engine to lose power
and produces high operating temperature. As with detonation,
preignition may also cause severe engine damage because
the expanding gases exert excessive pressure on the piston
while still on its compression stroke.
Figure 7-22. Eclipse 500 VLJ.
Detonation and preignition often occur simultaneously and
one may cause the other. Since either condition causes high
engine temperature accompanied by a decrease in engine
performance, it is often difficult to distinguish between the
two. Using the recommended grade of fuel and operating
the engine within its proper temperature, pressure, and rpm
ranges reduce the chance of detonation or preignition.
Full Authority Digital Engine Control
(FADEC)
FADEC is a system consisting of a digital computer and
ancillary components that control an aircraft’s engine
and propeller. First used in turbine-powered aircraft, and
referred to as full authority digital electronic control, these
sophisticated control systems are increasingly being used in
piston powered aircraft.
In a spark-ignition reciprocating engine, the FADEC uses
speed, temperature, and pressure sensors to monitor the status
of each cylinder. A digital computer calculates the ideal pulse
for each injector and adjusts ignition timing as necessary
to achieve optimal performance. In a compression-ignition
engine, the FADEC operates similarly and performs all of
the same functions, excluding those specifically related to
the spark ignition process.
FADEC systems eliminate the need for magnetos, carburetor
heat, mixture controls, and engine priming. A single throttle
lever is characteristic of an aircraft equipped with a FADEC
system. The pilot simply positions the throttle lever to a
desired detent, such as start, idle, cruise power, or max power,
and the FADEC system adjusts the engine and propeller
automatically for the mode selected. There is no need for the
pilot to monitor or control the fuel-air mixture.
During aircraft starting, the FADEC primes the cylinders,
adjusts the mixture, and positions the throttle based on engine
temperature and ambient pressure. During cruise flight, the
FADEC constantly monitors the engine and adjusts fuel flow
and ignition timing individually in each cylinder. This precise
control of the combustion process often results in decreased
fuel consumption and increased horsepower.
FADEC systems are considered an essential part of the
engine and propeller control and may be powered by the
aircraft’s main electrical system. In many aircraft, FADEC
uses power from a separate generator connected to the engine.
In either case, there must be a backup electrical source
available because failure of a FADEC system could result in a
complete loss of engine thrust. To prevent loss of thrust, two
separate and identical digital channels are incorporated for
redundancy. Each channel is capable of providing all engine
and propeller functions without limitations.
Turbine Engines
An aircraft turbine engine consists of an air inlet, compressor,
combustion chambers, a turbine section, and exhaust. Thrust
is produced by increasing the velocity of the air flowing
through the engine. Turbine engines are highly desirable
aircraft powerplants. They are characterized by smooth
operation and a high power-to-weight ratio, and they
use readily available jet fuel. Prior to recent advances in
material, engine design, and manufacturing processes, the
use of turbine engines in small/light production aircraft was
cost prohibitive. Today, several aviation manufacturers are
producing or plan to produce small/light turbine-powered
aircraft. These smaller turbine-powered aircraft typically
seat between three and seven passengers and are referred to
as very light jets (VLJs) or microjets. [Figure 7-22]
Types of Turbine Engines
Turbine engines are classified according to the type of
compressors they use. There are three types of compressors—
centrifugal flow, axial flow, and centrifugal-axial flow.
Compression of inlet air is achieved in a centrifugal flow
engine by accelerating air outward perpendicular to the
longitudinal axis of the machine. The axial-flow engine
compresses air by a series of rotating and stationary
airfoils moving the air parallel to the longitudinal axis. The
centrifugal-axial flow design uses both kinds of compressors
to achieve the desired compression.
The path the air takes through the engine and how power is
produced determines the type of engine. There are four types
of aircraft turbine engines—turbojet, turboprop, turbofan,
and turboshaft.
Turbojet
The turbojet engine consists of four sections—compressor,
combustion chamber, turbine section, and exhaust. The
compressor section passes inlet air at a high rate of speed to
Figure 7-23. Turbojet engine.
Fuel injector
Inlet
Combustion chamberCompressor
Turbine
Hot gases
Nozzle
Figure 7-24. Turboprop engine.
Combustion chamber
CompressorGear box
Exhaust
Fuel injector
Inlet
Prop
Turbine
the combustion chamber. The combustion chamber contains
the fuel inlet and igniter for combustion. The expanding
air drives a turbine, which is connected by a shaft to the
compressor, sustaining engine operation. The accelerated
exhaust gases from the engine provide thrust. This is a basic
application of compressing air, igniting the fuel-air mixture,
producing power to self-sustain the engine operation, and
exhaust for propulsion. [Figure 7-23]
Turbojet engines are limited in range and endurance. They
are also slow to respond to throttle applications at slow
compressor speeds.
Turboprop
A turboprop engine is a turbine engine that drives a propeller
through a reduction gear. The exhaust gases drive a power
turbine connected by a shaft that drives the reduction gear
assembly. Reduction gearing is necessary in turboprop
engines because optimum propeller performance is achieved
at much slower speeds than the engine’s operating rpm.
Turboprop engines are a compromise between turbojet
engines and reciprocating powerplants. Turboprop engines
are most efficient at speeds between 250 and 400 mph and
altitudes between 18,000 and 30,000 feet. They also perform
well at the slow airspeeds required for takeoff and landing and
are fuel efficient. The minimum specific fuel consumption
of the turboprop engine is normally available in the altitude
range of 25,000 feet to the tropopause. [Figure 7-24]
Turbofan
Turbofans were developed to combine some of the best
features of the turbojet and the turboprop. Turbofan engines
are designed to create additional thrust by diverting a
secondary airflow around the combustion chamber. The
turbofan bypass air generates increased thrust, cools the
engine, and aids in exhaust noise suppression. This provides
turbojet-type cruise speed and lower fuel consumption.
The inlet air that passes through a turbofan engine is usually
divided into two separate streams of air. One stream passes
through the engine core, while a second stream bypasses the
engine core. It is this bypass stream of air that is responsible
for the term “bypass engine.” A turbofan’s bypass ratio refers
to the ratio of the mass airflow that passes through the fan
divided by the mass airflow that passes through the engine
core. [Figure 7-25]
Turboshaft
The fourth common type of jet engine is the turboshaft.
[Figure 7-26] It delivers power to a shaft that drives
something other than a propeller. The biggest difference
between a turbojet and turboshaft engine is that on a
Figure 7-25. Turbofan engine.
Fuel injectorInlet
Combustion chamberCompressor
Turbine
Hot gases
Nozzle
Duct fan
Secondary air stream
Primary air stream
Figure 7-26. Turboshaft engine.
Combustion chamberCompressor ExhaustInlet
Free (power) turbine
Compressor turbine
Power shaft
turboshaft engine, most of the energy produced by the
expanding gases is used to drive a turbine rather than produce
thrust. Many helicopters use a turboshaft gas turbine engine.
In addition, turboshaft engines are widely used as auxiliary
power units on large aircraft.
Turbine Engine Instruments
Engine instruments that indicate oil pressure, oil temperature,
engine speed, exhaust gas temperature, and fuel flow are
common to both turbine and reciprocating engines. However,
there are some instruments that are unique to turbine engines.
These instruments provide indications of engine pressure
ratio, turbine discharge pressure, and torque. In addition,
most gas turbine engines have multiple temperature-sensing
instruments, called thermocouples, which provide pilots with
temperature readings in and around the turbine section.
Engine Pressure Ratio (EPR)
An engine pressure ratio (EPR) gauge is used to indicate the
power output of a turbojet/turbofan engine. EPR is the ratio
of turbine discharge to compressor inlet pressure. Pressure
measurements are recorded by probes installed in the engine
inlet and at the exhaust. Once collected, the data is sent to
a differential pressure transducer, which is indicated on a
flight deck EPR gauge.
EPR system design automatically compensates for the effects
of airspeed and altitude. Changes in ambient temperature
require a correction be applied to EPR indications to provide
accurate engine power settings.
Exhaust Gas Temperature (EGT)
A limiting factor in a gas turbine engine is the temperature
of the turbine section. The temperature of a turbine section
must be monitored closely to prevent overheating the turbine
blades and other exhaust section components. One common
way of monitoring the temperature of a turbine section is
with an EGT gauge. EGT is an engine operating limit used
to monitor overall engine operating conditions.
Variations of EGT systems bear different names based on
the location of the temperature sensors. Common turbine
temperature sensing gauges include the turbine inlet
temperature (TIT) gauge, turbine outlet temperature (TOT)
gauge, interstage turbine temperature (ITT) gauge, and
turbine gas temperature (TGT) gauge.
Torquemeter
Turboprop/turboshaft engine power output is measured
by the torquemeter. Torque is a twisting force applied to a
shaft. The torquemeter measures power applied to the shaft.
Figure 7-27. Dual-spool axial-flow compressor.
Low pressure
compressor (N1)
High pressure
compressor (N2)
Low pressure compressor drive shaft
High pressure compressor drive shaft
Turboprop and turboshaft engines are designed to produce
torque for driving a propeller. Torquemeters are calibrated
in percentage units, foot-pounds, or psi.
N1 Indicator
N1 represents the rotational speed of the low pressure
compressor and is presented on the indicator as a percentage
of design rpm. After start, the speed of the low pressure
compressor is governed by the N 1 turbine wheel. The N 1
turbine wheel is connected to the low pressure compressor
through a concentric shaft.
N2 Indicator
N2 represents the rotational speed of the high pressure
compressor and is presented on the indicator as a percentage of
design rpm. The high pressure compressor is governed by the
N2 turbine wheel. The N2 turbine wheel is connected to the high
pressure compressor through a concentric shaft. [Figure 7-27]
Turbine Engine Operational Considerations
The great variety of turbine engines makes it impractical to
cover specific operational procedures, but there are certain
operational considerations common to all turbine engines.
They are engine temperature limits, foreign object damage,
hot start, compressor stall, and flameout.
Engine Temperature Limitations
The highest temperature in any turbine engine occurs at the
turbine inlet. TIT is therefore usually the limiting factor in
turbine engine operation.
Thrust Variations
Turbine engine thrust varies directly with air density. As air
density decreases, so does thrust. Additionally, because air
density decreases with an increase in temperature, increased
temperatures also results in decreased thrust. While both
turbine and reciprocating powered engines are affected to
some degree by high relative humidity, turbine engines will
experience a negligible loss of thrust, while reciprocating
engines a significant loss of brake horsepower.
Foreign Object Damage (FOD)
Due to the design and function of a turbine engine’s air inlet,
the possibility of ingestion of debris always exists. This
causes significant damage, particularly to the compressor
and turbine sections. When ingestion of debris occurs, it is
called foreign object damage (FOD). Typical FOD consists
of small nicks and dents caused by ingestion of small objects
from the ramp, taxiway, or runway, but FOD damage caused
by bird strikes or ice ingestion also occur. Sometimes FOD
results in total destruction of an engine.
Prevention of FOD is a high priority. Some engine inlets
have a tendency to form a vortex between the ground and
the inlet during ground operations. A vortex dissipater may
be installed on these engines. Other devices, such as screens
and/or deflectors, may also be utilized. Preflight procedures
include a visual inspection for any sign of FOD.
Turbine Engine Hot/Hung Start
When the EGT exceeds the safe limit of an aircraft, it
experiences a “hot start.” This is caused by too much fuel
entering the combustion chamber or insufficient turbine rpm.
Any time an engine has a hot start, refer to the AFM/POH or an
appropriate maintenance manual for inspection requirements.
If the engine fails to accelerate to the proper speed after
ignition or does not accelerate to idle rpm, a hung or false start
has occurred. A hung start may be caused by an insufficient
starting power source or fuel control malfunction.
Compressor Stalls
Compressor blades are small airfoils and are subject to the
same aerodynamic principles that apply to any airfoil. A
compressor blade has an AOA that is a result of inlet air
velocity and the compressor’s rotational velocity. These two
forces combine to form a vector, which defines the airfoil’s
actual AOA to the approaching inlet air.
A compressor stall is an imbalance between the two vector
quantities, inlet velocity, and compressor rotational speed.
Compressor stalls occur when the compressor blades’ AOA
exceeds the critical AOA. At this point, smooth airflow
is interrupted and turbulence is created with pressure
fluctuations. Compressor stalls cause air flowing in the
compressor to slow down and stagnate, sometimes reversing
direction. [Figure 7-28]
Figure 7-28. Comparison of normal and distorted airflow into the
compressor section.
Normal inlet airflow
Distorted inlet airflow
Compressor stalls can be transient and intermittent or steady
and severe. Indications of a transient/intermittent stall are
usually an intermittent “bang” as backfire and flow reversal
take place. If the stall develops and becomes steady, strong
vibration and a loud roar may develop from the continuous
flow reversal. Often, the flight deck gauges do not show
a mild or transient stall, but they do indicate a developed
stall. Typical instrument indications include fluctuations
in rpm and an increase in exhaust gas temperature. Most
transient stalls are not harmful to the engine and often correct
themselves after one or two pulsations. The possibility of
severe engine damage from a steady state stall is immediate.
Recovery must be accomplished by quickly reducing power,
decreasing the aircraft’s AOA, and increasing airspeed.
Although all gas turbine engines are subject to compressor
stalls, most models have systems that inhibit them. One
system uses a variable inlet guide vane (VIGV) and variable
stator vanes that direct the incoming air into the rotor blades
at an appropriate angle. To prevent air pressure stalls,
operate the aircraft within the parameters established by the
manufacturer. If a compressor stall does develop, follow the
procedures recommended in the AFM/POH.
Flameout
A flameout occurs in the operation of a gas turbine engine in
which the fire in the engine unintentionally goes out. If the
rich limit of the fuel-air ratio is exceeded in the combustion
chamber, the flame will blow out. This condition is often
referred to as a rich flameout. It generally results from
very fast engine acceleration where an overly rich mixture
causes the fuel temperature to drop below the combustion
temperature. It may also be caused by insufficient airflow
to support combustion.
A more common flameout occurrence is due to low fuel
pressure and low engine speeds, which typically are
associated with high-altitude flight. This situation may also
occur with the engine throttled back during a descent, which
can set up the lean-condition flameout. A weak mixture can
easily cause the flame to die out, even with a normal airflow
through the engine.
Any interruption of the fuel supply can result in a
flameout. This may be due to prolonged unusual attitudes,
a malfunctioning fuel control system, turbulence, icing, or
running out of fuel.
Symptoms of a flameout normally are the same as those
following an engine failure. If the flameout is due to a
transitory condition, such as an imbalance between fuel
flow and engine speed, an airstart may be attempted once
the condition is corrected. In any case, pilots must follow
the applicable emergency procedures outlined in the AFM/
POH. Generally these procedures contain recommendations
concerning altitude and airspeed where the airstart is most
likely to be successful.
Performance Comparison
It is possible to compare the performance of a reciprocating
powerplant and different types of turbine engines. For
the comparison to be accurate, thrust horsepower (usable
horsepower) for the reciprocating powerplant must be used
rather than brake horsepower, and net thrust must be used
for the turbine-powered engines. In addition, aircraft design
configuration and size must be approximately the same.
When comparing performance, the following definitions
are useful:
• Brake horsepower (BHP)—the horsepower actually
delivered to the output shaft. Brake horsepower is the
actual usable horsepower.
• Net thrust—the thrust produced by a turbojet or
turbofan engine.
• Thrust horsepower (THP)—the horsepower equivalent
of the thrust produced by a turbojet or turbofan engine.
Equivalent shaft horsepower (ESHP)—with respect
to turboprop engines, the sum of the shaft horsepower
(SHP) delivered to the propeller and THP produced by the
exhaust gases.
Figure 7-29 shows how four types of engines compare in net
thrust as airspeed is increased. This figure is for explanatory
Figure 7-29. Engine net thrust versus aircraft speed and drag. Points
A through F are explained in the text below.
Net thrust
Airspeed
Aircraft drag
A B C D E F
Reciprocating
Turboprop
Turbofan
Turbojet
purposes only and is not for specific models of engines. The
following are the four types of engines:
• Reciprocating powerplant
• Turbine, propeller combination (turboprop)
• Turbine engine incorporating a fan (turbofan)
• Turbojet (pure jet)
By plotting the performance curve for each engine, a
comparison can be made of maximum aircraft speed variation
with the type of engine used. Since the graph is only a means
of comparison, numerical values for net thrust, aircraft speed,
and drag are not included.
Comparison of the four powerplants on the basis of net thrust
makes certain performance capabilities evident. In the speed
range shown to the left of line A, the reciprocating powerplant
outperforms the other three types. The turboprop outperforms
the turbofan in the range to the left of line C. The turbofan
engine outperforms the turbojet in the range to the left of
line F. The turbofan engine outperforms the reciprocating
powerplant to the right of line B and the turboprop to the
right of line C. The turbojet outperforms the reciprocating
powerplant to the right of line D, the turboprop to the right
of line E, and the turbofan to the right of line F.
The points where the aircraft drag curve intersects the net
thrust curves are the maximum aircraft speeds. The vertical
lines from each of the points to the baseline of the graph
indicate that the turbojet aircraft can attain a higher maximum
speed than aircraft equipped with the other types of engines.
Aircraft equipped with the turbofan engine attains a higher
maximum speed than aircraft equipped with a turboprop or
reciprocating powerplant.
Airframe Systems
Fuel, electrical, hydraulic, and oxygen systems make up the
airframe systems.
Fuel Systems
The fuel system is designed to provide an uninterrupted
flow of clean fuel from the fuel tanks to the engine. The
fuel must be available to the engine under all conditions
of engine power, altitude, attitude, and during all approved
flight maneuvers. Two common classifications apply to fuel
systems in small aircraft: gravity-feed and fuel-pump systems.
Gravity-Feed System
The gravity-feed system utilizes the force of gravity to
transfer the fuel from the tanks to the engine. For example, on
high-wing airplanes, the fuel tanks are installed in the wings.
This places the fuel tanks above the carburetor, and the fuel
is gravity fed through the system and into the carburetor. If
the design of the aircraft is such that gravity cannot be used
to transfer fuel, fuel pumps are installed. For example, on
low-wing airplanes, the fuel tanks in the wings are located
below the carburetor. [Figure 7-30]
Fuel-Pump System
Aircraft with fuel-pump systems have two fuel pumps. The
main pump system is engine driven with an electrically-
driven auxiliary pump 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 flight deck.
Fuel Primer
Both gravity-feed and fuel-pump systems may incorporate a
fuel primer into the system. The fuel primer is used to draw
fuel from the tanks to vaporize fuel directly into the cylinders
prior to starting the engine. During cold weather, when
engines are difficult to start, the fuel primer helps because
there is not enough heat available to vaporize the fuel in the
carburetor. It is important to lock the primer in place when
it is not in use. If the knob is free to move, it may vibrate
out of position during flight which may cause an excessively
rich fuel-air mixture. To avoid overpriming, read the priming
instructions for the aircraft.
Fuel Tanks
The fuel tanks, normally located inside the wings of an
airplane, have a filler opening on top of the wing through
which they can be filled. A filler cap covers this opening.
Figure 7-31. Fuel selector valve.
LEFT
19 gal
LEVEL
FLIGHT
ONLY
RIGHT
19 gal
LEVEL
FLIGHT
ONLY
OFF
TAKEOFF LANDING
ALL FLIGHT 38 GAL ATTITUDES
BOTH Figure 7-30. Gravity-feed and fuel-pump systems.
Gravity-feed system
Fuel-pump system
Engine-driven pump
Right tank
Right tank
Left tank
Left tank
BOTH
LEFT
RIGHT
OFF
BOTH
LEFT
RIGHT
OFF
Selector valve
Carburetor
Primer
Strainer
Vent
Selector valve
Carburetor
Electric pump
Strainer
Primer
The tanks are vented to the outside to maintain atmospheric
pressure inside the tank. They may be vented through the
filler cap or through a tube extending through the surface
of the wing. Fuel tanks also include an overflow drain that
may stand alone or be collocated with the fuel tank vent.
This allows fuel to expand with increases in temperature
without damage to the tank itself. If the tanks have been
filled on a hot day, it is not unusual to see fuel coming from
the overflow drain.
Fuel Gauges
The fuel quantity gauges indicate the amount of fuel
measured by a sensing unit in each fuel tank and is displayed
in gallons or pounds. Aircraft certification rules require
accuracy in fuel gauges only when they read “empty.” Any
reading other than “empty” should be verified. Do not depend
solely on the accuracy of the fuel quantity gauges. Always
visually check the fuel level in each tank during the preflight
inspection, and then compare it with the corresponding fuel
quantity indication.
If a fuel pump is installed in the fuel system, a fuel pressure
gauge is also included. This gauge indicates the pressure in
the fuel lines. The normal operating pressure can be found
in the AFM/POH or on the gauge by color coding.
Fuel Selectors
The fuel selector valve allows selection of fuel from various
tanks. A common type of selector valve contains four
positions: LEFT, RIGHT, BOTH, and OFF. Selecting the
LEFT or RIGHT position allows fuel to feed only from the
respective tank, while selecting the BOTH position feeds
fuel from both tanks. The LEFT or RIGHT position may be
used to balance the amount of fuel remaining in each wing
tank. [Figure 7-31]
Fuel placards show any limitations on fuel tank usage, such
as “level flight only” and/or “both” for landings and takeoffs.
Regardless of the type of fuel selector in use, fuel
consumption should be monitored closely to ensure that a
tank does not run completely out of fuel. Running a fuel tank
dry does not only cause the engine to stop, but running for
prolonged periods on one tank causes an unbalanced fuel load
between tanks. Running a tank completely dry may allow air
to enter the fuel system and cause vapor lock, which makes
it difficult to restart the engine. On fuel-injected engines, the
fuel becomes so hot it vaporizes in the fuel line, not allowing
fuel to reach the cylinders.