Fundamentals

M15
GAS TURBINE ENGINE
Rev.-ID: 1APR2013
Author: DaC
For Training Purposes Only
ELTT Release: Jun. 06, 2013

M15.11
Fuel Systems

EASA Part-66
CAT B1

M15.11_B1 E
Training Manual

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Lufthansa Technical Training
GAS TURBINE ENGINE EASA PART-66 M15
FUEL SYSTEMS
M15.11

M15 GAS TURBINE ENGINE

M15.11 FUEL SYSTEMS
FOR TRAINING PURPOSES ONLY!

FRA US/O-5 DaC May 21, 2013 ATA DOC Page 1

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FUEL SYSTEMS System Lay−out
M15.11

FUEL SYSTEM LAY-OUT

Introduction
On an aircraft we distinguish between the primary fuel system, which stores the
fuel and the secondary fuel system, which is on the engine. Here we will only
talk about the secondary fuel system.
One purpose of this system is to supply the fuel to the combustion chamber,
and the other main purpose is to control the quantity of fuel necessary for all
operating conditions of the engine.
The engine fuel system can be split into 2 subsystems: the fuel distribution
subsystem and the fuel control subsystem.
The engine fuel distribution subsystem has 3 main tasks:
S First it has to safely supply the fuel from the aircraft fuel system to the
combustion chamber.
S The second task is to pressurize the fuel sufficiently so that it can be
vaporized in the combustion chamber.
S The third task of the distribution system is to heat the fuel. This makes sure
that fuel flow to the fuel nozzles is not blocked by ice build−up.
FOR TRAINING PURPOSES ONLY!

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Figure 1 Fuel Distribution System Purpose

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FUEL SYSTEMS System Lay−out
M15.11

Distribution Lay-Out
Here you can see a simplified engine fuel distribution system.
This system usually starts directly behind the low pressure fuel shut-off
valve. This valve feeds the fuel into the main fuel supply line, which runs
from the wing to the engine accessory gearbox where you can find the next
component of this system.
This is the low pressure fuel pump. The low pressure fuel pump increases
the fuel pressure that comes from the tank boost pumps.
The fuel from the low pressure pump then enters the oil cooler. This
component has a dual function. The cold fuel cools the oil of the engine
lubrication system and by this process the fuel is heated to a temperature
above the freezing point of water. This prevents ice particles coming from the
fuel tanks and blocking the fuel filter.
So this is the reason why the fuel filter is located downstream of the oil cooler.
It is needed to protect the following components in the engine fuel system.
The next component behind the fuel filter is the high pressure fuel pump.
This pump increases the fuel pressure to the high level needed for proper fuel
vaporization in the combustion chamber. It is always equipped with a pressure
relief valve, which protects the components in the high pressure fuel system
against overpressure.
The fuel from the high pressure fuel pump then enters the fuel control unit.
The fuel control unit meters the fuel that is needed for combustion. It is also
responsible for supply and shut-off of fuel to the fuel nozzles at the combustion
chamber.
The fuel control unit needs some of the high pressure fuel as servo pressure to
operate the internal control mechanisms. To be sure that this servo fuel is
absolutely free of ice, some engines have an additional servo fuel heater.
FOR TRAINING PURPOSES ONLY!

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Figure 2 Fuel Distribution System

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Distribution Layout cont.

When the fuel leaves the fuel control unit, it has to pass through the fuel flow
transmitter. The fuel flow transmitter measures the actual fuel flow and
transmits signals to the cockpit for the fuel flow and fuel used indication.
Note, that all the components of the fuel distribution system, from the LP fuel
pump to the fuel flow transmitter, are in the area of the accessory gearbox at
the engine.
From the fuel flow transmitter the fuel is then routed to the fuel manifold,
which distributes the fuel to the individual fuel nozzles on the combustion
chamber.
FOR TRAINING PURPOSES ONLY!

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Figure 3 Fuel Distribution System

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Distribution Layout cont.

Some engines have the oil cooler downstream of the high pressure pump, as
shown in this example.
This arrangement has the advantage that it requires less external fuel lines,
because the low pressure pump and the high pressure pump are usually in one
housing, but the danger of fuel leaks in the oil cooler is much higher than in a
low pressure system.
FOR TRAINING PURPOSES ONLY!

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Figure 4 Oil Cooler in High Pressure System

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Distribution Operation
When the fuel boost pumps in the tanks are on and the LP fuel shut-off valve is
open, the fuel flows from the tank to the low pressure stage of the engine fuel
pump.
This low pressure fuel pump increases the fuel pressure from the tanks up to
approximately 175 psi and supplies the fuel across the oil cooler and fuel filter
to the high pressure stage of the fuel pump.
The pressure increase by the low pressure fuel pump is necessary so that the
high pressure stage does not have to draw the fuel from the tank by suction.
The low pressure fuel pump is needed to prevent cavitation at the inlet of the
high pressure fuel pump.
At maximum engine speed the high pressure fuel pump increases the fuel
pressure to approx. 900 psi. At this pump an overpressure relief valve is
needed to prevent damage to the following fuel system components.
The overpressure relief valve usually opens when, at a malfunction, the
pressure reaches approx. 1250 psi. At this point it releases some fuel back to
the inlet port of the high pressure fuel pump.
Fuel from the high pressure fuel pump then flows to the metering section of the
fuel control unit.
Note, that the pump always supplies more fuel than is needed for combustion.
The metering section lets only the metered fuel pass to the fuel nozzles. The
fuel that is not needed returns to the fuel pump by the bypass return line.
FOR TRAINING PURPOSES ONLY!

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Figure 5 Distribution System Operation

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Distribution Operation cont.

On some engines you find that this bypass fuel is used to cool the IDG oil
cooler, as shown in this example. When the bypass fuel passes through the
IDG oil cooler, it takes the heat from the IDG oil.
In low engine power conditions the hot bypass return fuel heats up the fuel
from the low pressure pump too much, so that the engine oil can not be cooled
sufficiently.
In this situation the bypass return fuel is fed back to the fuel tank by the fuel
recirculation system.
Some cold fuel from the low pressure stage is added to the hot bypass fuel so
that the fuel that enters the tank is not too hot.
FOR TRAINING PURPOSES ONLY!

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Figure 6 Fuel Recirculation

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FUEL SYSTEMS System Components
M15.11

FUEL SYSTEM COMPONENTS

Fuel Pump Operation

In this segment we will look at the build−up and operation of engine fuel
pumps.
Engine fuel pumps are usually made up of 2 stages: the low pressure stage
and the high pressure stage. The 2 stages are usually combined in a common
housing so that they need only 1 drive shaft.
Note, that the cut views show an engine fuel pump with the oil cooler and fuel
filter downstream of the high pressure stage.
The low pressure fuel pumps are usually impeller type pumps driven by the
accessory gearbox.
Impeller type pumps look similar to radial compressors. They have an impeller
wheel in the pump housing with an axial inlet port and radial outlet ports.
These pumps can supply a continuous flow of fluid, but they cannot create very
high pressure.
FOR TRAINING PURPOSES ONLY!

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Figure 7 Fuel Pumps

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Fuel Pump Operation cont.

High pressure fuel pumps are always positive displacement pumps. 2 basic
types of pumps are used on gas turbine engines:
S the gear type pump and
S the piston type pump.
The piston type pump has a pump housing with many pistons in a rotor
assembly. It usually also has a variable wobbleplate, which changes the stroke
of the pistons. Piston type pumps are used where very high pressures are
needed. They can create pressures of more than 2 000psi.
In the piston type pump the output depends on the engine speed and the
stroke of the pistons. The stroke of the pistons can be controlled via the
wobbleplate by a servo signal from the fuel control unit. Therefore this pump is
also used to meter the fuel for the combustion chamber.
If you want to know more about the operation of a piston type pump look at the
corresponding lesson in Module M11.11 Hydraulic power.
FOR TRAINING PURPOSES ONLY!

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Piston
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Wobble Plate

Figure 8 Piston Type Pump

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Fuel Pump Operation cont.

Gear type fuel pumps are most commonly used on turbofan engines. They
have 2 counter−rotating gears in a pump housing. The teeth of the gears carry
the fluid from the pump inlet to the outlet.
You have seen that the gear type pumps and piston type pumps are used as
high pressure pumps in engine fuel systems.
The next segment describes the build−up and operation of fuel filters and heat
exchangers.
FOR TRAINING PURPOSES ONLY!

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Figure 9 Gear Type Pump

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FUEL SYSTEMS System Components
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Fuel Filters
Fuel filters are necessary to remove contaminations from the fuel which might
block or reduce the flow passages of important system components.
You can find these filters in the low pressure part or in the high pressure part of
the engine fuel system. Some engines can even have low pressure filters and
high pressure filters.
Low pressure filters usually have disposable paper filter elements, and high
pressure filters have cleanable wire mesh filter elements.
Fuel filters can be integrated in the fuel pump housing, or they can be separate
components. These filters have
S an inlet port,
S a filter element in the filter bowl, and
S an outlet port.
This type of filter operates in the same way as a typical oil filter or hydraulic
filter.
FOR TRAINING PURPOSES ONLY!

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Figure 10 Fuel Filter

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Fuel Filters cont.

On some engines you can find fuel filters with 2 outlet ports:
S the main outlet port to the fuel control unit, and
S a servo outlet port to the servo fuel heater.
These filters also have 2 filter elements:
S the normal filter element in the filter bowl, and
S a filter screen for the servo fuel flow.
The filter screen for the servo fuel flow is usually finer than the normal filter
element.
It is often called a wash screen. This is because the normal discharge flow
washes away all particles which are caught by the screen when the servo fuel
passes through it.
Also note, that all filters in the fuel system are usually equipped with relief
valves, which open by differential pressure if the filter elements become
clogged.
FOR TRAINING PURPOSES ONLY!

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Figure 11 Fuel Filter with Servo Outlet

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Heat Exchangers
The heat exchangers that can be found in an engine fuel system are
S the engine oil cooler,
S the servo fuel heater, and
S the IDG oil cooler.
All these heat exchangers are usually fuel / oil heat exchangers.
FOR TRAINING PURPOSES ONLY!

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Figure 12 Heat Exchangers

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Heat Exchangers cont.

The heat exchangers in the engine fuel system can be either individual
components, or they can be installed on the fuel pump housing.
Here you can see 2 examples of engine oil coolers.
One is a separate component and the other one is installed on the engine fuel
pump.
If they are mounted on the fuel pump, less external fuel lines are needed. This
reduces the possibility of fuel leaks.
The servo fuel heater on an engine is often mounted directly onto the engine oil
cooler to save external lines.
The IDG oil cooler is always a separate component. You find it on the engine
wherever there is sufficient room for it.
FOR TRAINING PURPOSES ONLY!

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Figure 13 Heat Exchanger Locations

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Fuel Manifolds
The fuel supply line supplies the fuel from the fuel control unit at the accessory
gearbox to the fuel manifold at the combustion chamber. The fuel manifold
distributes the fuel to the individual fuel nozzles.
You usually find single fuel manifolds on modern gas turbine engines like this
one. You can easily see that the fuel manifold is a ring-shaped line which
surrounds the combustion case. This manifold supplies fuel to each individual
fuel nozzle.
The fuel supply lines and the fuel manifold carry fuel of a very high pressure.
To prevent fuel leaks in the hot environment of the HP compressor and
combustion case, the fuel lines are protected by shrouds.
Shrouds are used on fluid lines that are routed through critical areas of the
engine to catch fluid leaks and carry them away to the engine drain mast.
On older aircraft engines all the fuel lines and the fuel manifold are shrouded
as you can see on the left graphic.
FOR TRAINING PURPOSES ONLY!

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Figure 14 Fuel Manifold

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Fuel Manifolds cont.

The highest risk of leaks is at the connections of fluid lines.
Therefore, on modern engines only the connections of the fuel lines have
special leak protection. There are 2 methods to protect the connections of fluid
lines against leaks.
There are shrouded connections and double sealed connections.
Shrouded connections cannot prevent leaks, but they can catch leaks and
carry them away by the engine drain system.
The O-rings inside the shroud make sure that a leakage from the coupling nut
cannot get onto the hot combustion case.
A double sealed connection gives double safety against leakage.
A small transfer tube fits between the fuel manifold and the fuel nozzle. This
connection is equipped with an inner seal which is a pair of O−rings. A conical
seal between the fuel nozzle and the fuel manifold is the outer seal in this
double sealed connection.
FOR TRAINING PURPOSES ONLY!

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Fuel Manifold

Shroud
Fuel Manifold

Drain Line
FOR TRAINING PURPOSES ONLY!

Figure 15 Fuel Manifold Connections

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Fuel Manifolds cont.

On some older aircraft engines you can find 2 fuel supply lines and 2 fuel
manifolds instead of 1.
This design method was necessary, because the combustion chamber was
equipped with 2 types of fuel nozzles.
The primary manifold supplies the primary fuel nozzles and the secondary
manifold supplies the secondary fuel nozzles.
On modern aircraft you find this design method on APUs only. The reason for
primary and secondary fuel flow will be explained in the segment about the fuel
nozzles.
FOR TRAINING PURPOSES ONLY!

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Figure 16 Primary & Secondary Fuel Manifold

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Fuel Nozzles
The fuel nozzles supply the metered fuel into the forward part of the
combustion chamber. Their main function is to atomize the fuel so that it
vaporizes and burns as quickly as possible.
Atomizing means that the solid fuel flow is changed into a mist of millions of
microscopic fuel droplets. The smaller the droplets are, the quicker the fuel
vaporizes.
In this segment we look at 2 atomizing methods.
One uses the fuel spray nozzle and the other method uses the airspray nozzle.
FOR TRAINING PURPOSES ONLY!

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Figure 17 Atomization of Fuel

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Fuel Nozzles cont.

The so-called fuel spray nozzle is the most commonly used fuel nozzle on gas
turbine engines.
It atomizes the fuel by forcing it through a small orifice with high pressure. This
increases the outlet velocity of the fuel flow and tears it into very small fuel
droplets.
The simplest fuel spray nozzle is usually named a single flow nozzle or
simplex nozzle, which you can see in the cut-away view.
The fuel from the manifold flows through a swirl chamber before it reaches the
discharge orifice. This swirl chamber induces a swirl in the fuel to improve fuel
atomization.
The rate of swirl and the fuel pressure are the most important factors in good
atomization.
The disadvantage of the simplex nozzle is that it can only give good
atomization in a small fuel flow range.
This means, if such a nozzle gives good atomization for engine start and
ignition, it cannot supply sufficient fuel for take-off power because the
discharge orifice is too small.
FOR TRAINING PURPOSES ONLY!

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Nozzle with high fuel flow

FOR TRAINING PURPOSES ONLY!

Figure 18 Fuel Spray Nozzle

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Fuel Nozzles cont.

On the other hand, a nozzle that gives good atomization at high fuel flow rates
is very unsatisfactory at low engine speeds.
For a good atomization across a wide fuel flow range 2 sets of simplex nozzles
are necessary.
One set with small discharge orifices is used for engine start up to
approximately idle speed. These nozzles are usually called the primary fuel
nozzles.
A second set of fuel nozzles have larger discharge orifices to supply fuel for the
acceleration up to maximum power. These nozzles are called the secondary
fuel nozzles.
The primary and secondary fuel nozzles are usually supplied by independent
primary and secondary fuel manifolds.
FOR TRAINING PURPOSES ONLY!

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Nozzle with low fuel flow

FOR TRAINING PURPOSES ONLY!

Figure 19 Fuel Spray Nozzle - Low Fuel Flow

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Fuel Nozzles cont.

Modern engines usually do not have separate simplex nozzles for primary and
secondary flow. Here you can find the so-called duplex nozzles.
These are dual flow nozzles, which combine the 2 nozzles in 1 component.
This means a dual flow nozzle has 2 discharge orifices and 2 internal fuel
supply lines. The primary orifice is the smallest one, which you can find in the
center of the fuel nozzle, and the larger secondary orifice is concentrically
arranged around the primary orifice.
If you remember the operation of a secondary flow nozzle at low fuel
pressures, it must be clear to you that the secondary flow must be avoided in a
low pressure condition.
Therefore, the secondary fuel flow is controlled by a spring loaded pressurizing
valve.
This valve, which is also called the flow divider valve, is usually an internal
component of the fuel nozzles. It opens by the increasing fuel pressure when
the engine speed increases, and it closes when the engine speed decreases.
The flow divider valve can also be a single component as on some old aircraft
engines and on many APUs. It then supplies the primary and the secondary
fuel manifold.
FOR TRAINING PURPOSES ONLY!

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Figure 20 Duplex Nozzles & Flow Divider Valve

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Fuel Nozzles cont.

The 2 types of fuel spray nozzles also pick up some of the airflow that enters
the combustion chamber.
This airflow passes through slots and holes in the outer shell of the nozzle tip,
cooling the material of the fuel nozzle and blowing away all residual fuel from
the orifices when the engine is shut down.
This prevents the formation of carbon at the orifices.
Another method to prevent carbon formation is by check valves in the fuel
nozzle. The check valve closes when the fuel pressure in the fuel manifold
decreases after the HP fuel shut-off valve is closed.
This makes sure that no fuel enters the combustion chamber from the manifold
by gravity.
FOR TRAINING PURPOSES ONLY!

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Check Valve
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Figure 21 Fuel Spray Nozzle Cooling & Check Valve

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Fuel Nozzles cont.

Another method of fuel atomization is used in the so-called airspray nozzle. It
uses a high velocity of air instead of a high velocity of fuel to cause
atomization.
This method gives good atomization even at low fuel flows, but it always needs
a high airflow through the nozzle.
The airspray nozzle uses a large inner airflow, which rotates after it passes
through the inner swirler. When this airflow meets the fuel at the tip of the
nozzle, it tears the solid fuel flow into small droplets.
This effect is further improved by an outer airflow which passes through an
outer swirler. In this nozzle the fuel flow is atomized by the airflow which
passes through it.
The airspray nozzle is always a single flow nozzle, because it can supply
sufficient fuel for all operating conditions of the engine.
In the same way as on the fuel spray nozzle the bypassing airflow cools the
nozzle and prevents formation of carbon at the tip of the nozzle.
FOR TRAINING PURPOSES ONLY!

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Figure 22 Air Spray Nozzle

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FUEL METERING SYSTEM

Introduction
The main purpose of the fuel control unit is to meter the fuel necessary for all
operating conditions of the engine.
Engine operating conditions are:
S starting,
S idle speed operation,
S acceleration,
S constant speed operation,
S deceleration, and
S engine shut-down.
The operating conditions can be split in 2 categories which are important for
fuel metering. These categories are
S steady state operation, and
S transient operation.
Steady state operation means that the speed or the thrust of the engine is kept
constant.
Transient operation means that the speed or the thrust of the engine is
increased or decreased.
Steady state conditions are idle speed and constant speed operations.
Transient conditions are acceleration, deceleration, engine starting and
shut-down.
FOR TRAINING PURPOSES ONLY!

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Figure 23 Engine Operating Conditions

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Fuel Metering Demands

The relation between the fuel flow to the fuel nozzles and the resulting core
engine speed can be shown by a diagram like this one. It shows the fuel flow
on the vertical axis and the engine speed on the horizontal axis.
For idle speed the engine needs a low fuel flow, and for maximum N2 it needs
a high fuel flow. If you connect all the points on the diagram you get the steady
state curve, which shows the fuel demand for each engine speed.
Fuel metering for steady state operation is very simple, because the fuel
control unit only has to supply the necessary fuel to keep the selected engine
speed.
Fuel metering for transient operations is more difficult, because the fuel control
unit must change the speed as fast as possible, but it must also make sure that
the speed changes as safely as possible. This means that the fuel control unit
must make sure that during speed changes the engine operating limits are not
reached.
These limits are:
S the overtemperature limit,
S the overspeed limit,
S the compressor stall limit, and
S the flame out limit.
FOR TRAINING PURPOSES ONLY!

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Figure 24 Fuel Metering Demands & Operating Limits

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Idle Speed Control

The control of the engine speed takes place in the computing section of the
fuel control unit. This section receives all external signals which are necessary
for control. The main signal for idle speed control comes from the thrust lever in
the cockpit. When the thrust lever is in the idle position, the fuel control unit
operates in the idle speed control condition.
The idle speed is the lowest speed at which an engine operates. It must be as
low as possible to save fuel, but it must be high enough to give a minimum of
thrust, because the aircraft uses idle thrust for taxiing manoeuvres on ground.
This engine speed is often called the minimum idle speed. The value of the idle
speed, however, depends mainly on the type of engine. It is usually in the
range of 50 to 65% N2.
The computing section in the fuel control unit gets a demand signal from the
throttle lever. It now compares the actual N2 from the engine with the idle
demand signal to control the metering valve in the metering section.
The minimum idle speed gives a minimum idle thrust, but the thrust of the
engine does not only depend on the rotor speed. The thrust changes when the
ambient conditions change, and it changes when the engine supplies bleed air
for the pneumatic system.
The idle thrust decreases when the ambient temperature increases or the
ambient pressure decreases. To compensate for these thrust changes, the fuel
control unit receives signals about the ambient temperature and pressure.
These signals come either from pressure or temperature ports at the engine
inlet, or on other engines it comes from the compressor inlet temperature
sensor at the inlet of the high pressure compressor. This is possible because
the ambient temperature is the most important factor on ground and the
compressor inlet temperature is always fixed in relation to the engine inlet
FOR TRAINING PURPOSES ONLY!

temperature.

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Figure 25 Idle Speed Control

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Idle Speed Control cont.

To compensate for thrust changes because of engine bleed supply, the fuel
control unit receives the compressor discharge pressure (CDP).
When the engine supplies bleed air to the pneumatic system, the CDP
decreases and the fuel control unit increases the fuel supply to increase the
idle speed.
This also makes sure that the engine always supplies sufficient air pressure to
the pneumatic system at idle speed.
FOR TRAINING PURPOSES ONLY!

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Figure 26 Influence of Bleed Air Supply

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Idle Speed Control cont.

On large high-bypass turbofan engines the disadvantage of minimum idle
speed is that it takes a long time for the engine to accelerate from idle to
go-around power because of the high inertia of the fan rotor system.
These engines have a second idle speed, which is higher than the minimum
idle speed. This so-called approach idle, or flight idle speed, makes sure that in
an emergency the engine can accelerate to go-around thrust as quickly as
possible.
To switch from minimum idle to approach idle, the fuel control unit receives
signals about the aircraft configuration which either come from the air ground
logic, or from other suitable configuration switches.
FOR TRAINING PURPOSES ONLY!

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Figure 27 Approach Idle

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Acceleration Control
When the pilot pushes the thrust levers forward, he transmits a speed change
signal to the fuel control unit on the engine. This signal from the thrust lever to
the fuel control unit is usually called the N2 command signal.
The computing section in the fuel control unit makes sure that the engine
accelerates to the selected speed as quickly and as safely as possible.
The fuel control unit meters the fuel for acceleration from idle to a selected part
power condition.
Acceleration control first of all means that the fuel control unit must increase
the fuel flow to the combustion chamber. This is possible because the fuel
pump always supplies much more fuel than necessary for combustion.
The pump supply line shows how much fuel flow the pump supplies at each
engine speed.
You can see in this example that at idle the fuel pump supplies approx. 3 times
more fuel than needed.
FOR TRAINING PURPOSES ONLY!

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Figure 28 N2 Command
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Acceleration Control cont.

For a short acceleration time the N2 command signal tries to fully open the fuel
metering valve to get all the fuel that the pump supplies. This condition must be
avoided because it can damage the engine.
Too much fuel during acceleration can lead to compressor stall, because the
gas volume in the combustion chamber increases so rapidly that it cannot
leave the engine fast enough via the turbine.
As a result of this danger, the fuel control unit makes sure that the fuel flow
cannot reach the stall margin for a given engine speed.
The fuel control unit stops the opening of the fuel metering valve if the fuel flow
comes close to the stall limit, and then waits for the engine speed to increase.
When the speed is increased, there is less danger of compressor stall, so the
fuel control unit gradually increases the fuel flow again. You can see that the
fuel control unit meters the fuel for the acceleration process, so there is always
a safe distance to the compressor stall limit.
The fuel control unit stops the opening of the fuel metering valve shortly after
the necessary fuel flow for the selected speed is reached. It then closes the
fuel metering valve very slowly until the engine has reached the selected
speed.
The line that shows the fuel supply during acceleration is usually called the
acceleration line.
To observe the stall limit during acceleration control, the fuel control unit needs
the actual engine speed N2 and the compressor inlet temperature CIT. For
acceleration to maximum power the fuel control unit also needs the compressor
discharge pressure CDP. The CDP signal limits the maximum fuel flow to
prevent overpressure and overtemperature in high power conditions.
FOR TRAINING PURPOSES ONLY!

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Figure 29 Limits for Acceleration Control

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Deceleration Control
When you pull the thrust levers, you want the engine to decelerate as quickly
as possible without shutting down.
This means that the fuel control unit must decrease the fuel flow as far as
possible, but it must also make sure that the flame in the combustion chamber
does not go out. This limitation is shown by the flame out limit in the diagram.
When the fuel flow for a given engine speed comes close to the flame out limit,
the fuel control unit stops the closing of the fuel metering valve. It then waits for
the engine speed to decrease, and it then continues to close the fuel metering
valve with the decreasing engine speed. The deceleration line shows that the
fuel control unit always keeps a safe distance from the flame out limit.
Flame out occurs if the air/fuel ratio becomes too large. Therefore the fuel
control unit needs parameters that determine the airflow through the
compressor during engine deceleration.
These parameters again are:
S the engine speed,
S the compressor inlet temperature, and
S the compressor discharge pressure.
FOR TRAINING PURPOSES ONLY!

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Figure 30 Deceleration Control

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Constant Speed Control

Constant speed control is the simplest fuel metering principle for the fuel
control unit. The fuel control unit only has to compare the demanded speed
with the actual speed and keep it constant.
The fuel control unit must keep the demanded speed constant, because
changing ambient conditions or load changes of the bleed load, electrical load,
or hydraulic load can affect the speed of the engine.
The fuel control unit senses the speed change via the speed signal from the
accessory gearbox and changes the fuel flow to bring the engine speed back to
the demanded speed.
Constant speed control is a very simple method, but remember that constant
speed does not mean constant thrust.
As a result most modern engines have a constant thrust control, which you will
see in the next segment.
FOR TRAINING PURPOSES ONLY!

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Figure 31 Constant Speed Control

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Constant Thrust Control

Most modern turbofan engines have constant thrust fuel metering systems for
steady state operation.
The pilot still selects the thrust with the thrust lever as usual, but this signal is
used to control the power of the engine. Therefore these engines have an
additional power management system, which is either a separate computer, or
an integral part of the fuel control unit.
The power management system makes sure that the engine always supplies
the same constant thrust for the given thrust lever position.
The power management system uses the thrust lever position as the thrust
demand signal. This position is usually called the thrust lever angle or (TLA) in
short. Each thrust lever angle always represents a fixed thrust value, as shown
in this diagram.
The power management system uses the thrust lever angle to calculate which
fan rotor speed N1, or on other engines which engine pressure ratio is needed
to get the selected thrust. This calculated thrust is the thrust command signal
that is given to the computing section.
FOR TRAINING PURPOSES ONLY!

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Figure 32 Constant Thrust Control

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Constant Thrust Control cont.

The following example shows how constant thrust is controlled on an engine
that uses the fan speed N1 to set the thrust.
With a constant fan speed, the thrust changes with the ambient conditions.
Therefore, the power management system needs information about the
ambient pressure and the ambient temperature to correct the N1 thrust
command.
For a given thrust lever angle the power management calculates the N1
command as shown.
When the ambient conditions change, the density of the air changes. The
power management system increases the N1 command if the air density is low,
and it decreases the N1 command if the density is high.
The computing section in the fuel control unit compares the N1 command
signal with the actual N1 speed signal and, if necessary, it sends control
signals to the metering section to get the commanded N1, which is equal to the
commanded thrust.
FOR TRAINING PURPOSES ONLY!

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TLA

Figure 33 Constant Thrust Control

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HYDROMECHANICAL FUEL CONTROLS

Introduction
The fuel control unit of an engine is usually mounted to the engine fuel pump
on the accessory gearbox. On most turbofan engines the fuel control unit is a
hydromechanical component.
In a hydromechanical fuel control unit all the control functions are done by fluid
pressures and mechanical components.
The engine manufacturers use many different names for their fuel control units
to point out special tasks and capabilities of these components.
You can find names like main engine control (MEC) or fuel flow regulator, but
to avoid confusion the name fuel control unit is used throughout this lesson.
FOR TRAINING PURPOSES ONLY!

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Figure 34 Fuel Control Units

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Introduction cont.
There are 2 different types of fuel control units:
S speed governed fuel control units, which you usually find on older engine
types, or
S constant thrust fuel control units used on more modern turbofan engines.
Speed governed fuel control units mainly use the N2 demand signal and the N2
speed feedback signal for steady state fuel metering.
Constant thrust fuel control units use similar signals like the speed governed
fuel control units, and they additionally use thrust feedback signals like N1
speed, ambient pressure, and temperature signals.
FOR TRAINING PURPOSES ONLY!

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Figure 35 Types of Fuel Control Units

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Introduction cont.
You can see that this fuel control unit has many pipes and push-pull cables
connected to it. Some of them are needed to transmit the throttle demand
signal, the temperature signal, the pressure signal, and the speed feedback
signals for fuel metering, and others are needed to control the engine
compressor.
The thrust lever signal is usually a mechanical deflection of a small power lever
at the fuel control unit. This is either done by a rack and pinion transmission or
by rods and levers. The N2 speed feedback signal is usually transmitted via a
mechanical driveshaft from the main fuel pump.
Some fuel control units even get N1 speed feedback signals. This signal is a
fuel pressure signal which comes from a hydromechanical N1 speed sensor.
The temperature signals like CIT or fan inlet temperature come from
hydromechanical temperature sensors. These temperature sensors convert the
air temperatures to fuel pressure signals.
The fuel control unit receives pressure signals like CDP or ambient pressure
via air sense lines. Pressure sensors in the fuel control unit convert the
pressure signals into mechanical signals.
So in summary hydromechanical fuel control units use hydraulic, mechanical,
and pneumatic signals for their operation.
FOR TRAINING PURPOSES ONLY!

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Figure 36 Signals for the Fuel Control Unit

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Fuel Control Unit Organization

To help you to understand the operation of a fuel control unit we can split it into
2 major sections:
S the fuel metering section, and
S the computing section.
The computing section can be further split into
S a governing section, and
S a limiting section.
These 3 main sections of course are not individual chambers inside the
housing of the fuel control unit. They represent the main tasks of a fuel control
unit. These are:
S fuel metering,
S power control, and
S engine protection.
The metering section makes sure that the necessary fuel gets to the fuel
nozzles and all the fuel, which is not needed for combustion, returns to the fuel
pump.
The governing section makes sure that the selected power is controlled.
The limiting section of the fuel control unit monitors the governing section and
makes sure that the engine always operates within safe limits.
FOR TRAINING PURPOSES ONLY!

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Figure 37 Fuel Control Unit Organization

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Fuel Metering Section

A simplified fuel metering section has 3 main components. These are
S the fuel metering valve,
S the bypass valve, and
S the high pressure shut-off valve.
The fuel metering valve controls the fuel flow to the combustion chamber. It is
activated by an actuator which receives the opening pressure from the limiting
section.
The bypass valve returns all the fuel that is not needed for combustion back to
the fuel pump. In this example it also has a second function. It controls a
constant pressure difference across the fuel metering valve. It is activated by
fuel pressure from upstream and downstream of the fuel metering valve.
The high pressure fuel shut-off valve is used to supply or cut off the fuel to
the combustion chamber. It is activated by the pilots and operated by fuel
pressure.
FOR TRAINING PURPOSES ONLY!

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Figure 38 Metering Sections Components

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Fuel Metering Section cont.

Simply speaking you may say that the fuel flow through the metering valve
depends on
S the opening area of the valve,
S the pump delivery pressure, and
S the pressure behind the valve.
The indicator symbols show the fuel pressure upstream and downstream of the
valve.
With a high pressure difference across the valve there is also a high fuel flow.
The flow quantity through a valve changes with
S the flow area and with
S the differential pressure across the valve.
For correct fuel metering one of the 2 parameters must be kept at a constant
value. Usually the pressure difference across the valve is kept constant all the
time, so that the fuel flow only depends on the flow area, or in other words on
the valve position.
The fuel metering section always has a differential pressure valve which
controls a constant pressure difference across the fuel metering valve.
Often this differential pressure valve is combined with the bypass valve, as you
can see in this example.
FOR TRAINING PURPOSES ONLY!

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Figure 39 Metering Section Operation

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Fuel Metering Section cont.

When the metering valve opens, the pressure in front of the valve decreases
and the pressure behind the valve increases. This change in differential
pressure acts on the piston of the bypass valve and moves it to the left.
This closes the flow area for the bypass fuel, which subsequently increases the
fuel pressure in front of the metering valve until the original differential pressure
is reached again.
When the metering valve closes, the pressure in front of the valve increases
and behind the valve decreases. This change in differential pressure acts on
the piston of the bypass valve and moves it to the right.
This opens the flow area for the bypass fuel, which decreases the fuel pressure
in front of the metering valve until the original differential pressure is reached
again.
The bypass valve makes sure that there is always a constant differential
pressure across the fuel metering valve.
With an adjustment screw at the spring of this valve you can change the
differential pressure setting. This is needed to adjust the fuel control unit to
fuels with different specific gravity.
Bellows between the adjustment screw and the spring of the bypass valve are
needed to compensate gravity changes because of different fuel temperatures.
FOR TRAINING PURPOSES ONLY!

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Figure 40 Metering Section Operation

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Fuel Metering Section cont.

On engines with piston type fuel pumps the fuel metering system is a little bit
different.
The differential pressure valve has a servo pressure line that is connected to
the control actuator of the wobble plate at the fuel pump. There is no bypass
fuel return flow, because the piston type fuel pump can supply the exact
quantity of fuel that is needed for combustion.
Instead of the fuel metering valve the fuel control unit has a so-called throttle
valve.
When the throttle valve closes, the pressure difference across the valve
increases and the piston of the differential pressure valve moves to the right.
This movement opens a flow passage for the control pressure, and the control
actuator moves the wobble plate so that the pump supplies less fuel.
Less fuel supply causes lower supply pressure, which also decreases the
differential pressure again so that the piston of the differential pressure valve
returns to the neutral position.
When the throttle valve opens, the pressure difference across the valve
decreases and the differential pressure valve moves to the left.
This movement opens a flow passage for the control pressure and the actuator
moves the wobble plate so that the pump supplies more fuel.
More fuel supply causes higher supply pressure, which increases the
differential pressure again so that the piston of the differential pressure valve
returns to the neutral position.
FOR TRAINING PURPOSES ONLY!

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Figure 41 Metering with Piston Type Pumps

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Governing Section
The governing section in a fuel control unit controls the activation of the fuel
metering valve.
Its main component is the N2 governor, which is rotated by the mechanical
drive from the accessory gearbox. The governor also receives input signals
from the power lever via the speed setting lever.
The power lever on the fuel control unit is in a mid-position if the thrust lever in
the cockpit is in idle position. The power lever can move counter clockwise for
forward thrust, or clockwise for reverse thrust.
The power lever rotates a throttle cam clockwise, or counter clockwise. The
speed setting lever follows the contour of the throttle cam and compresses the
spring of the N2 governor. This signal moves the pilot valve inside the N2
governor downwards.
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Figure 42 Governing Section

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Governing Section cont.

A simplified governing section operates as follows:
The speed setting lever pushes down the spring and the pilot valve of the N2
governor.
This opens a flow passage for the servo fuel and the servo fuel opens the fuel
metering valve.
More fuel is supplied to the combustion chamber and the engine speed will
increase.
With increasing engine speed the centrifugal forces on the flyweights increase
more and more until they are strong enough to overcome the spring force.
In this condition the flyweights deflect outwards and by doing so they pull the
pilot valve back to the neutral position.
When the pilot valve is in the neutral position, the control pressure for the fuel
metering valve is captured and the fuel metering valve stays in its last position.
In an overspeed condition the flyweights pull the pilot valve further upwards.
Some of the control pressure for the fuel metering valve can escape and the
fuel metering valve closes a little bit.
The engine speed decreases and the flyweights return to the neutral position.
You usually find 2 adjustment screws in the governing section. There is the idle
adjustment screw to adjust the minimum idle speed of the engine.
With a part power adjustment screw you can synchronize all the engines on
one aircraft.
This makes sure that at a given thrust lever angle all engines have the same
speed.
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Figure 43 Governing Section

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Limiting Section
Flyweight governors have a major disadvantage:
They cannot schedule safe fuel flows when confronted with conditions, in which
the actual engine speed is much different from the desired speed.
When the force balance between the flyweights and the governor spring is
upset too much, the fuel metering valve goes full open or full closed. Obviously
this is not an acceptable fuel flow.
Therefore, the limiting section has authority over the governor to limit the
movement of the fuel metering valve under these conditions. The limiting
section also makes sure that the engine operating limits like the compressor
stall limit, the overspeed limit, the overboost limit, and the flame out limit are
not reached.
A typical limiting section has the following main components:
S a limit pilot valve between the N2 governor and the fuel metering valve,
S a so-called 3−D cam,
S a CDP cam, and
S interconnecting mechanical control linkage.
The limit pilot valve controls the servo pressure to the fuel metering valve.
The surface of the 3−dimensional cam represents the stall limit and the
maximum speed limit of the engine. The cam is twisted by changes in the
engine speed, and it is moved axially by changes in the compressor inlet
temperature. A feeler pin follows the surface of the 3−D cam and transmits its
position to the limit control linkage.
The CDP cam is twisted by changes in the compressor discharge pressure.
The surface of the cam, which is scanned by a feeler pin, represents the value
of the compressor discharge pressure. The CDP feeler pin transmits its
FOR TRAINING PURPOSES ONLY!

position to the limit control linkage.

The mechanical limit control linkage connects the main components of the
limiting section to control the position of the limit pilot valve. It also receives
the position feedback signal from the fuel metering valve.

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Figure 44 Limiting Sections

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Limiting Section cont.

When the governor pilot valve is opened by the thrust lever input signal, fuel
servo pressure flows to the fuel metering valve.
The fuel metering valve opens and the feedback linkage lifts the piston of the
limit pilot valve.
This shuts off the fuel servo pressure from the fuel metering valve. The limit
pilot valve stops the opening movement of the fuel metering valve.
The fuel flow to the combustion chamber is increased.
The engine speed increases and at the same time the compressor discharge
pressure increases.
Note, that the engine speed is still not high enough to balance the flyweights,
but the increasing CDP has an effect on the fuel metering now.
When the compressor discharge pressure increases, the CDP cam twists
clockwise and pushes the feeler pin downwards.
By this movement the limit pilot valve opens slightly, and servo fuel pressure
gets to the fuel metering valve to further open it until this opening is stopped
again by the feedback linkage.
The engine speed increases further, and the above mentioned sequence
continues until the flyweights in the N2 governor cut off the servo fuel from the
limit pilot valve.
When the CDP increases above the maximum limit, the limit pilot valve moves
up and releases the opening pressure of the fuel metering valve.
The fuel metering valve closes a little bit until the feedback linkage has closed
the limit control valve again.
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Figure 45 Limiting Section Operation

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Limiting Section cont.

The 3−D cam is used during acceleration to prevent the engine reaching the
stall limit of the HP compressor or the overspeed limit.
You can see that the feeler pin at the 3−D cam moves the feedback linkage
when the 3−D cam is twisted by the N2 signal.
The feeler pin also moves the feedback linkage when the 3−D cam is shifted
back and forward by the CIT signal.
As a conclusion we can say, that all external input signals like CDP, N2, or CIT
effect the fuel metering.
The compressor discharge pressure is transmitted to the CDP sensor on the
fuel control unit. This signal twists the CDP cam by a servo piston.
The engine speed N2 acts on a flyweight governor. This signal twists the 3−D
cam by a servo piston.
The compressor inlet temperature is sensed by a hydromechanical
temperature sensor. The CIT signal moves the 3−D cam in axial direction by a
servo piston.
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Figure 46 Limiting Section Operation

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Limiting Section cont.

The CIT sensor is usually installed in the fan frame in front of the HP
compressor.
2 fuel pressure lines connect the CIT sensor with the fuel control unit. The
sensor changes the fuel pressure in these lines depending on the compressor
inlet temperature.
The CDP sensor is usually an internal component of the fuel control unit. It
receives the compressor discharge pressure via a sense line from the
combustion case.
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Figure 47 CDP & CIT Sensor Location

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Constant Thrust Control

This segment describes improved hydromechanical fuel control units that are
used to achieve thrust control.
For thrust control the changes in air density must be compensated. A density
compensation mechanism changes the power lever input signal to the N2
governor when the density changes.
The density compensation mechanism decreases the power lever input signal
to the N2 governor when the density increases, and it increases the power
lever input signal when the density decreases. To operate correctly, the density
compensation needs external signals.
The fuel control unit receives information about the ambient temperature and
pressure for the density compensation.
In our example these signals are the static pressure Ps12. The density
compensation results in a corrected N2 speed governing but the N2, as you
know, is not always in a fixed relation to the thrust of the engine.
This means to control a constant thrust, the fuel control unit also needs
information about the thrust of the engine like the N1 or the engine pressure
ratio.
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Figure 48 Constant Thrust Fuel Control

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Constant Thrust Control cont.

Most fuel control units use an external power management computer to keep
the selected thrust constant.
The power management computer, which is called the PMC, is an electronic
computer usually installed on the fan stator case of the engine.
The power management computer calculates the necessary N1 based on the
throttle lever input and on the ambient conditions.
The PMC sends a control signal to a torque motor on the fuel control unit if the
actual N1 is different from the calculated N1.
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Figure 49 Power Management Computer

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Constant Thrust Control cont.

On some fuel control units, as shown here, the torque motor acts on the
governor input lever to override the power lever input signal and the
mechanical density correction.
On other fuel control units the torque motor acts on an additional pilot valve
which can override the N2 governor pilot valve and the limit pilot valve.
Torque motor controlled fuel control units can react much faster, because the
power management computer uses electrical signals from an electrical engine
inlet temperature sensor and from the fan speed sensor. The ambient pressure
is also converted into an electrical signal by an internal pressure transducer.
Note, that a torque motor controlled hydromechanical fuel control unit does not
depend on the power management computer. It can still do the fuel metering
for all operating conditions, even if the PMC or an electrical sensor fails,
because by hydromechanical sensors it also receives the ambient pressure,
the fan inlet temperature, and on some engines even the fan speed N1.
This type of fuel control unit is actually the first step to a full authority digital
engine control also called FADEC.
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Figure 50 Torque motor operation

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FADEC SYSTEM

Introduction
The term FADEC is an abbreviation for ’Full Authority Digital Engine Control’.
As the name indicates, in a FADEC system a digital computer has full authority
over the engine control functions. The digital computer is the heart of the
FADEC system.
It is usually named the electronic control unit (ECU) or on other engines it is
named the electronic engine control (EEC). To avoid confusion, we use the
term ECU throughout this segment.
The second main component of the FADEC system is the fuel metering unit
(FMU). This component is also named the hydromechanical unit, or HMU on
other engines. To avoid confusion, we use the term FMU throughout this
segment.
The terms FMU and HMU do not have a letter “C”. This indicates that the FMU
cannot control. It only receives orders from the ECU to move the fuel metering
valve.
Engines with a FADEC system do not have a hydromechanical fuel control unit
installed.
You usually find the electronic control unit on the fan stator case of the engine.
The fuel metering unit is at the same location as the fuel control unit on older
engines.
To operate correctly the electronic control unit needs the demand signal from
the thrust lever in the cockpit.
It also needs to know the engine speeds and all important air temperatures and
air pressures in the engine, and it certainly needs electrical power supply for its
FOR TRAINING PURPOSES ONLY!

operation.
This power supply either comes from the aircraft, or it can also come from a
small permanent magnet alternator on the engine accessory gearbox.
The ECU also needs a feedback signal about the opening condition of the fuel
metering valve.

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Figure 51 ECU & FMU Location

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Introduction cont.
The ECU can fulfil many tasks in addition to fuel metering and engine limit
protection.
It performs full power management and gives optimum thrust control for all
operating conditions. It also controls other engine subsystems like
S the compressor stall protection system,
S the turbine and compressor clearance control system,
S the thrust reverser system,
S the engine starting system, and
S the engine indication system.
With all the data that the ECU receives, it permanently monitors the engine
operation and the important system components and gives fault messages to
the centralized aircraft maintenance computer to indicate faulty components.
On some aircraft there are so-called engine interface units installed. These
EIUs transmit the data between the FADEC system and the aircraft. They also
control the power supply from the aircraft to the ECU.
A typical FADEC system has
S an electronic control unit,
S a fuel metering unit,
S many electrical sensors on the engine
S and many control possibilities for other engine sub systems.
The FADEC system is a centralized computer with all necessary sensors which
controls the engine and all its important sub systems.
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Figure 52 FADEC System

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ECU Organization
The FADEC system must be fully redundant.
This means that each electronic control unit has 2 independent computers.
The 2 computers are called channel A and channel B.
The 2 computers are made by a dual set of electronic cards, 1 set for each
channel. These cards are installed in a common housing.
In this ECU one set of cards is the channel A computer, and the other set of
cards is the channel B computer.
Each channel of the ECU receives individual signals from the aircraft and from
the engine sensors.
Signals from the engine to the ECU are either electric signals or pneumatic
signals. For safety reasons all electrical signals are duplicated and transmitted
individually to the 2 ECU channels.
There are 2 temperature pick-ups in the T12 temperature sensor. Each pick-up
is connected with an individual wire to the ECU.
All electrical sensors on the engine have individual pick-ups that are connected
to the respective ECU channel.
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Figure 53 ECU Organization

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ECU Organization cont.

Pressure signals from the engine are usually pneumatic signals.
The ECU receives the pneumatic signals via pressure sense lines from the
engine. There is usually only 1 pressure sense line from the respective engine
station routed to the ECU.
The air pressures are converted into digital signals by pressure transducers
inside the ECU.
There are always 2 individual transducers for each pressure sense line. This
means each channel has its own pressure transducer.
Electrical signals to the ECU can be analog signals, which are transmitted via
standard electrical wires, or they can be digital signals transmitted via ARINC
data busses.
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Figure 54 Signals to / from the ECU

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ECU Operation
If we take a look inside a typical electronic control unit, we can split it into a
power management section and a governing and limiting section.
Software in the ECU performs all the fuel metering and constant thrust control.
For this control the ECU receives basically the same data as the
hydromechanical constant thrust control.
The power management calculates the thrust command based on the thrust
lever angle, ambient air temperature, and pressure such as T12 and Ps12.
It also uses the Mach number for information about the aircraft speed. With the
information about the Mach number, the ECU corrects the change in thrust due
to the influence of the airspeed.
Another basic input for power management is made by the so-called thrust
rating plug. This is a connector from the engine to the ECU used for engine
identification.
By the connection with the thrust rating plug the ECU knows on which engine
type it is installed. This means that a standard ECU can be used for all thrust
variants of one engine family like the CFM 56−5A1 or CFM 56−5A3 engine.
The thrust rating plug makes sure that the ECU controls the engine to the
correct maximum take-off thrust.
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Figure 55 ECU Operation

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ECU Operation cont.
The power management section transmits the thrust command signal to the
governing and limiting section. This signal is either a N1 command signal or a
EPR command signal.
The governing section compares the thrust command signal with the actual N1
signal and controls an error signal to the fuel metering valve if the command
signal is different to the actual N1 signal.
The limiting section receives the N2, CDP, and CIT to make sure that the
engine operational limits are not exceeded.
The T25 signal represents the compressor inlet temperature. It is used in the
limiting section to protect the engine against compressor surge.
The Ps3 signal represents the compressor discharge pressure. It is used to
protect the engine against overboost.
The governing and limiting section receives a position feedback signal from the
fuel metering valve to control the necessary fuel flow.
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Figure 56 ECU Operation

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FMU / HMU Organization

The main components of a typical fuel metering unit are
S the fuel metering valve,
S the bypass valve,
S the HP shut-off valve, and
S a servo valve with a torque motor.
A fuel metering unit has almost the same components as the metering section
of a hydromechanical fuel control unit.
The torque motor receives control signals from the electronic control unit to
move the fuel metering valve. A position sensor continuously measures the
position of the fuel metering valve and transmits a feedback signal to the ECU.
You can see that this fuel metering unit still has a bypass valve. This valve is
necessary to keep the pressure difference across the fuel metering valve
constant so that the fuel flow is proportional to the opening area of the fuel
metering valve.
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Figure 57 FMU Organization

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HMU / FMU Organization cont.

Some fuel metering units additionally have an overspeed governor. This
governor reduces the fuel flow through the fuel metering valve if the electrical
overspeed protection by the ECU fails.
You probably remember that the FADEC system also controls other
subsystems of the engine.
Some of these subsystems operate with high fuel pressure from the engine fuel
pump. Therefore, on some fuel metering units you can find additional servo
valves with torque motors. These servo valves control
S variable stator vanes,
S variable bleed valves, and
S high pressure and low pressure turbine clearance control valves.
A fuel metering unit with all these additional servo valves is usually called the
hydromechanical unit (HMU).
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Figure 58 HMU Organization

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HMU / FMU Operation

When the ECU needs to change the speed of the engine, it transmits a control
current to the torque motor of the fuel metering valve.
The torque motor moves the servo valve to control the fuel pressure to the fuel
metering valve actuator.
The ECU continuously compares the actual N1 with the N1 command, and it
increases the fuel flow as long as the commanded N1 is higher than the actual
N1. If the commanded N1 is reached, the servo valve moves to the neutral
position and the fuel metering valve stays in its position.
In an overspeed condition the ECU first tries to decrease the fuel flow via the
fuel metering valve. If the ECU fails at a slightly higher overspeed, the
overspeed governor acts as a back-up.
When the flyweights deflect, the governor pilot valve releases the downstream
pressure of the bypass valve. The upstream pressure pushes the bypass valve
fully open so that more fuel returns to the fuel pump. As a result less fuel flows
across the fuel metering valve and the engine speed will decrease.
Note, that no adjustments are necessary on the fuel metering unit before or
after its installation on the engine.
The idle speed, for example, is programmed in the ECU. This means that the
ECU will change the position of the fuel metering valve until the programmed
idle speed is reached.
Part power adjustments are not necessary, because the ECU makes sure that
for a given thrust lever angle the engine always supplies the same thrust.
Also gravity adjustment is not necessary any more for the same reason.
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Figure 59 FMU Operation

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Engine / Aircraft Interface

Some aircraft with FADEC systems have additional electronic components for
data transfer between the ECU and the aircraft.
You can find these engine interface units in the electronic compartment of the
aircraft. As you can see here, there are 2 engine interface units in the
electronic compartment of this aircraft. 1 for each engine.
There are only 2 direct connections between the aircraft and the electronic
control unit. These signals, which are usually independent for safety reasons,
are the thrust lever signal to the ECU and the engine indication data from the
ECU.
All the other signals between aircraft and ECU are routed via the engine
interface unit like the ignition power supply and an electrical power supply for
the ECU.
Note, that during engine operation the electrical power supply to the ECU is
just a back-up to prevent loss of electrical power. Normally a permanent
magnet alternator on the engine accessory gearbox supplies the ECU with
electrical power.
The engine interface unit also receives discrete inputs from control switches in
the cockpit. It digitizes these inputs and sends them to the ECU via the data
bus. It also transfers all data between the ECU and other aircraft computers.
The advantage of the EIU in a FADEC system is that it limits the number of
signal wires between the aircraft and engine, because 1 single data bus can
transmit all digital data. This saves weight and reduces the risk of wiring faults.
FOR TRAINING PURPOSES ONLY!

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Figure 60 Engine/Aircraft Interface

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FADEC System Operation

The heart of the FADEC system is the electronic control unit.
The 2 channels in an ECU operate totally independently from each other. This
is possible because each channel has its own redundant power supply.
1 power supply comes from the aircraft, which is used for engine starting and
as a back-up. With the power supply from the permanent magnet alternator the
ECU is almost fully independent from the aircraft. It only needs the thrust lever
input signal.
The data from the aircraft is used to improve the operation of the FADEC
system, but the ECU can operate without it.
During operation of the ECU, the 2 channels are operating but only 1 channel
at a time has the authority to control the engine. This channel is called the
channel in command. It generates the control signals.
The other channel is in standby. The 2 channels are alternately in command.
The switch-over from one channel to the other one is usually at the beginning
of each engine start sequence.
FOR TRAINING PURPOSES ONLY!

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FUEL SYSTEMS FADEC
M15.11
FOR TRAINING PURPOSES ONLY!

Figure 61 Channel in Command

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FADEC System Operation cont.

During engine operation the 2 channels A and B always collect the data from
the engine and aircraft and they always communicate with each other via a
cross channel data link.
This internal communication is important to find out if the 2 channels are
operating correctly and if all input data is correct. This means that the channel
in command compares its own input data with the respective data from the
neighbor channel to find out if they are correct.
Let us now look at an example to see this operation.
Here channel B is in command and the two T25 signals for channel A and
channel B are different. Channel B now must find out which T25 signal is
correct. Therefore it uses other data to calculate T25.
Channel B calculates T25 based on the fan inlet temperature T12 and the fan
speed N1. This is possible because the temperature increase across the LP
compressor is always in a fixed relation to the fan speed.
Channel B uses its own signal if the calculated T25 is similar to it, but channel
B ignores its own T25 signal and uses the signal from channel A instead if the
calculated T25 is similar to the signal from channel A.
If 1 channel is faulty and unable to control, the command is taken over by the
other channel. This change in command is automatic, based on the fault status
of each channel. It is usually not visible to the operating personnel, because
the engine continues its operation.
To completely loose a control function, more than 1 fault is necessary.
If, for example, the variable bleed valves remain open because of a control
fault, the ECU tries to operate the engine with reduced efficiency. This means
that the engine is still operating, but the ECU cannot run the engine at optimum
efficiency.
FOR TRAINING PURPOSES ONLY!

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GAS TURBINE ENGINE EASA PART-66 M15
FUEL SYSTEMS FADEC
M15.11
FOR TRAINING PURPOSES ONLY!

Figure 62 ECU Data Management

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FUEL SYSTEMS FADEC
M15.11

FADEC System Operation cont.

During automatic engine start on ground, the ECU controls all the necessary
activities and continuously monitors the start sequence.
If something goes wrong during engine start, the ECU can interrupt and repeat
the start sequence. During engine operation the ECU can detect compressor
stalls, flame outs, and other abnormal conditions.
If a flame out occurs, the ECU automatically activates the ignition to restart the
combustion.
FOR TRAINING PURPOSES ONLY!

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FUEL SYSTEMS FADEC
M15.11
FOR TRAINING PURPOSES ONLY!

Figure 63 Starting Control

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TABLE OF CONTENTS
M15 GAS TURBINE ENGINE . . . . . . . . . . . . . 1 HMU / FMU OPERATION . . . . . . . . . . . . . . . . . . . . . . . . . . 118
ENGINE / AIRCRAFT INTERFACE . . . . . . . . . . . . . . . . . . 120
M15.11 FUEL SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . 1 FADEC SYSTEM OPERATION . . . . . . . . . . . . . . . . . . . . . 122
FUEL SYSTEM LAY-OUT . . . . . . . . . . . . . . . . . . . . . . . . . . 2
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
DISTRIBUTION LAY-OUT . . . . . . . . . . . . . . . . . . . . . . . . . . 4
DISTRIBUTION OPERATION . . . . . . . . . . . . . . . . . . . . . . 10
FUEL SYSTEM COMPONENTS . . . . . . . . . . . . . . . . . . . . 14
FUEL PUMP OPERATION . . . . . . . . . . . . . . . . . . . . . . . . . 14
FUEL FILTERS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
HEAT EXCHANGERS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
FUEL MANIFOLDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
FUEL NOZZLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
FUEL METERING SYSTEM . . . . . . . . . . . . . . . . . . . . . . . . 46
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
FUEL METERING DEMANDS . . . . . . . . . . . . . . . . . . . . . . 48
IDLE SPEED CONTROL . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
ACCELERATION CONTROL . . . . . . . . . . . . . . . . . . . . . . . 56
DECELERATION CONTROL . . . . . . . . . . . . . . . . . . . . . . . 60
CONSTANT SPEED CONTROL . . . . . . . . . . . . . . . . . . . . 62
CONSTANT THRUST CONTROL . . . . . . . . . . . . . . . . . . . 64
HYDROMECHANICAL FUEL CONTROLS . . . . . . . . . . . 68
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
FUEL CONTROL UNIT ORGANIZATION . . . . . . . . . . . . 74
FUEL METERING SECTION . . . . . . . . . . . . . . . . . . . . . . . 76
GOVERNING SECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
LIMITING SECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
CONSTANT THRUST CONTROL . . . . . . . . . . . . . . . . . . . 96
FADEC SYSTEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
ECU ORGANIZATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
ECU OPERATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
FMU / HMU ORGANIZATION . . . . . . . . . . . . . . . . . . . . . . 114

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TABLE OF FIGURES
Figure 1 Fuel Distribution System Purpose . . . . . . . . . . . . . . . . . . . . . . . . 3 Figure 36 Signals for the Fuel Control Unit . . . . . . . . . . . . . . . . . . . . . . . . 73
Figure 2 Fuel Distribution System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Figure 37 Fuel Control Unit Organization . . . . . . . . . . . . . . . . . . . . . . . . . 75
Figure 3 Fuel Distribution System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Figure 38 Metering Sections Components . . . . . . . . . . . . . . . . . . . . . . . . 77
Figure 4 Oil Cooler in High Pressure System . . . . . . . . . . . . . . . . . . . . . . 9 Figure 39 Metering Section Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
Figure 5 Distribution System Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Figure 40 Metering Section Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
Figure 6 Fuel Recirculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Figure 41 Metering with Piston Type Pumps . . . . . . . . . . . . . . . . . . . . . . . 83
Figure 7 Fuel Pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Figure 42 Governing Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
Figure 8 Piston Type Pump . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Figure 43 Governing Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
Figure 9 Gear Type Pump . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Figure 44 Limiting Sections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
Figure 10 Fuel Filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Figure 45 Limiting Section Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
Figure 11 Fuel Filter with Servo Outlet . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Figure 46 Limiting Section Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
Figure 12 Heat Exchangers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Figure 47 CDP & CIT Sensor Location . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
Figure 13 Heat Exchanger Locations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Figure 48 Constant Thrust Fuel Control . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
Figure 14 Fuel Manifold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 Figure 49 Power Management Computer . . . . . . . . . . . . . . . . . . . . . . . . . 99
Figure 15 Fuel Manifold Connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 Figure 50 Torque motor operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
Figure 16 Primary & Secondary Fuel Manifold . . . . . . . . . . . . . . . . . . . . . 33 Figure 51 ECU & FMU Location . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
Figure 17 Atomization of Fuel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 Figure 52 FADEC System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
Figure 18 Fuel Spray Nozzle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 Figure 53 ECU Organization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
Figure 19 Fuel Spray Nozzle - Low Fuel Flow . . . . . . . . . . . . . . . . . . . . . 39 Figure 54 Signals to / from the ECU . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
Figure 20 Duplex Nozzles & Flow Divider Valve . . . . . . . . . . . . . . . . . . . . 41 Figure 55 ECU Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
Figure 21 Fuel Spray Nozzle Cooling & Check Valve . . . . . . . . . . . . . . . 43 Figure 56 ECU Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
Figure 22 Air Spray Nozzle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 Figure 57 FMU Organization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
Figure 23 Engine Operating Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 Figure 58 HMU Organization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
Figure 24 Fuel Metering Demands & Operating Limits . . . . . . . . . . . . . . 49 Figure 59 FMU Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
Figure 25 Idle Speed Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 Figure 60 Engine/Aircraft Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
Figure 26 Influence of Bleed Air Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 Figure 61 Channel in Command . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
Figure 27 Approach Idle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 Figure 62 ECU Data Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
Figure 28 N2 Command . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 Figure 63 Starting Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
Figure 29 Limits for Acceleration Control . . . . . . . . . . . . . . . . . . . . . . . . . . 59
Figure 30 Deceleration Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
Figure 31 Constant Speed Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
Figure 32 Constant Thrust Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
Figure 33 Constant Thrust Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
Figure 34 Fuel Control Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
Figure 35 Types of Fuel Control Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

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