Fundamentals
M15
GAS TURBINE ENGINE
M15.18
Auxiliary Power Units (APUs)
EASA Part-66
CAT A
M15.18_A E
Rev.-ID: 1APR2013
Author:
DaC
For Training Purposes Only
ELTT Release: Jul. 10, 2013
Training Manual
For training purposes and internal use only.
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M15
GAS TURBINE ENGINE
M15.18
AUXILIARY POWER UNITS
EASA PART-66 M15
M15.18
FOR TRAINING PURPOSES ONLY!
Lufthansa Technical Training
GAS TURBINE ENGINE
AUXILIARY POWER UNITS
FRA US/O-5
DaC
May 21, 2013
ATA DOC
Page 1
EASA PART-66 M15
Purpose
M15.18
PURPOSE
Auxiliary Power Unit
The auxiliary power unit is a small gas turbine engine on the aircraft, which
supplies pneumatic bleed air and electrical power for the different aircraft
systems.
This gas turbine engine is often called the APU. It is mainly used on the ground
when the aircraft engines are not running.
On modern twin-engine aircraft the APU must also provide bleed air and
electrical power in flight as a back-up source in case of engine bleed air and/or
engine AC generator failure.
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01|APU Purpose|L1|A/B1
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EASA PART-66 M15
Purpose
M15.18
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Auxiliary Power Unit (APU)
01|APU Purpose|L1|A/B1
Page 3
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Purpose
M15.18
APU Sections
We can split the APU into 3 main sections:
S the power section
S the bleed section
S the accessory gearbox section.
The power section of the APU drives the compressor and the gearbox. This
gearbox drives all APU accessories like
S the fuel pump
S the oil pumps
S the cooling fan
S the AC−generator
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On very large APUs you may even find 2 AC−generators on the gearbox.
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02|APU Sections|L1|A/B1
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EASA PART-66 M15
Purpose
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APU Main Sections
02|APU Sections|L1|A/B1
Page 5
EASA PART-66 M15
Purpose
M15.18
APU Main Components
The bleed section of the APU must make sure that sufficient bleed air is
supplied to the aircraft pneumatic system.
On most modern APUs you will find a separate compressor for this task. This
compressor is usually called the ”load compressor”.
As on all gas turbine engines, the power section has
S a compressor
S a combustion chamber
S and a turbine.
The compressor of the power section is mainly used to provide the air for the
combustion. It can be either a single stage or a multiple stage compressor.
On some APUs this compressor must also provide the bleed air for the aircraft
pneumatic system.
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EASA PART-66 M15
Purpose
M15.18
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APU Sections
03|APU Main Comp|L1|A/B1
Page 7
EASA PART-66 M15
Purpose
M15.18
APU Main Components cont.
On APUs you will usually find centrifugal flow compressors. These
compressors have good performance, a long lifetime, and they are resistant
against foreign object damage known as F O D (pronounce as F. O. D.).
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Purpose
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APU Sections
04|APU Main Comp|L1|A/B1
Page 9
EASA PART-66 M15
Purpose
M15.18
APU Main Components cont.
The combustion chamber of a modern APU is usually an annular type, but on
older APUs you may also find can−type combustion chambers.
The third main component of the APU power section is the turbine. You will find
centrifugal turbines on small APUs or axial turbines with 2 or more stages on
large APUs.
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EASA PART-66 M15
Purpose
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APU Main Components
05|APU Main Comp|L1|A/B1
Page 11
EASA PART-66 M15
Purpose
M15.18
APU Bleed Air Supply
The APU must supply sufficient bleed air for the aircraft pneumatic system.
The air pressure is usually 30 to 45 psi.
There are 2 different methods of APU bleed air supply:
S One uses the compressor of the power section and
S the other one uses an individual load compressor.
APUs with load compressors are very fuel efficient. This is because the load
compressors can be switched off if bleed load is not required by the aircraft.
This means that EGT limits are not reached so easy because no air is taken
from the combustor. This improves the lifetime of the APU.
The air flow into the load compressor is controlled by variable inlet guide
vanes. These inlet guide vanes work like a modulating and shut−off valve. They
can move into any position between open and closed.
This is another advantage because the airflow can be changed on demand
from the pneumatic system.
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Purpose
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APU Load Compressor
06|APU Bleed Air Sup|L1|A/B1
Page 13
EASA PART-66 M15
Purpose
M15.18
APU Electrical Power Supply
Another major task of the APU is to provide electrical power as a back−up
source for the aircraft.
This electrical power supply comes from generators which are driven via the
APU gearbox.
Most APUs have 1 generator which is the same type as the engine generators.
You may also find 2 generators on very large APUs.
Engine driven generators need a constant speed drive to run at a fixed speed if
the electrical network requires a constant frequency.
APU generators do not need a constant speed drive because the APU is
always controlled to a constant speed by the APU fuel control unit.
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07|APU Elec Pwr Sup|L1|A/B1
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EASA PART-66 M15
Purpose
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APU Generators
07|APU Elec Pwr Sup|L1|A/B1
Page 15
EASA PART-66 M15
APU Installation
M15.18
APU INSTALLATION
APU Compartments
The APU compartment is located in the aircraft tail section.
The tail cone is attached to the fuselage structure and arranged as a support
and fairing for the APU.
The APU and its components are enclosed by compartments:
The equipment compartment, the APU compartment and on some aircraft
there is also a muffler compartment.
The APU equipment compartment houses the APU equipment like, for
example:
S the air−intake actuator,
S the APU fire extinguisher bottle,
S the fuel supply line and
S the bleed air duct.
The APU compartment houses the APU. It is entirely fire proof and serves as a
fire compartment. The APU fire walls prevent any possible fire propagation to
the fuselage.
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01|Introduction|L1|A/B1
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APU Installation
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Fire Wall
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APU Compartments
01|Introduction|L1|A/B1
Page 17
EASA PART-66 M15
APU Installation
M15.18
APU Access Doors
The APU doors give access for servicing and maintenance.
There are usually 2 doors located on the bottom of the rear fuselage. On small
aircraft there is only 1 door.
Latches in the lower inboard edge of the access door hold and secure the
doors closed.
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APU Installation
M15.18
Access Door
Latch
Figure 9
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APU Access Doors
03|APU Access Doors|L1|A/B1
Page 19
EASA PART-66 M15
APU Installation
M15.18
APU Mounts
There are 3 APU mounts that support the APU.
The mounts are named the right aft mount, the left aft mount, and the forward
mount.
On large aircraft most APU mounts are connected by APU mount links.
APU mount links are rods or tubes that transmit the forces from the mounts to
the aircraft structural points.
3 mounts support the APU. 1 mount is always a fixed mount for strength and
stability.
The other 2 mounts are flexible because they must permit limited movement for
thermal expansion.
All 3 mounts transmit vertical loads and forces from the APU to the aircraft
structure.
2 mounts transmit axial loads and forces from the APU to the aircraft structure.
Only 1 mount transmits lateral loads and forces from the APU to the aircraft
structure.
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APU Installation
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Figure 10
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Forces Transmitted by APU Mounts
05|APU Mounts|L1|A/B1
Page 21
EASA PART-66 M15
APU Installation
M15.18
APU Air Intake System
The major components of the air intake system are:
S the air intake door,
S the air intake door actuator,
S the air intake nose and
S the air inlet duct.
The air intake door is also named the air intake flap. It can be a lower intake
door which you typically find on Airbus aircraft.
The APU air intake can also be on the right hand upper side of the fuselage
which is typical for Boeing aircraft.
The APU air intake door supplies air to the APU, or it closes off the APU air
inlet if the APU is not running.
The door also prevents birds and debris from entering the APU inlet and
reduces aerodynamic drag in flight.
The air inlet door is operated by an electrical actuator.
The APU air intake door actuator is normally driven by 28 volt DC power.
In addition all actuators have a manual override to open the air intake door if
the electrical motor does not operate.
A door position switch monitors the position of the APU air intake door. This
makes sure that the APU can only run if the air intake door is open.
The door actuator rod transmits the actuator movement to the air intake door.
The actuator rod extends to open the air intake door or the actuator rod
retracts to close it.
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EASA PART-66 M15
APU Installation
M15.18
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APU Air Intake System
09|APU Air Intake System|L1|A/B1
Page 23
EASA PART-66 M15
APU Controls
M15.18
APU CONTROLS
APU Start - Activation
The APU supplies the AC generator with a constant speed. This constant
speed demand makes it very easy for APU control to be automatic.
An electronic APU control unit controls the APU in 3 separate operating
modes.
S The start mode controls the APU from start activation until it has reached
100 % speed.
S The Normal Speed Mode keeps the APU at 100 % speed regardless of load
changes.
S The APU shut−down mode monitors and controls APU shut−down.
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EASA PART-66 M15
APU Controls
M15.18
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APU Operating Modes
01|APU Start−Act|L1|A/B1
Page 25
EASA PART-66 M15
APU Controls
M15.18
APU Start - Activation cont.
The start mode is always activated manually by switches on the overhead
panel in the cockpit.
The control switches look different on different aircraft types, but they have the
same function.
On an Airbus there are 2 individual switches,
S the APU master switch, and
S the APU start switch.
On Boeing aircraft there is only 1 APU switch with an additional start position.
This switch serves as a master switch and as a start switch.
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APU Controls
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APU Start Mode
02|APU Start−Act|L1|A/B1
Page 27
EASA PART-66 M15
APU Controls
M15.18
APU Start - Activation cont.
2 actions are necessary to start the APU:
S Start initiation and
S Start activation.
Start initiation means that electrical power is supplied to the APU control unit.
When the master switch is set to ON, the APU control unit is supplied with
electrical power and first performs a pre-start test. This pre-start test makes
sure that the APU control unit itself operates properly and that all important
sensor signals are available.
The APU control unit tests the electrical circuits and the condition of the
sensors, for example the APU speed sensor, the exhaust gas temperature
probes, the low oil pressure switch, and the oil temperature sensor.
When the pre-start test is successfully performed, the APU air inlet door is
commanded to open, the APU fuel supply is prepared, the fuel shut-off valve is
opened, and the APU fuel boost pump is switched on.
When the master switch is on, the APU indication is available in the cockpit.
The speed indicator and the EGT indicator show zero and the air inlet door is
open.
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APU Controls
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APU Start Initiation
03|APU Start−Act|L1|A/B1
Page 29
EASA PART-66 M15
APU Controls
M15.18
Normal Operation Control
After the APU reaches its operational speed the control becomes very simple.
The APU control unit must now only make sure that the speed for the
generator drive remains constant at 100% and that no excessive EGT is
reached.
The APU control unit uses 2 signals for constant speed control.
S These are the APU speed signal, and
S the torque motor signal.
The APU control unit maintains a constant speed by comparing the actual APU
speed with the necessary speed. It then changes the torque motor signal to
change the fuel metering.
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APU Controls
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Normal Speed Mode
07|Normal Ops Ctrl|L1|A/B1
Page 31
EASA PART-66 M15
APU Controls
M15.18
Normal APU Shut-Down
There are 3 different ways to shut−down the APU.
S The normal way is to manually turn off the APU master switch.
S Automatic shut−down is activated by the APU control unit because of
malfunctions in important APU components.
S An emergency shut−down is activated, for example, by the fire switch or by
an alternate emergency shut−down switch.
The 3 shut−down methods differ in the components which initiate the
shut−down. They also differ in the time that it takes to stop the APU.
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APU Controls
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Different APU Shut-Downs
11|Nor APU Shut−Dwn|L1|A/B1
Page 33
EASA PART-66 M15
APU Controls
M15.18
Normal APU Shut-Down cont.
When the master switch is set to OFF, the APU is prepared for a safe
shut-down.
This means that the APU is only allowed to shut-down after it has operated for
a sufficient time without pneumatic or electric load.
This cool-down period differs between zero and 120 s and can be changed by
maintenance on some APUs, if necessary.
When the cool-down period has passed, the control unit closes off the fuel
supply to the combustion chamber so that the APU comes to a stop.
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APU Controls
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Normal APU Shut-Down
12|Nor APU Shut−Dwn|L1|A/B1
Page 35
EASA PART-66 M15
APU Controls
M15.18
Automatic APU Shut-Down
Automatic shut-down is always activated by the APU control unit. It can be
activated in any operational mode.
This protects the APU from damage if operating limits are exceeded or
important APU components are defective.
In automatic shut-down the stop command is activated immediately without any
cool-down period.
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APU Controls
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Automatic APU Shut-Down
13|Auto APU Shut−Dwn|L1|A/B1
Page 37
EASA PART-66 M15
APU Controls
M15.18
Emergency APU Shut-Down
In case of an APU emergency shut-down the APU must be switched off
immediately without any cooling-down period.
This emergency shut-down is usually initiated by switches on the aircraft like
the APU fire switch or external emergency shut-down switches.
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APU Controls
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Figure 19
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Emergency APU Shut-Down
15|Emg APU Shut−Dwn|L1|A/B1
Page 39
EASA PART-66 M15
APU Controls
M15.18
Emergency APU Shut-Down cont.
The external emergency shut-down switches are in areas of the aircraft where
they are easily accessible to the ground personnel, for example:
S at the nose landing gear,
S in the main wheel well,
S or on the refuelling station depending on aircraft type or size.
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APU Controls
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Figure 20
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External Emergency Shut-Down Switches
16|Emg APU Shut−Dwn|L1|A/B1
Page 41
EASA PART-66 M15
APU Controls
M15.18
Emergency APU Shut-Down cont.
An automatic APU emergency shut-down is activated in a fire condition when
the aircraft is on ground.
On modern aircraft types the APU fire is then extinguished automatically by a
special fire extinguishing system.
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APU Controls
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Fire Extinguisher
Bottle
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APU Automatic Fire Shut-Down
17|Emg APU Shut−Dwn|L1|A/B1
Page 43
EASA PART-66 M15
Primary APU Fuel System
M15.18
PRIMARY APU FUEL SYSTEM
Organization
The primary fuel system of the APU is also named the low pressure fuel
system. This system starts at the fuel tanks and ends at the fuel control unit of
the APU.
The secondary fuel system, which is also named the high pressure fuel
system, starts at the fuel control unit and ends at the fuel nozzles on the
combustion chamber.
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01|Organization|L1|A/B1
Page 44
EASA PART-66 M15
Primary APU Fuel System
M15.18
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Figure 22
HAM US/F-4
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01.04.2008
Primary APU Fuel System
01|Organization|L1|A/B1
Page 45
EASA PART-66 M15
Primary APU Fuel System
M15.18
Operation
The APU fuel feed system has a separate APU fuel boost pump, but there are
also 2 tank boost pumps in each tank.
The APU fuel boost pump is needed to give positive fuel supply if the tank
boost pumps are not available. This positive fuel pressure in the APU fuel feed
line is very important to prevent cavitation at the high pressure fuel pump of the
APU.
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03|Operation|L1|A/B1
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EASA PART-66 M15
Primary APU Fuel System
M15.18
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Figure 23
HAM US/F-4
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01.04.2008
Primary APU Fuel Supply
03|Operation|L1|A/B1
Page 47
EASA PART-66 M15
Primary APU Fuel System
M15.18
Operation cont.
On Airbus aircraft you also find a control push button on the inner firewall of the
APU compartment.
This push button is named the APU fuel line vent switch. If you push the vent
push button the APU fuel boost pump is started and the APU fuel shut-off valve
opens.
The vent push button bypasses the APU master switch. This is an advantage
for maintenance because they can purge the APU fuel feed line without a
person in the cockpit.
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06|Operation|L1|A/B1
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EASA PART-66 M15
Primary APU Fuel System
M15.18
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Figure 24
HAM US/F-4
SwD
01.04.2008
APU Fuel Line Vent Push button
06|Operation|L1|A/B1
Page 49
EASA PART-66 M15
Air System
M15.18
AIR SYSTEM
Introduction
The APU air system is divided into two sub-systems,
S the cooling air system and
S the bleed air system.
The bleed air system has three main tasks;
S bleed supply,
S surge protection, and
S if the APU has a load compressor, load compressor control.
The bleed supply system controls the supply of the APU bleed air to the aircraft
pneumatic system.
The surge protection system prevents any surge condition of the APU
compressors.
If the APU has a load compressor this system also belongs to the air bleed
system. The load compressor control system controls the amount of air going
over the load compressor to meet the bleed demand of the aircraft pneumatic
system. The control system also prevents surging.
The other sub-system of the APU air system is the APU cooling system. This
system uses air from the APU air intake to cool the APU, the APU
compartment, and air cooled components like the oil cooler or the AC
generator.
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EASA PART-66 M15
Air System
M15.18
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Air Intake
Figure 25
HAM US/F-4
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Bleed Air System
01|Introduction|L1|A/B1
Page 51
EASA PART-66 M15
Air System
M15.18
Load Compressor Control System Introduction
Many APUs have a load compressor system because it reduces the fuel
consumption of the APU.
In most operating conditions the APUs can supply electrical power and
pneumatic bleed air at the same time, but this is not always necessary.
If only one power source is needed, the other one can be switched off. When
the APU bleed is switched off, the EGT decreases.
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EASA PART-66 M15
Air System
M15.18
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Figure 26
HAM US/F-4
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01.04.2008
APU Load Compressor
05|Load Compr Ctrl Syst|L1|A/B1
Page 53
EASA PART-66 M15
Air System
M15.18
Load Compressor Control System Introduction cont.
The load compressor cannot be switched off, because it is fixed to the rotor
shaft. The air inlet into the load compressor is closed off by the inlet guide
vanes. This means that air cannot enter the load compressor.
The main components of the load compressor system are:
S the variable inlet guide vanes,
S the inlet guide vane actuator with a mechanical transmission assembly,
and
S the APU control unit.
The variable inlet guide vanes control the quantity of airflow into the load
compressor. They can move to any position between open and closed.
The inlet guide vane actuator moves the inlet guide vanes to the correct
position. The actuator deflects the IGVs via a mechanical transmission.
The APU control unit sends control signals to move the inlet guide vane
actuator to the correct position.
It also receives feedback signals about the true position of the actuator.
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EASA PART-66 M15
Air System
M15.18
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Figure 27
HAM US/F-4
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01.04.2008
Load Compressor Main Components
06|Load Compr Ctrl Syst|L1|A/B1
Page 55
EASA PART-66 M15
Air System
M15.18
Surge Protection Methods on APU’s
APU compressors can surge, especially when bleed load changes.
Compressor surging can be prevented by bleeding off the air that is blocking
the compressor discharge.
There are two surge protection methods used on APUs. The first method uses
the anti-surge system. This system is installed on APUs, which do not have a
load compressor.
The anti-surge system has an anti−surge valve which always bleeds off air
when surging is possible.
The anti-surge valve is normally opened during flight to prevent surging. This
method is not very fuel efficient because air is also bled off when there is no
surging.
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Air System
M15.18
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Figure 28
HAM US/F-4
SwD
01.04.2008
APU with Anti - Surge Valve
10|Surge Prot Meth|L1|A/B1
Page 57
EASA PART-66 M15
Air System
M15.18
Surge Protection Methods on APU’s cont.
A better surge protection method is used on APUs with load compressors.
Here a surge protection system monitors the airflow in the load compressor
discharge duct and opens a surge control valve if the compressor surges.
As a general rule we can say: The surge control valve opens if the airflow
behind the load compressor decreases or stops.
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EASA PART-66 M15
Air System
M15.18
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Figure 29
HAM US/F-4
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01.04.2008
APU with Surge Control Valve
11|Surge Prot Meth|L1|A/B1
Page 59
EASA PART-66 M15
Oil System
M15.18
APU OIL SYSTEM
Organization
The oil system of a typical APU has the same main components and functions
as all oil systems on gas turbine engines, but there are two differences on APU
oil systems, which you do not find on engine oil systems.
APU oil systems have a de-oiling valve which helps to improve the APU start
sequence, and on many APUs the oil system is also used to cool the APU
generator.
An APU oil system always has:
S a reservoir,
S an oil supply pump,
S an oil cooler,
S an oil filter,
S oil supply lines, and
S oil scavenge lines.
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01|Organization|L1|A/B1
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EASA PART-66 M15
Oil System
M15.18
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Figure 30
HAM US/F-4
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01.04.2008
APU Oil System Layout
01|Organization|L1|A/B1
Page 61
EASA PART-66 M15
Oil System
M15.18
APU Oil Vent System Operation cont.
The air/oil separator has vanes, which are mounted on a drive gear in the APU
gearbox. As you can imagine it separates the air/oil mist by rotating action.
Here you can see an enlarged view of the air/oil separator in the APU
gearbox.
The oil mist enters the separator from the left and is accelerated by the vanes
on the separator. The rotation of the air/oil separator forces the oil to the
gearbox and lets the air escape through the hollow drive shaft into an external
pipe, which leads to the APU exhaust duct.
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09|APU OilVent Sys Ops|L1|A/B1
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EASA PART-66 M15
Oil System
M15.18
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Figure 31
HAM US/F-4
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01.04.2008
Air / Oil Seperator
09|APU OilVent Sys Ops|L1|A/B1
Page 63
EASA PART-66 M15
Oil System
M15.18
APU Generator Oil Cooling System
Most modern APUs have oil cooled generators which use oil from the APU oil
system for cooling purposes. You can see that these generators are connected
to the APU oil supply system, but they usually have an independent oil
scavenge system.
The generator oil scavenge system has:
S a scavenge pump which is usually installed in the APU gearbox,
S a separate oil scavenge filter, and
S a scavenge oil return line to the APU gearbox.
The generator oil cooling system is closely monitored for malfunction. It has an
oil temperature sensor installed in the APU generator. This sensor monitors the
generator oil temperature and transmits it to the APU control unit.
If the generator oil temperature becomes too high, the APU control unit initiates
an automatic APU shut-down.
The generator scavenge filter is equipped with a filter bypass valve and a
differential pressure switch.
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11|APU Gen Oil Cooling
Sys|L1|A/B1
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EASA PART-66 M15
Oil System
M15.18
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Figure 32
HAM US/F-4
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01.04.2008
Generator Oil Cooling System
11|APU Gen Oil Cooling
Sys|L1|A/B1
Page 65
EASA PART-66 M15
Ignition & Starting
M15.18
IGNITION & STARTING
Starting System Introduction
The components which are necessary for an APU start are:
S the APU control switches,
S the APU control unit,
S the APU starter motor,
S the aircraft battery,
S the APU starter lead which transmits the battery power to the APU starter
motor, and
S the APU start relay which connects or/and disconnects the battery power to
the starter motor.
You find the control switches for APU start in the cockpit. These switches are
used to prepare and activate the APU start sequence or shut-down of the APU.
The master switch supplies electrical power to the APU control unit which
starts its power-up test. After a successful test it opens the APU fuel shut-off
valve, and the APU fuel pump starts running if the fuel pressure is too low, and
it opens the intake door.
A signal from the start switch then activates the start sequence.
You usually find the APU control unit in the tail section of the aircraft. The APU
control unit receives the start signal from the start switch and closes the APU
start relay.
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Page 66
EASA PART-66 M15
Ignition & Starting
M15.18
APU Air
Intake Door
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Figure 33
HAM US/F-4
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01.04.2008
APU Starting Preparation
01|Start Sys Intro|L1|A/B1
Page 67
EASA PART-66 M15
Ignition & Starting
M15.18
Starting System Introduction cont.
The APU start relay is usually installed in the electronic equipment
compartment of the aircraft.
It is closed by the APU control unit as long as the APU starter needs power
from the aircraft battery.
APUs are usually started by DC motors which you can find on the APU
gearbox.
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Ignition & Starting
M15.18
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Figure 34
HAM US/F-4
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01.04.2008
APU Starting Components
02|Start Sys Intro|L1|A/B1
Page 69
EASA PART-66 M15
Ignition & Starting
M15.18
APU Starter Description
The main components of a typical APU starter are the DC motor and the
starter clutch.
The starter motor is usually a brush type DC motor. It has:
S a housing with a mounting flange,
S two terminals for the power leads,
S brushwear indicators, and
S an electrical connector for feedback signals to the APU control unit.
The brushwear indicators show if the brushes on the starter motor are worn
out. These indicators are usually small inspection windows. Behind the
windows there are indicator pins which show the length of the brushes.
On most modern APUs you also find electrical brushwear sensors. These
electrical sensors send signals to the APU control unit when the brushes reach
their wear limits.
On many APUs there is another signal which is send from the starter motor to
the APU control unit. With this feedback signal the APU control unit knows how
much voltage is available at the APU starter motor terminals. The voltage
feedback signal is used for trouble shooting to find out which component has
caused an APU start problem. For example, a slow APU start with a low
voltage at the starter terminals indicates that the problem is in the starter power
supply and NOT in the APU starter motor.
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03|APU Starter Descr|L1|/A/B1
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EASA PART-66 M15
Ignition & Starting
M15.18
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Figure 35
HAM US/F-4
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01.04.2008
APU Starter Main Components
03|APU Starter Descr|L1|/A/B1
Page 71
EASA PART-66 M15
Ignition & Starting
M15.18
Start System Operation
We will now look at a typical APU start procedure.
When the APU start switch is activated the APU control unit energizes the
starter relay. The starter relay closes and DC power from the aircraft battery is
available at the APU starter motor.
The starter motor drives the APU rotor.
At starter cut-off speed the APU control unit opens the starter relay. This
disconnects the battery from the APU starter motor.
The APU continues to accelerate without starter support.
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Ignition & Starting
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Speed Sensor
Figure 36
HAM US/F-4
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01.04.2008
APU Start System Operation
06|Start Sys Ops|L1|A/B1
Page 73
EASA PART-66 M15
Ignition & Starting
M15.18
APU Starter Limitations
The APU starter motor needs a very high current to accelerate the APU rotor.
This means that the starter motor and also the battery become very hot,
especially at the beginning of the start sequence.
Therefore, after each APU start they must cool down sufficiently before you
can use them again.
You must be very careful during maintenance activities when you have to start
the APU more frequently.
There are three starter limitations that must always be observed during an APU
start.
These limitations are:
S the starter duty time,
S the starter cool down time, and
S the starter duty cycle.
The starter limitations make sure that the starter equipment does not get
overstressed and overheated.
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EASA PART-66 M15
Ignition & Starting
M15.18
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Figure 37
HAM US/F-4
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01.04.2008
APU Starter Limitations
07|APU Starter Limit|L1|A/B1
Page 75
EASA PART-66 M15
Ignition & Starting
M15.18
APU Ignition System Operation
The main components of a typical APU ignition system are:
S the APU control unit,
S the ignition exciter,
S the ignition leads, and
S the igniter plugs.
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EASA PART-66 M15
Ignition & Starting
M15.18
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Figure 38
HAM US/F-4
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01.04.2008
APU Ignition System Components
08|APU Ign Sys Comp|L1|A/B1
Page 77
EASA PART-66 M15
Ignition & Starting
M15.18
APU Ignition System Operation cont.
The APU ignition system is usually powered by direct current. The DC power
supply for the ignition exciter comes from the APU control unit.
The ignition exciter converts low voltage DC power into the high voltage for the
igniter plugs. If you want more information on ignition exciter operation, refer to
the Ignition lesson of Module M15.13.
The ignition system is activated in parallel with the APU start system.
On some APUs the ignition system is switched on by the APU speed signal,
and on other APUs the ignition is switched on directly by the start command
signal.
When ignition is on, the fuel/air mixture in the combustion chamber ignites. The
APU ignition is always switched off by the APU control unit if the APU reaches
a sufficient speed. This happens either at starter cut-out speed or when the
APU reaches operational speed.
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EASA PART-66 M15
Ignition & Starting
M15.18
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Figure 39
HAM US/F-4
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01.04.2008
APU Ignition System Operation
09|APU Ign Sys Ops|L1|A/B1
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EASA PART-66 M15
Start & Shut-Down
M15.18
APU START & SHUT-DOWN
APU Start Checklist
Before you start the APU you must make sure that all safety checks are
performed. These safety checks are carried out with an APU start checklist.
We will now follow a typical checklist and look at each task step by step.
We have already performed all checks that are necessary prior to energizing
the electrical network on the aircraft. These checks make sure that no aircraft
system is activated inadvertently after electrical power is put on.
If all APU circuit breakers are closed, you can be sure that all electrical circuits
for APU operation are supplied. The battery switches must always be in the ON
position. You must also check the battery voltage. On this aircraft, for example,
it must be more than 23V.
The APU generator push button switch must be OFF to make sure that the
APU generator is not connected to the aircraft electrical circuits by mistake.
The fire warning test makes sure that the fire warning system operates
properly. It also tests the electrical circuits to the squibs on the fire extinguisher
bottle.
If external power is available, the fire warning test is performed correctly if the
aural fire warning sounds, the red light in the APU fire switch illuminates, and
the squib and discharge lights in the agent push button illuminate. If external
power is not available, only the red light in the APU fire switch and the squib
and discharge lights in the agent push button illuminate.
The APU bleed switch must be OFF to prevent the APU bleed valve opening
and the cross-bleed valve selector is put to AUTO to make sure that the APU
supplies the complete pneumatic system when the APU bleed switch is set to
ON.
One of these fuel boost pump switches must be put ON to supply fuel to the
APU.
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01|APU Start Checklist|L1|A/B1
Page 80
EASA PART-66 M15
Start & Shut-Down
M15.18
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HAM US/F-4
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01.04.2008
APU Start Checklist
01|APU Start Checklist|L1|A/B1
Page 81
EASA PART-66 M15
Start & Shut-Down
M15.18
Start Indications
Before you start the APU you must be familiar with the indications you will see
during an APU start. The most important indications during APU start are:
S the battery voltage,
S the speed indication, and
S the EGT indication.
On some APUs you must also look at the oil quantity indication.
The battery voltage is shown next to the battery switches.
During APU start the decrease in battery voltage shows that the APU starter is
accelerating the APU.
The battery voltage first decreases, and then after starter cut-out it increases
again. This shows that the starter motor draws energy from the battery.
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Start & Shut-Down
M15.18
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Figure 41
HAM US/F-4
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01.04.2008
Start Indications
02|Start Indications|L1|A/B1
Page 83
EASA PART-66 M15
Start & Shut-Down
M15.18
Start Indications cont.
The indications on the ECAM are only available if the AC power is switched
ON.
The increase in EGT indicates that the fuel supply to the combustion chamber
is available, because the APU control unit opened the APU fuel solenoid valve.
The fuel solenoid valve is usually opened by the speed signal in the APU start
sequence at the same time that the ignition is switched on.
During the APU start sequence you must make sure that the APU shows a
smooth acceleration. The smooth acceleration indicates that the starter is
operating properly and the fuel metering from the fuel control unit is correct.
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EASA PART-66 M15
Start & Shut-Down
M15.18
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Figure 42
HAM US/F-4
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01.04.2008
Start Indications
03|Start Indications|L1|A/B1
Page 85
EASA PART-66 M15
Start & Shut-Down
M15.18
APU Shut-Down Checklist
This condition shows an APU running with the APU bleed switch ON. We now
go through the steps to shut-down this APU using the checklist.
First set the APU bleed switch to OFF.
Next set the APU master switch to OFF, but note that the APU will not shut
down immediately. The APU shuts down when the master switch is set to OFF
and the cool-down period is completed.
Then all fuel pumps must be switched off.
The following steps are now different from aircraft to aircraft. In this example
you must also check that the batteries are OFF and that all circuit breakers are
in the required condition.
If there is an emergency, the quickest way to shut down the APU is to activate
the fire switch.
FOR TRAINING PURPOSES ONLY!
Lufthansa Technical Training
GAS TURBINE ENGINE
AUXILIARY POWER UNITS
HAM US/F-4
SwD
01.04.2008
04|APU Shut−Down|L1|A/B1
Page 86
EASA PART-66 M15
Start & Shut-Down
M15.18
FOR TRAINING PURPOSES ONLY!
Lufthansa Technical Training
GAS TURBINE ENGINE
AUXILIARY POWER UNITS
Figure 43
HAM US/F-4
SwD
01.04.2008
APU Checklist
04|APU Shut−Down|L1|A/B1
Page 87
EASA PART-66 M15
Start & Shut-Down
M15.18
Start and Shut-Down Simulations
Use the checklist for the APU start and shut-down tasks.
Make sure that you do not overheat the APU starter motor. If you have reached
the starter duty cycles, you must click on the clock symbol before you can do
the next APU start.
FOR TRAINING PURPOSES ONLY!
Lufthansa Technical Training
GAS TURBINE ENGINE
AUXILIARY POWER UNITS
HAM US/F-4
SwD
01.04.2008
05|APU Shut−Down|L1|A/B1
Page 88
EASA PART-66 M15
Start & Shut-Down
M15.18
FOR TRAINING PURPOSES ONLY!
Lufthansa Technical Training
GAS TURBINE ENGINE
AUXILIARY POWER UNITS
Figure 44
HAM US/F-4
SwD
01.04.2008
APU Checklist
05|APU Shut−Down|L1|A/B1
Page 89
M15.18 A E
TABLE OF CONTENTS
M15
GAS TURBINE ENGINE . . . . . . . . . . . . .
1
M15.18
AUXILIARY POWER UNITS . . . . . . . . . . . . . . .
1
PURPOSE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
AUXILIARY POWER UNIT . . . . . . . . . . . . . . . . . . . . . . . . .
APU SECTIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
APU MAIN COMPONENTS . . . . . . . . . . . . . . . . . . . . . . . .
APU BLEED AIR SUPPLY . . . . . . . . . . . . . . . . . . . . . . . . .
APU ELECTRICAL POWER SUPPLY . . . . . . . . . . . . . . .
2
2
4
6
12
14
APU INSTALLATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
APU COMPARTMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . .
APU ACCESS DOORS . . . . . . . . . . . . . . . . . . . . . . . . . . . .
APU MOUNTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
APU AIR INTAKE SYSTEM . . . . . . . . . . . . . . . . . . . . . . . .
16
16
18
20
22
APU CONTROLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
APU START - ACTIVATION . . . . . . . . . . . . . . . . . . . . . . . .
NORMAL OPERATION CONTROL . . . . . . . . . . . . . . . . . .
NORMAL APU SHUT-DOWN . . . . . . . . . . . . . . . . . . . . . . .
AUTOMATIC APU SHUT-DOWN . . . . . . . . . . . . . . . . . . . .
EMERGENCY APU SHUT-DOWN . . . . . . . . . . . . . . . . . .
24
24
30
32
36
38
PRIMARY APU FUEL SYSTEM . . . . . . . . . . . . . . . . . . . .
ORGANIZATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
OPERATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
44
44
46
AIR SYSTEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
LOAD COMPRESSOR CONTROL SYSTEM INTRODUCTION
SURGE PROTECTION METHODS ON APU’S . . . . . . .
50
50
52
56
APU OIL SYSTEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ORGANIZATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
APU GENERATOR OIL COOLING SYSTEM . . . . . . . . .
60
60
64
IGNITION & STARTING . . . . . . . . . . . . . . . . . . . . . . . . . . .
STARTING SYSTEM INTRODUCTION . . . . . . . . . . . . . .
APU STARTER DESCRIPTION . . . . . . . . . . . . . . . . . . . . .
66
66
70
START SYSTEM OPERATION . . . . . . . . . . . . . . . . . . . . .
APU STARTER LIMITATIONS . . . . . . . . . . . . . . . . . . . . . .
APU IGNITION SYSTEM OPERATION . . . . . . . . . . . . . .
72
74
76
APU START & SHUT-DOWN . . . . . . . . . . . . . . . . . . . . . . .
APU START CHECKLIST . . . . . . . . . . . . . . . . . . . . . . . . . .
START INDICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
APU SHUT-DOWN CHECKLIST . . . . . . . . . . . . . . . . . . . .
START AND SHUT-DOWN SIMULATIONS . . . . . . . . . . .
80
80
82
86
88
Page i
M15.18 A E
TABLE OF CONTENTS
Page ii
M15.18 A E
TABLE OF FIGURES
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
Auxiliary Power Unit (APU) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
APU Main Sections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
APU Sections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
APU Sections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
APU Main Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
APU Load Compressor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
APU Generators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
APU Compartments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
APU Access Doors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Forces Transmitted by APU Mounts . . . . . . . . . . . . . . . . . . . . .
APU Air Intake System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
APU Operating Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
APU Start Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
APU Start Initiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Normal Speed Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Different APU Shut-Downs . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Normal APU Shut-Down . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Automatic APU Shut-Down . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Emergency APU Shut-Down . . . . . . . . . . . . . . . . . . . . . . . . . . .
External Emergency Shut-Down Switches . . . . . . . . . . . . . . .
APU Automatic Fire Shut-Down . . . . . . . . . . . . . . . . . . . . . . . .
Primary APU Fuel System . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Primary APU Fuel Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
APU Fuel Line Vent Push button . . . . . . . . . . . . . . . . . . . . . . . .
Bleed Air System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
APU Load Compressor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Load Compressor Main Components . . . . . . . . . . . . . . . . . . . .
APU with Anti - Surge Valve . . . . . . . . . . . . . . . . . . . . . . . . . . . .
APU with Surge Control Valve . . . . . . . . . . . . . . . . . . . . . . . . . .
APU Oil System Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Air / Oil Seperator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Generator Oil Cooling System . . . . . . . . . . . . . . . . . . . . . . . . . .
APU Starting Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
APU Starting Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
APU Starter Main Components . . . . . . . . . . . . . . . . . . . . . . . . .
3
5
7
9
11
13
15
17
19
21
23
25
27
29
31
33
35
37
39
41
43
45
47
49
51
53
55
57
59
61
63
65
67
69
71
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
36
37
38
39
40
41
42
43
44
APU Start System Operation . . . . . . . . . . . . . . . . . . . . . . . . . . .
APU Starter Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
APU Ignition System Components . . . . . . . . . . . . . . . . . . . . . .
APU Ignition System Operation . . . . . . . . . . . . . . . . . . . . . . . . .
APU Start Checklist . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Start Indications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Start Indications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
APU Checklist . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
APU Checklist . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
73
75
77
79
81
83
85
87
89
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TABLE OF FIGURES
Page ii
M15.18 A E
TABLE OF FIGURES
Page iii
M15.18 A E
TABLE OF FIGURES
Page iv