Training Manual
Fundamentals
Jet
Aircraft
Maintenance
Fundamentals
ATA 72
Basic Engine
JAR-66 Cat.A
Book No:
JAMF ATA 72 CAT.A
Lufthansa
Technical Training GmbH
Lufthansa Base
Issue: March 2001
For Training Purposes Only
 Lufthansa 1995
For training purpose and internal use only.
Copyright by Lufthansa Technical Training GmbH.
All rights reserved. No parts of this training
manual may be sold or reproduced in any form
without permission of:
Lufthansa Technical Training GmbH
Lufthansa Base Frankfurt
D-60546 Frankfurt/Main
Tel. +49 69 / 696 41 78
Fax +49 69 / 696 63 84
Lufthansa Base Hamburg
Weg beim Jager 193
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Tel. +49 40 / 5070 24 13
Fax +49 40 / 5070 47 46
ATA 72
FUNDAMENTALS
BASIC ENGINE
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BASIC ENGINE
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Page: 1
FUNDAMENTALS
ATA 72 Lesson 8
MAINTENANCE PRACTICES
BLADE DAMAGE LIMITS
During engine operation foreign objects can be sucked in by the engines.
These objects can impact with the fan blades and damage them. Therefore,
ground surface around an operating engine must be clean to prevent any
foreign object damage.
A damaged fan blade can still be used if damage does not exceed acceptable
limits or if the damaged area is reworked.
When the damaged area of the blade is reworked, a smooth transition between
the reworked surface and the surrounding undamaged areas of the blade is
ensured.
There are many examples of fan blade damage. Let us look at three of them in
more detail. Typical surface damages are:
-- dents
-- scratches.
A nick is a sharp--bottomed depression with rough outer edges.
A dent is a sharp--bottomed depression. The damaged material is distorted but
not separated.
A scratch is a narrow, shallow mark or line. It is caused by a sharp pointed
object striking the surface of the blade.
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FUNDAMENTALS
ATA 72 Lesson 8
NICK
DENT
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SCRATCH
Figure 1
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Different Fan Blade Damages
Page: 3
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ATA 72 Lesson 8
blade damage limits cont.
Damage on a fan blade can only be reworked if they are within certain limits as
shown in the Aircraft Maintenance Manual. In the manual you will find limits for
the damage depth, damage length and the total damage permitted per blade.
Note also, that because of the load distribution during operation these damage
limits are different for specific areas across the blade. The fan blade is therefore divided into specific areas.
In areas of the blade where stress is low, the damage permitted can be larger
than in areas where stress on the blade is high. For example, there is very
high stress in the blade root area and, therefore, no damage is permitted in this
area.
The fan blade is also divided vertically into damage limit areas. These are the
leading edge area, the surface area and the trailing edge area.
For Training Purposes Only
Example:
On critical areas of the blade root and the blade shroud where stress is very
high, scratches may not be deeper than 0,076 millimeters.
At the leading edge and the trailing edge of the blade root and shroud,
scratches may not be deeper than 0.152 millimeters.
In the middle of the blade scratches may only be 0.381 millimeters deep.
In areas, where stress on the blade is low, scratches may be 1,016 millimeters
deep.
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FUNDAMENTALS
ATA 72 Lesson 8
LEADING EDGE
SURFACE EDGE
AREA
TRAILING
EDGE
AREA
AREA
For Training Purposes Only
AREA
AREA
AREA
Figure 2
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Fan Blade Areas and Damage Limits
Page: 5
FUNDAMENTALS
ATA 72 Lesson 8
blade damage limits cont.
If there is a nick, dent or scratch on a fan blade that is within the limits, the
damage is reworked. This process is called blending.
In this example you can see a damaged surface after the blending process has
taken place. The highlighted area shows the material removed from the original
damage area.
The removed material reduces the weight of the fan blade and causes
imbalance.
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FUNDAMENTALS
ATA 72 Lesson 8
For Training Purposes Only
REMOVED
MATERIAL
Figure 3
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Fan Blade Blending
Page: 7
FUNDAMENTALS
ATA 72 Lesson 8
FAN BLADE MOMENT WEIGHTS
Imbalance to the fan rotor occurs if you remove material from a blade. Another
cause of imbalance is if you have to replace fan blades. You get an imbalance
because of the difference in moment weights between the fan blades.
A rotor can run free of vibration if the fan blades, which are opposite to each
other, create identical centrifugal forces.
The centrifugal forces however depend on 3 parameters. These are:
-- the rotor speed,
-- the blade mass
-- and the distance of the center of gravity of the fan blade from the
rotational axis.
The moment weight is the product of the blade mass and the distance of the
center of gravity from the rotational axis.
It is worked out by a special blade scale.
The moment weight changes with the blade mass and with the position of the
center of gravity of the fan blade.
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FUNDAMENTALS
ATA 72 Lesson 8
MOMENT WEIGHT = BLADE MASS X DISTANCE
CENTRIFUGAL
FORCE
CENTER
OF
GRAVITY
For Training Purposes Only
CENTER
OF
GRAVITY
CENTRIFUGAL
FORCE
Figure 4
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Moment Weight Calculation
Page: 9
FUNDAMENTALS
ATA 72 Lesson 8
fan blade moment weights cont.
Fan blades have a different moment weight even if they have the same mass.
This is because the center of gravity of each fan blade is different because of
manufacturing tolerances.
Note also that the moment weight can be given by different dimensions on
different types of engines.
You may find the moment weight given
-- in gram inches
-- or in centimeter grams
-- or in ounce inches.
You may remember, that fan blades cannot be manufactured to have the same
moment weight.
For example, you will find that the fan blades on a CFM56--3 engine have
moment weights that vary between 29001 gram inches and 33951 gram
inches. This means that you will usually not find two fan blades with the same
moment weight.
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ATA 72 Lesson 8
MOMENT
WEIGHT
30543 gram inch
MOMENT
WEIGHT
30561 gram inch
MOMENT
WEIGHT
24987 cmg
MOMENT
WEIGHT
30453 gram inch
Figure 5
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MOMENT
WEIGHT
25781 cmg
MOMENT
WEIGHT
2627,390 oz inch
MOMENT
WEIGHT
2619,980 oz inch
Different Moment Weight Dimensions
Page: 11
FUNDAMENTALS
ATA 72 Lesson 8
fan blade moment weights cont.
You can usually see the moment weight at the blade root.
On the blade root you will find several numbers.
-- There is a part number which is the same for all the fan blades on this
engine type,
-- a serial number which identifies each individual blade that has been
manufactured.
-- And finally there is the moment weight.
Sometimes the moment weight is crossed out and another number is shown.
If the moment weight of a fan blade has changed because of blending, the
original weight is crossed out and the new moment weight is marked on the
root next to it.
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FUNDAMENTALS
ATA 72 Lesson 8
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Figure 6
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Several Numbers on Fan Blade Root
Page: 13
FUNDAMENTALS
ATA 72 Lesson 8
COMPUTERIZED FAN BLADE DISTRIBUTION
All the blades with their individual moment weights are generally matched on
the fan disk by computerized balancing.
The computer works out the best position for each blade on a fan disk.
A numbering system is used to place the fan blades on the disk.
Fan blade number 1 can be identified by an index mark on the fan disk.
From fan blade number 1 onwards the other fan blades are numbered in the
direction of the rotation.
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FUNDAMENTALS
ATA 72 Lesson 8
BLADE NO. 1
BLADE NO. 1
For Training Purposes Only
OFF-SET
HOLE
Figure 7
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Fan Blade Numbering System
Page: 15
FUNDAMENTALS
ATA 72 Lesson 8
computerized fan blade distribution cont.
We will now look at an example of how the fan blades are matched on a fan
rotor. All moment weights of the necessary 36 fan blades for a fan rotor are
given to a computer.
The computer works out the best position for the fan blades and prints a report
like the one shown in this table. The table tells you the position of each fan
blade on the fan disk.
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FUNDAMENTALS
ATA 72 Lesson 8
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Figure 8
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Aug 98
Computerized Fan Blade Distribution
Page: 17
FUNDAMENTALS
ATA 72 Lesson 8
FAN BLADE CHANGE
If you have to change single fan blades then two methods are proposed by the
engine manufacturers.
The best method is to replace the blade with a new blade that has a moment
weight which is as close as possible to the old one.
If the difference in moment weight between the old and new blade is small
enough (as specified by the manufacturer) then often no further balancing is
needed.
If the difference is larger than specified by the manufacturer, then you can use
balance weights.
Balance weights are used to balance the difference in the moment weight of
the old and new fan blades.
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ATA 72 Lesson 8
For Training Purposes Only
MOMENT WEIGHT
30561 gram inch
MOMENT WEIGHT
30453 gram inch
Figure 9
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Aug 98
Single Fan Blade Replacement Methods
Page: 19
FUNDAMENTALS
ATA 72 Lesson 8
fan blade change cont.
These balance weights are installed on the fan disk or in the spinner cone.
Many different balance weights can be installed into the spinner cone. This
means that, on these engines, you must put the spinner cone back in exactly
the same position as before. That is why an off--set mounting hole is used to
find the correct position.
If you must use balance screws then you must calculate which screws to use.
The calculation method is explained in the relevant maintenance manuals.
The other method for changing single fan blades is to always change a pair of
blades with similar moment weights even if only one blade is damaged.
This means that you replace the damaged blade and also the blade which is
opposite to the damaged one.
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ATA 72 Lesson 8
BALANCE SCREWS
FAN
DISK
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SPINNER
CONE
Figure 10
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Balance Weights
Page: 21
FUNDAMENTALS
ATA 72 Lesson 8
ENGINE BORESCOPING
In this segment, we will show the principles of one kind of visual inspection of
engine components, known as borescope inspection.
A borescope inspection is used to inspect the internal gas path components of
the engine.
These inspections are mostly necessary after FOD ingestion or an overheat
condition of the engine.
If you need to inspect an engine, you will find that with a torch you can only
look into the engine inlet or turbine outlet, but with a borescope you can inspect
the internal components of the engine without taking the engine apart.
Note that a borescope inspection is only done by a especially trained people,
because it requires a lot of experience, but very often a second person is
needed for the borescope inspection as you will see later.
This type of inspection also makes the maintenance concept of on--condition
monitoring possible. This means that parts can be left on the engine as long as
possible. They do not have to be replaced after a fixed operating time even if
they are still serviceable.
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FUNDAMENTALS
ATA 72 Lesson 8
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Figure 11
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Principles of Borescope Inspection
Page: 23
FUNDAMENTALS
ATA 72 Lesson 8
engine borescoping cont.
Now, let#s take a look at the basic borescope equipment.
-- There is the borescope itself,
-- the light source,
-- and the light transmitting cable.
You will also find that the borescope can have either a rigid probe or a flexible.
probe.
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ATA 72 Lesson 8
LIGHT
SOURCE
RIGID PROBE
BORESCOPE
RIGID BORESCOPE
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LIGHT
TRANSMITTING
CABLE
FLEXIBLE
PROBE
FLEXIBLE BORESCOPE
Figure 12
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Aug 98
Borescope Equipment
Page: 25
FUNDAMENTALS
ATA 72 Lesson 8
engine borescoping cont.
You can see the internal components of the borescope which help to transmit
the picture in the field of vision to the eyepiece.
These internal components include optical parts like
-- lenses,
-- prisms,
-- objectives
-- and light guiding glass fibres.
To light up the area inside the engine, a high intensity light is transmitted from
the light source to the probe tip. In flexible probes the light and also the picture
is transmitted by the glasfibres.
Most typical borescopes have different angles of view and different adapters for
the eyepiece. These adapters are interchangeable and have attachments for
cameras and video cameras.
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FUNDAMENTALS
ATA 72 Lesson 8
LIGHT
SOURCE
ADJUSTABLE EYE
PIECE
BORESCOPE
FIELD OF VISION
PRISM
OBJECTIVE
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GLASSFIBRE
FORWARD
FORE-OBLIQUE
LENSES
LIGHT
TRANSMITTING
CABLE
RIGHT-ANGLE RETROSPECTIVE
CAMERA
Figure 13
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BORESCOPE
ADAPTER
Borescope Internal Parts
Page: 27
FUNDAMENTALS
ATA 72 Lesson 8
engine borescoping cont.
A modern type of a borescope is the videoscope. Here the optical components
are replaced by electronic components.
The videoscope has a hand--held monitor, with a flexible probe and a light
source with a computer.
As you can see, the image and data measurements are visible on the monitor.
You control the probe tip with the joystick on the monitor handle.
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ATA 72 Lesson 8
FLEXIBLE
PROBE
HANDHELD
MONITOR
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LIGHT SOURCE
WITH COMPUTER
Figure 14
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Aug 98
Videoscope
Page: 29
FUNDAMENTALS
ATA 72 Lesson 8
engine borescoping cont.
If you have to insert a borescope into an engine, you need to find a hole big
enough to let it through.
In the compressor and turbine case of modern engine there are many
borescope ports, usually 1 port per stage.
You will find these borescope ports on the compressor and turbine usually at
the 3 o#clock or 9 o#clock position. You can get the precise position from the
maintenance manual.
You can also use alternate openings by removing external parts, such as igniter
plugs or fuel nozzles.
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ATA 72 Lesson 8
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Figure 15
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Borescope Ports
Page: 31
FUNDAMENTALS
ATA 72 Lesson 8
engine borescoping cont.
As you can see here, the borescope is almost completely inserted in the stator,
between two stator vanes of the compressor or the turbine.
The mechanic can see the rotor blades in front of the borescope when the
viewing direction is facing forward as shown in this position.
The mechanic can then turn the borescope around to see the rotor blades
behind the borescope.
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ATA 72 Lesson 8
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Figure 16
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Viewing Directions
Page: 33
FUNDAMENTALS
ATA 72 Lesson 8
engine borescoping cont.
When one mechanic looks at the rotor blades, another mechanic turns the
corresponding rotor system slowly.
The N1 rotor is usually turned directly by hand.
The N2 rotor can be turned by a hand cranking drive which is connected to the
accessory gearbox or the transfer gearbox.
You do not need a second person to crank the N2 rotor if you install an
automatic cranking drive. This drive has an electric motor which can be
controlled by a foot operated toggle switch.
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FUNDAMENTALS
ATA 72 Lesson 8
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Figure 17
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Rotor Cranking
Page: 35
FUNDAMENTALS
ATA 72 Lesson 8
THIS PAGE INTENTIONALLY LEFT BLANK
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TABLE OF CONTENTS
ATA 72 BASIC ENGINE . . . . . . . . . . . . . . . . . . . . . .
1
MAINTENANCE PRACTICES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
BLADE DAMAGE LIMITS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
FAN BLADE MOMENT WEIGHTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
COMPUTERIZED FAN BLADE DISTRIBUTION . . . . . . . . . . . . . . . . . .
FAN BLADE CHANGE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ENGINE BORESCOPING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2
2
8
14
18
22
Page i
TABLE OF FIGURES
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Figure 10
Figure 11
Figure 12
Figure 13
Figure 14
Figure 15
Figure 16
Figure 17
Different Fan Blade Damages . . . . . . . . . . . . . . . . . . . . . . .
Fan Blade Areas and Damage Limits . . . . . . . . . . . . . . . .
Fan Blade Blending . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Moment Weight Calculation . . . . . . . . . . . . . . . . . . . . . . . . .
Different Moment Weight Dimensions . . . . . . . . . . . . . . . .
Several Numbers on Fan Blade Root . . . . . . . . . . . . . . . .
Fan Blade Numbering System . . . . . . . . . . . . . . . . . . . . . .
Computerized Fan Blade Distribution . . . . . . . . . . . . . . . .
Single Fan Blade Replacement Methods . . . . . . . . . . . . .
Balance Weights . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Principles of Borescope Inspection . . . . . . . . . . . . . . . . .
Borescope Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Borescope Internal Parts . . . . . . . . . . . . . . . . . . . . . . . . . .
Videoscope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Borescope Ports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Viewing Directions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Rotor Cranking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3
5
7
9
11
13
15
17
19
21
23
25
27
29
31
33
35
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