393SYS
Airport Engineering Practice
Lecture 1
Fundamentals of
Maintenance
1
Airport Engineering Practices
Course Examiner : Andrew LAYFIELD
Lecturers : Andrew LAYFIELD and Ivan LI
Week
Lecturer
Details
1-4
Andrew
LAYFIELD
Room P6955
Tel. 2784-4772
Email :
dcandrew@cityu.edu.hk
Ivan LI
5 - 13
Room P6855
Tel. 2788-8637
Email :
dcivan@cityu.edu.hk
2
Airport Engineering Practices
Course Text Book
Publisher : McGraw-Hill
Edition : 1st, 2004
ISBN : 007142251X
ISBN : 978-0771422512
Covers aircraft maintenance
programme development and
operations from a managerial
as well as a technical
perspective.
3
Airport Engineering Practices
Indicative Syllabus
Week #
Topic
Reading Assignment
1
Fundamentals of Maintenance
Chapters 1 & 2
2
Aviation Industry Certification Requirements
Chapter 4
3
Documentation for Maintenance
Chapter 5
4
Requirements of Maintenance Programme
Chapter 6
5
Engineering Services
6
Aviation law / Airline maintenance Costs
7
Quality Assurance
8
ISO9000 Quality Standard
TBA
9
Quality Management Systems
TBA
10
Quality Audits
TBA
11
Quality Control
Chapter 18
12
Statistical Quality Control Tools & Techniques
13
Reliability
Chapters 8 and 9
TBA
Chapter 17
TBA
Chapters 19 & n3
4
Airport Engineering Practices
Assessment
Exam :
50%
Course Work :
50%
Coursework : Andrew ~ 15% Test (Week 5)
Ivan
~ 35% TBA
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1.0 Why We Have to Do Maintenance
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1.0 Why We Have to Do Maintenance
The Three Laws of Thermodynamics
1st Law: Energy can neither be created or
destroyed
2nd Law: All spontaneous events act to
increase total entropy .
3rd Law: Absolute zero is removal of all
thermal molecular motion
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1.0 Why We Have to Do Maintenance
The second law of thermodynamics states that, in a closed
system, the entropy increases.
Cars rust, dead trees decay, buildings collapse; all these
things are examples of entropy in action, the spontaneous
movement from order to disorder.
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1.0 Why We Have to Do Maintenance
S=Q/T
S = Entropy
Q = the heat content of the system
T = the temperature of the system
There is also another formula for entropy which uses
probabilities – the probability that a particular state of a
system will occur at random.
Highly ordered states are very improbable.
The main point here is that entropy is something that can
be quantified !
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1.0 Why We Have to Do Maintenance
Note that –
- the increase in
entropy gives a
direction to time
because entropy
increases with time
- recognizing ordered
and disordered states
is a function of human
consciousness.
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1.0 Why We Have to Do Maintenance
Aircraft
systems
degrade
with time
– they
wear out !
Their
entropy
increases.
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1.0 Why We Have to Do Maintenance
The Role of the Engineer
Perfect systems can be designed on paper but perfect systems
cannot be built in the real world.
A design engineer may be limited from making the perfect
design by –

imperfections in the natural world

technology

ability

economics
Very often there is just not enough money to build a (nearly)
perfect system.
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1.0 Why We Have to Do Maintenance
However, the designer is obliged to build the best system
possible within the constraints which exist.
A project manager for a new aircraft will be responsible for
the project budget.
He will ask his design manager, “how much will it cost to
build this ?”
The design manager might say $120M.
The project manager, however, might be limited to a budget
of $100M for the production cost of each of the new
aircraft.
In this case, the design manager must redesign the new
aeroplane so it can be built for $100M.
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1.0 Why We Have to Do Maintenance
That usually means – reduced tolerances, cheaper materials,
and, consequently, more entropy.
More entropy in the design will mean more maintenance is
required.
The design engineer’s main problem is then to minimize (not
eliminate) the entropy of the system he or she is designing
while staying within the required constraints.
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1.0 Why We Have to Do Maintenance
The Role of the Mechanic
Entropy exists in every system and the entropy of a system is
always increasing.
This means not only that a real system which starts off being
new will have some entropy already built into it, but that the
entropy will increase with time.
Some components or systems will deteriorate from use, or
even lack of use.
Misuse by an operator may cause some deterioration.
Therefore, while the engineer’s job is to minimize the
entropy of a system during design, the mechanics job is to
fight the continual increase in the entropy of a system during
its lifetime.
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1.0 Why We Have to Do Maintenance
This graph shows the level of perfection of a typical system :
100%
ENTROPY
ATTAINABLE LEVEL OF
PERFECTION
100% perfection is
at the very top of
the y-axis.
The x-axis shows
time.
PERFECTION
NATURAL DECAY OF SYSTEM
(Increasing Entropy)
The solid curve
shows the level of
perfection of a real
world system.
Note that the curve
turns downwards
with time.
TIME
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1.0 Why We Have to Do Maintenance
When the system deteriorates to some lower level of perfection, it is
necessary to perform some corrective action - adjusting, servicing, etc.
100%
ENTROPY
PERFECTION
ATTAINABLE LEVEL OF
PERFECTION
NATURAL DECAY OF SYSTEM
(Increasing Entropy)
TIME
System
needs to be
corrected
when it
reaches
some lower
level.
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1.0 Why We Have to Do Maintenance
In other words, at some point, we have to perform some
maintenance to restore the system to its designed-in level of
perfection.
We need to reduce the entropy to its original level.
This is called preventative maintenance and is usually
performed at regular intervals.
This is done to prevent deterioration of the system to an
unusable level.
Preventative maintenance is also sometimes referred to as s
scheduled maintenance.
This schedule could be daily, every flight, every 200 flight
hours, or every 100 cycles ( a cycle is a takeoff and landing).
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1.0 Why We Have to Do Maintenance
Considering the number of components on a modern
aircraft, maintenance is a complex, on-going process.
For this reason, we will see that aircraft maintenance
must be approached systematically.
Definition
 systematic
 adjective
done using a fixed and organized plan
e.g. “the systematic collection and
analysis of information”
systematically adverb
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1.0 Why We Have to Do Maintenance
This graph shows the system restored to its normal level (curves a and
b):
100%
ENTROPY
DESIGNED-IN LEVEL OF
PERFECTION
a
At other times the
system breaks down
completely (curve
d).
b
PERFECTION
(OR
RELIABILITY)
Sometimes the
system can
deteriorate rapidly
in service to a low
level of perfection
(curve c).
c
POINT AT WHICH
SCHEDULED
MAINTENANCE IS DONE
d
TIME
In these cases, the
maintenance
actions require
extensive testing,
trouble shooting,
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adjustment.
1.0 Why We Have to Do Maintenance
This graph shows the system restored to its normal level (curves a and
b):
Very often, the
100%
ENTROPY
a
b
PERFECTION
(OR
RELIABILITY)
replacement or
complete overhaul of
parts or subsystems
is required.
DESIGNED-IN LEVEL OF
PERFECTION
c
POINT AT WHICH
SCHEDULED
MAINTENANCE IS DONE
d
TIME
Since these
breakdowns occur at
various
unpredictable
intervals, the
maintenance actions
required to correct
the problems are
referred to as
unscheduled
maintenance.
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1.0 Why We Have to Do Maintenance
Reliability
100%
ENTROPY
a
b
PERFECTION
(OR
RELIABILITY)
The level of
perfection of a
system can also be
referred to as the
reliability of the
system.
DESIGNED-IN LEVEL OF
PERFECTION
c
POINT AT WHICH
SCHEDULED
MAINTENANCE IS DONE
d
TIME
The designed-in
level of perfection is
known as the
inherent reliability
of that system.
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1.0 Why We Have to Do Maintenance
Redesign
IMPROVED RELIABILITY
DUE TO REDESIGN
DESIGNED-IN LEVEL OF
RELIABILITY
100%
ENTROPY
B A
C
D
RELIABILITY
POINT AT WHICH
SCHEDULED
MAINTENANCE IS DONE
TIME
In this diagram,
curve A is one of the
original curves from
the first diagram.
However, the system
has now been
redesigned to a
higher level of
perfection – see
curves B, C and D.
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1.0 Why We Have to Do Maintenance
IMPROVED RELIABILITY
DUE TO REDESIGN
DESIGNED-IN LEVEL OF
RELIABILITY
100%
ENTROPY
B A
C
D
RELIABILITY
POINT AT WHICH
SCHEDULED
MAINTENANCE IS DONE
TIME
The redesigned
system now starts
off at a higher level
of reliability.
However, over time,
the system will still
deteriorate from this
new initial level of
perfection.
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1.0 Why We Have to Do Maintenance
IMPROVED RELIABILITY
DUE TO REDESIGN
DESIGNED-IN LEVEL OF
RELIABILITY
100%
ENTROPY
B A
C
D
RELIABILITY
POINT AT WHICH
SCHEDULED
MAINTENANCE IS DONE
TIME
The rate of decay
may change as a
result of the
redesign.
If the decay is
steeper, as in (B), the
point at which
preventative
maintenance needs
to be performed
might occur sooner.
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1.0 Why We Have to Do Maintenance
IMPROVED RELIABILITY
DUE TO REDESIGN
DESIGNED-IN LEVEL OF
RELIABILITY
100%
ENTROPY
So, for case (B),
maintenance will be
needed more often.
B A
C
D
RELIABILITY
POINT AT WHICH
SCHEDULED
MAINTENANCE IS DONE
TIME
The inherent
reliability is
increased but more
maintenance is
required to maintain
that level of
reliability.
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1.0 Why We Have to Do Maintenance
IMPROVED RELIABILITY
DUE TO REDESIGN
DESIGNED-IN LEVEL OF
RELIABILITY
100%
ENTROPY
B A
C
D
RELIABILITY
POINT AT WHICH
SCHEDULED
MAINTENANCE IS DONE
TIME
Conversely, if the
decay rate is the
same as before, as in
case (C), or less
steep as in case (D),
then the maintenance
interval would be
increased and the
overall amount of
preventative
maintenance might
be reduced.
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1.0 Why We Have to Do Maintenance
The question to be considered here is : Does the reduction in
maintenance justify the cost of the redesign ?
This is a question for the designers to consider, not the
maintenance people.
One of the major factors in redesign is cost.
This graph shows two opposing relationships :
INCREASE IN PERFECTION IS
LOGARITHMIC
INCREASE IN COST IS
EXPONENTIAL
PERFECTION VS COST
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1.0 Why We Have to Do Maintenance
INCREASE IN PERFECTION IS
LOGARITHMIC
INCREASE IN COST IS
EXPONENTIAL
PERFECTION VS COST
The upper curve is logarithmic. It represents the increasing
perfection attained with more sophisticated design efforts – the
closer we get to perfection, the harder it is to make a
substantial increase.
The lower curve represents the cost of the efforts to improve
the system. This is an exponential curve. The more we try to
approach perfection, the more it is going to cost us.
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1.0 Why We Have to Do Maintenance
INCREASE IN PERFECTION IS
LOGARITHMIC
INCREASE IN COST IS
EXPONENTIAL
PERFECTION VS COST
Designers are limited in their goal of perfection by both
entropy and costs.
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1.0 Why We Have to Do Maintenance
Failure Rate Patterns
Not all systems or components fail at the same rate nor do
they all exhibit the same pattern of wear out and failure.
This is important because the nature of the maintenance to be
performed on these systems and components is related to their
failure rates and failure patterns.
United Airlines did some studies on lifetime failure rates and
found six basic patterns.
These are shown in the following table.
Note :
1.
The vertical axes show failure rates, not reliability ! The higher the vertical
position, the worse the failure rate.
2.
The horizontal axes show time.
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1.0 Why We Have to Do Maintenance
A. Infant mortality; constant or slightly rising
failure rate; definite wear out period (4 %).
B. No infant mortality; slightly rising failure
rate; definite wear out period (2 %).
C. No infant mortality; slightly rising failure
rate; no definite wear out period (5 %).
D. Increasing failure rate at outset; constant
or slightly rising failure rate; no definite wear
out period (7 %).
E. No infant mortality; constant failure rate
throughout life; no definite wear out period
(14 %).
F. Infant mortality; constant failure rate
throughout life; no definite wear out period
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(68 %).
1.0 Why We Have to Do Maintenance
A. Infant mortality; constant or slightly rising
failure rate; definite wear out period (4 %).
Curve A shows what is commonly referred to as a “bathtub”
curve.
This failure rate pattern exhibits a high rate of failure during the
early portion of the system’s/component’s life known as infant
mortality.
Some systems/components exhibit high early failure rates for
several reasons – poor design, improper parts, incorrect usage.
Once the bugs are worked out and the equipment settles into its
“pattern”, the failure rate levels off or rises only slightly over time.
Near the end of its life, the curve shows a rapid rise in failure rate as
the materials of the system/component reach some kind of physical
limit.
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1.0 Why We Have to Do Maintenance
The United Airlines study showed that only about 11 %
of the items included in the experiment (curves A, B and
C) would benefit from setting operating limits, or from
applying a repeated check of wear conditions. The other 89
% would not.
Consequently, time of failure, or deterioration beyond
useful levels could be predicted on only 11 % of the
items.
Only components with definite life time limits and/or wear
out periods will benefit from scheduled maintenance !
The required maintenance activity for these items can be
spread out over the available time to even out the work
load.
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1.0 Why We Have to Do Maintenance
For the other 89 %, these items will have to be operated
to failure before replacement or repair is done.
This is unpredictable and would result in a need for
maintenance at odd times – i.e.. unscheduled
maintenance.
These characteristics of failure make it necessary to
approach maintenance in a systematic manner, to reduce
periods of unscheduled maintenance.
The aviation industry has developed three management
techniques for handling in-service interruptions which
occur where items must be operated to failure before
maintenance can be done.
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1.0 Why We Have to Do Maintenance
1. Redundant Systems
In redundant systems, if one unit fails, one or more backups
are available to immediately take over the function :
Example 1
Aircraft elevator
hydraulic
actuator.
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1.0 Why We Have to Do Maintenance
Example 2
Aircraft flight data recorders.
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1.0 Why We Have to Do Maintenance
Another example would be the VHF radios used in
commercial aircraft.
A unique feature of redundant units also affects the
maintenance requirements.
If both the primary and the backup units are
instrumented such that the flight crew is aware of any
malfunction, no prior maintenance check is required.
On the other hand, if neither system is so instrumented,
maintenance personnel would need to perform some
check on both the primary and backup systems.
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1.0 Why We Have to Do Maintenance
2. Line Replaceable Unit (LRU)
An LRU is a component or system that has been
designed in such a manner that the parts that most
commonly fail can be quickly removed and replaced on
the vehicle.
The vehicle can then continue in without any significant
interruption in service.
The failed part can be discarded or repaired later.
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1.0 Why We Have to Do Maintenance
3. Minimum Equipment List (MEL)
This list allows a vehicle to be dispatched into service
with certain items inoperative – provided that the loss of
the function does not affect the safety and operation of
the flight.
These items are carefully determined by the aircraft
manufacturer and must also be sanctioned by the
appropriate regulatory authority during the early stages
of vehicle design and test.
The manufacturer issues a master minimum equipment
list (MMEL) which indicates all equipment and
accessories available for a particular aircraft model.
The airline then customizes this document to produce its
own MEL.
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1.0 Why We Have to Do Maintenance
Many of the MEL items are associated with redundant
systems.
The MEL allows deferral of maintenance without upsetting
the mission requirements.
The maintenance, however, must be performed within certain
prescribed periods – typically 1, 3, 10 or 30 days.
MELs may be part of
laptop based computer
software for aircraft
maintenance.
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1.0 Why We Have to Do Maintenance
Summary
Why We Have To Do Maintenance

Entropy and the Laws of Thermodynamics

The Role of the Engineer

The Role of the Mechanic

Perfection and Reliability

Failure Rate Patterns

Management Techniques for In-Service Interruptions –

Redundant Systems

Line Replaceable Units

Minimum Equipment List
42
What you need to know for the exam !

Explain what entropy is and its relationship to the need for maintenance.

What is the role of an engineer in the context of entropy ?

What is the role of a mechanic in the context of entropy ?

With the aid of a sketch diagram, explain the relationship between
entropy or perfection and time.

With the aid of a sketch diagram illustrating the relationship between
reliability and time, explain what is meant by the “designed-in level of
perfection” and the “point at which scheduled maintenance is done”.

With the aid of a sketch diagram, explain the relationship between
perfection and cost.

Give some examples of failure rate patterns. What is the “bath tub” curve ?

Explain each of the three management techniques which have been
developed by the aviation industry for handling in-service interruptions
due to equipment failures. What failure rate curves characterize these types
43
of failures ?