BACHELOR’S THESIS
2006:18 HIP
JOHAN WIKSTÉN MARCUS JOHANSSON
B. Sc. PROGRAMME Aeronautical Engineering
Luleå University of Technology Department of Civil and Environmental
Division of Operation and Maintenance
2006:18 HIP • ISSN: 1404 - 5494 • ISRN: LTU - HIP - EX - - 06/18 - - SE
Maintenance and Reliability
With Focus on Aircraft Maintenance
and Spares Provisioning
SAMMANFATTNING
Den här rapporten är ett resultat av det examensarbete som utförts i ämnet underhållssäkerhet, med sina relaterade områden. En ingående studie i logistisk support företogs också, för att fördjupa förståendet av dess roll i, och hur detta påverkar, underhålls procedurerna. Då ämnet underhåll är stort, har fokus lagts på att undersöka möjligheten att med hjälp av befintliga arkiv, förbättra underhålls procedurerna och i viss mån även reservdelsförsörjningen. Arbetet gäller underhålls procedurer inom flygindustrin och har utförts i enlighet med gällande regler och föreskrifter från luftfartsmyndigheten.
Arbetet har utförts på Avdelningen för Operation and Maintenance Enginering vid LTU, under en tio veckors period vårterminen 2006. Uppdragsgivare var Luleå Flygteknik, som tillhandahöll material och expertis inom flygunderhåll och dess procedurer. Författarna vill här ta tillfället i akt att tacka alla som på ett eller annat sätt bidragit till, eller medverkat, i arbetet. Ett speciellt tack tillskrivs följande personer för deras medverkan och för delgivandet av expertis och kunskaper under arbetets gång, dessa personer är: Prof. Uday Kumar (examinator LTU), Aditya Parida (handledare LTU), Arne Nissen (Adjunkt LTU) och Jan Viklund (uppdragsgivare och handledare LFT).
Luleå Maj 2006
Johan Wikstén Marcus Johansson
SUMMARY
This report is the result of a study performed on maintenance reliability and, its, related issues. Studies of logistic support issues have also been performed to understand its important role in maintenance practice. Due to the extensive topics of maintenance, the focus in this study has been set to the investigation of the possibilities to make use of existing records for improvement in maintenance procedures and in some manner, also spare parts provisioning. The study focuses on the maintenance procedures in the aviation industry and, has been carried out in accordance with aviation regulating authority.
The study has been performed at the Division of Operation and Maintenance Engineering at Luleå University of Technology (LTU) during ten weeks in the spring semester of 2006. Sponsor for the study was Luleå FlygTeknik (LFT) which provided material and expertise in aviation maintenance and its procedures.
The authors wish to thank every person who has, in one way or another, been contributing or involved in this study. A especially thank you, are directed to the following persons for their participation and for sharing their expertise during the study, they are: Prof. Uday Kumar (examiner LTU), Aditya Parida (supervisor LTU) and Jan Viklund (sponsor and supervisor LFT).
Luleå May 2006
Johan Wikstén Marcus Johansson
ABBREVIATIONS AND EXPLANATIONS
AMP Aircraft Maintenance Plan
ATA Air Transportation Association
of AmericaEASA European Aviation Safety Agency
FFOP Failure Free Operating Period FMEA Failure Mode and Effect Analysis
LFT Luleå FlygTeknik
MDT Mean Down Time
MFOP Maintenance Free Operating Period
MLDT Mean Logistic Delay Time
MMH Maintenance Man Hour
MPD Maintenance Plan Document
MRB Maintenance Review Board
MSG Maintenance Steering Group
MTBCF Mean Time Between Critical Failure
MTBF Mean Time Between Failure
MTBM Mean Time Between Maintenance
MTBO Mean Time Between Overhaul
MTBUR Mean Time Between Unscheduled Removal
MTTF Mean Time To Failure
MTTR Mean Time To Repair
PART A sub part of EASA regulation document
RCM Reliability Centered Maintenance
TFh Total Flight hours
VAT Value Added Tax
CHAPTER 1 1
INTRODUCTION 1
1.1BACKGROUND 1
1.2PROBLEM STATEMENT 2
1.3PURPOSE 2
1.4AIM 2
1.6SCOPE AND LIMITATIONS 2
1.7METHODOLOGY 2
CHAPTER 2 4
LITERATURE STUDIES AND INTERVIEWS 4
2.1RELIABILITY 4
2.1.1PROBABILITY THEORY 4
2.1.2PROBABILITY/RELIABILITY DISTRIBUTION 5
2.1.3RELIABILITY MEASUREMENTS 6
2.1.4FAILURE FUNCTION 6
2.1.5RELIABILITY FUNCTION 7
2.2MAINTENANCE 8
2.2.1MAINTAINABILITY AND ITS CONCEPTS 9
2.2.2MAINTAINABILITY MEASUREMENTS 9
2.2.3MAINTENANCE AND ITS CONCEPTS 10
2.2.4MAINTENANCE COST 12
2.3OPTIMIZATION OF MAINTENANCE 13
2.3.1PREVENTIVE MAINTENANCE AND RELIABILITY 13
2.3.2MSG3,RELIABILITY CENTERED MAINTENANCE 15
2.3.3RCMPROCESS 16
2.4LOGISTICS 19
2.4.1HISTORY 19
2.5INTEGRATED LOGISTIC SUPPORT 19
2.5.1MAINTENANCE PLANNING 20
2.5.2SUPPLY SUPPORT 20
2.5.3SUPPORT AND TEST EQUIPMENT 21
2.5.4MANPOWER AND PERSONNEL 21
2.5.5TRAINING AND TRAINING DEVICES 22
2.5.6TECHNICAL DATA 22
2.5.7COMPUTER RESOURCES SUPPORT 23
2.5.8HANDLING,STORAGE, AND TRANSPORTATION 23
2.5.9FACILITIES 24
2.5.10DESIGN INTERFACE 24
2.6AVIATION LOGISTICS 24
2.7INTERVIEWS 25
CHAPTER 3 27
AUTHORITY REGULATIONS 27
3.1CERTIFYING AUTHORITY 27
3.2MAINTENANCE REVIEW BOARD (MRB) 29
3.3AIRCRAFT MAINTENANCE PROGRAM (AMP) 30
CHAPTER 4 31
MAINTENANCE TASK COMPILING 31
4.1MAINTENANCE TASK COMPILING 31
4.2SAMPLE AIRCRAFT FLEET 32
4.3DETERMINE THE SCHEDULED MAINTENANCE 32
4.4DETERMINE THE UNSCHEDULED MAINTENANCE 32
4.5MAINTENANCE DATA ANALYSIS 34
4.5.1ANALYSIS OF FAILURE OCCURRENCE 34
4.5.2ANALYSIS OF PREVENTIVE MAINTENANCE 35
CHAPTER 5 38
DISCUSSION 38
5.1ANALYSIS REFLECTIONS 38
5.2LOGISTIC SOFTWARE 40
5.3SPARES PROVISIONING OPTIMIZATION 40
5.3.1ANALYSIS OF PREVIOUS EXAMPLES 41
CHAPTER 6 42
RECOMMENDATIONS 42
CHAPTER 7 43
CONCLUSION 43
CHAPTER 8 44
REFERENCES 44
8.1LITERATURE 44
8.2WEB 45
APPENDIX 46
Chapter 1
INTRODUCTION
1.1 Background
Maintaining a repairable system can be a complex task from economical and reliability point of view. High reliability is most essential which may not be connected with high costs. Due to the multidisciplinary characteristic of maintenance which consists of management planning, equipment, facilities, inventory and human resources, it is hard to overview and manage the maintenance tasks. If a system is not maintained correctly, it will sooner or later cease to perform its required function with loss of safety and availability, besides reliability which will lead to several losses like economical, structural and in the worst case fatal injuries to humans.
Within the aviation industry, the operational requirements are high and because of the consequences of failure, the maintenance procedures governed by strict legislation under strict directive from regulating authority. These directives are developed in a structured way of national requirements like international safety standards. The scheduled maintenance of an aircraft contains hundreds of timely/cyclic based inspections and replacements of parts. These can be divided into about 40, or more, groups with different periodic intervals, which are developed from maintenance strategy like MSG3. Even if a system is maintained, deviations will occur, these deviations must be considered separately since they have a tendency to occur randomly at times and these are the unscheduled maintenance tasks. All implemented maintenance tasks, scheduled as well as unscheduled, must be recorded and archived.
For small and medium companies, it seems like this information is rarely used to
improve the organization. Even if there are already developed software’s available on
the market, which takes care of probability calculations of failures and spare part
optimization, the burden of license fees is too high for the small and medium
maintenance organizations to justify the use of these. Savings can be made by using
the knowledge of previous maintenance data, although the use of an adopted software
would be more effective, a simpler and cheaper analytic method can improve the
logistic tasks like maintenance planning, spares, etc, and will also affect positively on
aircraft availability and unpredicted expenses.
1.2 Problem statement
The cost of maintenance in the aviation industry is high and there is always a continuous process of cutting costs, which has to be done without interfering with safety and airworthiness. Due to the demands from the authorities, there is a source of information in the maintenance records. Our task is to compile these information’s and investigate for analyzing it and further, to give suggestions of how to implement these statistical results in the maintenance and logistic planning.
1.3 Purpose
The main purpose of this work is to develop a methodology that will, facilitate improvement of the logistic function of spare part handling, analyze the existing source of information that has been collected in compliance with regulating authorities. Additionally, this study may affect areas such as aircraft maintenance planning, availability and reliability. Altogether, they will have a long term effect on the safety and economy, we will also enhance our knowledge in the topic of
maintenance reliability.
1.4 Aim
The aim of this study is to improve maintenance decision making, using the existing maintenance data.
1.6 Scope and limitations
This study is limited to small and medium maintenance organizations in the aviation industry. To keep this study within reasonable limits, we will only focus on the stock holding of spare parts and study planned and unplanned maintenance tasks of three sample aircrafts, where the unplanned maintenance is restricted to the following ATA-groups (Air Transport Association): ATA25, 32, 72, 76 and 79, and no notice is taken of the delegation within the subgroups of these.
1.7 Methodology
To reach the aim previously stated, we will do the literature study on maintenance reliability and logistic support theory, besides, analyze the authority regulation by studying PART-M, PART-145 and a MRB/MPD-AMP system from the aviation industry including the ATA-100 system. We will also make interviews with, for this topic, relevant people. We will identify the demanded maintenance tasks performed during one year, divided into four quarters, for three sample aircrafts which are recognized in different lifetime periods with few, mid and many running hours to give some idea of the different characteristics of failure due to age. The process of this study is illustrated in Figure 1.1.
2
Identification of problem and
purpose
Literature studies
MRB/MPD AMP-system
studies
Recommendation
Conclusion
Report writing Analyze the data
and tasks
Discussion Identifying maintenance tasks and data
Compiling maintenance
tasks
Figure 1.1 The process of the study
The maintenance data will be collected, classified and analyzed to give an overview
of the study. Out of this, we hope to develop a theoretical method to achieve the
above stated aim and be able to give recommendations that will affect on the
reliability, safety and economy for the small or medium sized maintenance
organization. Further, we will give suggestions regarding further investigations of the
subject.
Chapter 2
LITERATURE STUDIES AND INTERVIEWS
2.1 Reliability
Ever since the very beginning of the industrial era, customers have demanded more, better and faster deliveries of products and services, all these at lower costs. In other words, they want to get value for their money spent. This problem is due to the fact that one produces, what someone else consumes, the consumer has to rely on the producer, that the product will deliver value as expected and the producer must as far as possible try to fulfill the consumers needs and demands, this is important due to the competitive market.
“Operators want infinite performance, at zero life-cycle costs, with 100% availability from the day they take delivery to the day they dispose it.”
(Kumar et al., 2000)
This is of course the ideal request, but of course impossible to achieve. The operators demand is to get as close as possible to this extreme, or at least closer than their competitors. One step to reach a high level of availability is to increase the reliability of the products, although this on its own, one can’t fulfill all those demands, but it is a link in the chain consisting of reliability, maintenance and logistic support, where maintenance comes as a natural part of reliability.
2.1.1 Probability theory
Probability theory is applicable in various situations where the outcome is uncertain, such as in experiments, trials and repeated processes etc. where predictions have to be made. This is a topic that plays a leading role in modern science. In fact it was previously developed as a tool to guess the outcome of some games of chance, however, subchapter 2.1 is not intended to describe this topic in detail with theorems and proofs, but just give some brief overview of concepts in probability that may be applied to problems during this study considering reliability, maintenance and logistic support.
2.1.2 Probability/Reliability distribution
There are some theoretical probability distributions that imply to reliability study and analysis. However, we will only explain functions which are applicable for the continuous time variable.
Exponential distribution is well defined by a single parameter that controls the scale of the distribution. The exponential distribution is positively skewed, with the smallest possible value, zero. A unique property of the exponential distribution is that it is the only continuous distribution that has memory less property, that mean the conditional probability depends only on remaining duration and is independent of the current age of the item. This property is exploited to a great extend in reliability theory.
The general formula for the probability density function of the exponential distribution is
( ) t e
tf = λ
−λ0 < t < ∞
The general formula for the reliability function of the exponential distribution is
( ) t e
tR =
−λwhere
MTBF
= 1 λ
(Leitch, 1995)
Poisson discrete distribution
isa theoretical distribution where the number of occurrences is paired with an event of a given period of time and its probability. This is useful when it’s not possible to observe a finite sequence of trials. The observations take place over a continuum, such as time. The Poisson distribution is used to model the number of events occurring within a given time interval.
The formula for the Poisson probability distribution function is
) !
( x
t e P
x x
μ ⋅
−μ= μ is defined as the expected value λt
(Billinton et al., 1983)
The formula for the Poisson cumulative distribution function is:
∑
=− ⋅
= t
i
i
i t e
F
0 !
)
( μ
μ
(NIST/SEMATECH, web)
5
2.1.3 Reliability measurements
Broadly the reliability metrics are classified as: 1. Basic Reliability Measures, 2.
Mission Reliability Measures, 3. Operational Reliability Measures and 4. Contractual Reliability Measures. Basic Reliability Measures are used to predict system’s ability to operate without maintenance and logistic support. Reliability measures like reliability function and failure function fall under this category. Mission Reliability Measures are used to predict systems ability to complete mission. Reliability measures such as mission reliability, Maintenance Free Operating Period (MFOP), Failure Free Operating Period (FFOP), and hazard function fall under this category.
Operational Reliability Measures are used to predict the performance of the system when operated in a planned environment including the combined effect of design, quality, environment, maintenance, support policy, etc. Measures such as Mean Time Between Maintenance (MTBM), Mean Time Between Overhaul (MTBO), Maintenance Free Operating Period (MFOP), Mean Time Between Critical Failure (MTBCF) and Mean Time Between Unscheduled Removal (MTBUR) fall under this category. Contractual Reliability Measure is used to define measure and evaluate the manufacturers program. Contractual reliability is calculated by considering design and manufacturing characteristics. Basically it is the inherent reliability characteristic.
Measures such as Mean Time To Failure (MTTF), Mean Time Between Failure (MTBF) and Failure rate fall under this category. (Kumar et al., 2000)
2.1.4 Failure function
Failure function is a basic reliability measure and is defined as the probability that an item will fail before or at the moment of operating time t. Here time t is used in a generic sense and it can have units such as number of landings, flying hours, number of cycles, etc., depending on the operational profile and the utilization of the system.
Failure function is equal to the probability that the time-to-fail random variable will
be less than or equal a particular value t. The failure function is usually represented as
F(t). (Kumar et al., 2000) Failure function is an increasing function where F(0) = 0
and F(∞) = 1.
Applications of the failure function:
F(t) is the probability that an item will fail by time t, F(t) is the fraction of items that will fail by time t and 1-F(t) is the probability that an item will survive up to time t.
Failure Function
Increasing function Probability of failure by given age
Fraction of items that fail by given
age
Figure 2.2 Properties of failure function 2.1.5 Reliability function
Reliability is the ability of the item to maintain the required function for a specified period of time under given operating conditions. Reliability function R(t) is defined as the probability that the system will not fail during the stated period of time t, under stated operating conditions.
Applications of the reliability function:
R(t) is the probability that an individual item survives up to time t, R(t) is the fraction of items in a population that survives up to time t and R(t) is the basic function used for many reliability measures and system reliability prediction.
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2.2 Maintenance
Maintenance and maintainability has always been important to the industry as it affects the performance as well as the finance. For commercial airlines, maintenance costs are approximately 10% of the airlines total costs. Operators would like their systems to be functional and available for safe operation when ever required. To achieve this there are in fact several ways to go, one of them is to construct systems that are extremely reliable which consequently will result in higher acquisition cost, another way is to design systems that are easy and quick to repair when they fail, or some combination of these two described methods would do. Maintenance is a necessary action to sustain and restore the performance, reliability and safety of the item.
The definition of maintenance can be described as:
“The combination of all technical and administrative actions, including supervision actions, intended to remain an item in, or restore it to, a state in which it can perform a required function”. (BS 4778, 1991)
The main objective of maintenance is to ensure the availability of the system. For aircraft, maintenance is an essential part of airworthiness. Further more, critical failures are under no circumstances allowed to occur during flight. Terminologically, maintenance is divided in corrective and predictive maintenance in accordance to ISO/SS 13306 standard, predictive maintenance is further divided into preventive maintenance (time-based) and conditional maintenance (condition-based). SeeFigure 2.3
Maintenance
Corrective Maintenance
Predictive Maintenance Condition
Based
Predetermined (Time based)
Scheduled Scheduled,
continuous or on request Immediate
Deferred
After a detected fault Before a detected fault
2.2.1 Maintainability and its concepts
Even if the ideal situation would be that an item could maintain its performance and/or function for ever, this can in reality never be achieved. Therefore, it must be maintained, thus for the user it is important to know; when and how often maintenance tasks should be performed, how they should be performed, how many people will be needed and what skills and how much training they will need, how much restoration will cost, how long the system will be down, and what facilities and equipment will be required.
In the aviation industry, several of these questions are strictly regulated by the authorities. It is a common misperception that maintainability is simply the ability to reach a component for required maintenance tasks, which is called accessibility. There are other factors like, visibility, testability, simplicity and interchangeability which must be considered. Additionally, one must consider the environment in which it operates.
2.2.2 Maintainability measurements
The maintainability measure can be represented using the probability that the maintenance task will be completed by a stated time. Since the time used for maintenance is a random variable one can use the cumulative distribution function of the elapsed time to find the percentage of maintenance task that will be completed within a specific time.
Mean Time to Repair (MTTR) is a method to measure maintainability. MTTR is the expected value of items repair time. Maintenance Man Hour (MMH) is also known as maintenance labour hours and is an estimate of the expected total man hour needed to accomplish the maintenance, the MMH value can be larger than MTTR due to the fact that it is often possible and sometimes even necessary to employ more than one person on a given activity or task. Another characteristic of system design pertaining to maintainability is in optimizing the mix between preventive and corrective maintenance. In the aviation industry it is a common way to reach this mix by built in redundancy, both doubled and tripled, which allows certain components to fail during flight, this is mostly used where the items have an unpredictable life time or constant hazardous rate, like electronic components, but also some components pertaining high level of risk when failing. However redundant components will add to the cost of the system and often reduces its load carrying capacity. Corrective maintenance can be expensive if the failure causes damage to other components. Additionally it also causes loss in availability, so the main objective for the maintainability engineer is to strike the balance between these maintenance issues. Prognostics can help but these too have their own problems of reliability.
9
2.2.3 Maintenance and its concepts
Maintenance can be explained as all actions taken to keep a system running and to ensure that it is keeping an acceptable standard in which it is able to operate at the required levels of efficiency and effectiveness. The objectives of maintenance are to:
1. Reduce consequences of failure.
2. Extend the life of the system, by keeping the system in a proper condition for a longer time. In other words, to increase the “up” time of the system.
3. Ensure that the system is fit and safe to use.
4. Ensure that the condition of the system meets all regulatory requirements.
5. Maintain the value of the system.
6. Maintain reliability and achieve a high level of safety.
7. Maintain the systems availability and therefore minimize the life cycle cost.
The purpose of maintenance is to keep systems in a state of functioning in accordance with their design and to restore them to a similar state as and then required. (Kumar et al., 2000)
As said before, the maintenance tasks can be classified into three categories namely
corrective, preventive and conditional maintenance task. Corrective is performed after
a failure has occurred, while preventive and conditional maintenance is performed by
schedule. The difference between these last two classes may some times be difficult to
see, since both of them mostly are scheduled. The main difference is that in
preventive maintenance, the item is either replaced or repaired in accordance to a
predetermined maintenance plan, while conditional maintenance represents scheduled
inspections where, as an outcome, replacement or repair may be required.
Corrective maintenance task is a set of activities which is performed with the intension of restoring the functionality or performance i.e. after failure. The duration of corrective maintenance task, DMT
c, represents the elapsed time needed for successful completion of the task. See Figure 2.4. Corrective maintenance task is also referred to as an unscheduled or unplanned maintenance task.
Figure 2.4 Corrective maintenance task activities (adapted, source, Kumar et al., 2000)
Item failed Fault location Disassembly
Repair or Replacement
Assembly Test and Check
Disassembly Verification MTBF
DMT
c?
Corrective maintenance task activities CMT
start CMT complet
Preventive maintenance task is a maintenance activity that is performed in order to reduce the probability of failure of an item/system or to maximize the operational benefit. The duration of the preventive maintenance task, DMT
p, represents the elapsed time needed for the successful completion of the task. See Figure 2.5
Figure 2.5 Preventive maintenance task activities (adapted, source, Kumar et al., 2000)
Disassembly
Replacement
Assembly Test and Check
Verification MTBF
DMT
pPreventive maintenance task activities PMT
start PMT
complete
Tp
11
Conditional maintenance task recognizes that a change in condition and/or performance is likely to precede a failure so the maintenance task should be based on the actual condition of the item. Preventive action is taken when it is believed that an incipient failure has been detected. Thus through monitoring of some condition parameters it would be possible to identify the most suitable instant of time at which preventive maintenance tasks should take place. The duration of conditional maintenance task, DMT
m, represents the elapsed time needed for the successful completion of the task. See Figure 2.6
Figure 2.6 Conditional maintenance task activities (adapted, source, Kumar et al., 2000)
Conditional maintenance task activities
Inspection/
Examination
Data collection
Condition assessment Condition
interpretation Decision
making
DMT
mCOT
start COT
complete
FMT
I/FMTE
2.2.4 Maintenance cost
The cost of maintenance task is the cost associated with each corrective or preventive task whether time-based or condition-based. The expected corrective costs are the total cost of maintenance resources needed to repair or replace the failed items.
Similarly, the expected preventive maintenance cost is the total of maintenance
resources needed to inspect and/or examine an item before a failure takes place and to
replace any items failed. The sum of the corrective and preventive maintenance cost is
the total cost throughout a systems life and the overhead costs, which consists of all
costs other than direct material, labour, and plant equipment. The cost of maintenance
can be divided in two categories: Direct cost of maintenance task and indirect cost of
maintenance task. Direct costs consists of cost of spare parts, material, personnel,
tools and support, equipment, facilities and technical data. Indirect costs consists the
cost of management and administration staff needed for the successful completion of
the task, also costs related to a complete or partial loss due to availability. It also
includes overhead costs as heating, insurance, taxes, facilities, electricity, telephone
and IT. These costs are difficult to deduce to a specific cost carrier. As a conclusion of
maintenance cost one can say that the total cost of maintenance task, is the sum of
2.3 Optimization of maintenance
The objective of maintenance optimization models is to determine the optimum maintenance tasks that minimize the downtime while providing the most effective use of systems in order to secure the desired results at the lowest possible costs, taking all possible constraints into account. The models can be either quantitative or procedure based such as reliability centered maintenance, age related or total productive maintenance (Kumar et al., 2000) The most frequent criteria for developing maintenance models are:
1. Minimizing; maintenance cost, down time and time to repair.
2. Maximizing; revenue, profit, time between failure and availability.
3. Achieving required level of reliability and safety.
The need of adequate maintenance planning is the most common cause of developing mathematical models and their algorithms. These should provide optimal solutions to the following question: when should an item be repaired, replaced, inspected or examined? The mathematical model provides answer to the above question, based on information available and chosen criteria. Modeling of the relation between the operating cost per unit time or the system availability and the preventive maintenance interval is a traditional method in use. An additional measure which also has to be considered are Mean Down Time (MDT), which represents the total time the aircraft is out of operation due to maintenance.
2.3.1 Preventive maintenance and reliability
It is a fact that in some cases preventive maintenance induces an increase of hazardous rate, for example considering items, which do not wear out, scheduled replacements may actually likely induce failures. But items subject to failure mechanisms such as wear, corrosion, fatigue, etc, should be considered for preventive maintenance. The hazard function could be either decreasing, increasing or be constant. Then the hazard rate for an item is decreasing due to operating time, maintenance aimed to restoring it to as new condition is actually not advisable.
Hazard rate
Operating time R
Figure 2.7 Decreasing hazard rate
13
An items time to failure, is exponential distributed, if it has a constant hazardous rate.
That mean, the probability of failure during the next time increment remains unchanged throughout the lifetime of the item, indicating that it is as good as new, no matter how long it has operated. Preventive maintenance is irrelevant in this case though a replacement will make no difference to the failure probability.
Figure 2.8 Constant hazard rate
Hazard rate
Operating time R
Scheduled replacement at any time will theoretically improve the reliability of a system which has an increasing hazard function. In order to optimize preventive replacement, it is therefore necessary to know the following for each item, the time- to-failure distribution, the cost of failure and the cost of scheduled replacement, and the effectiveness of maintenance after scheduled replacement.
Figure 2.9 Increasing hazard rate
Hazard rate
Operating time
R
2.3.2 MSG3, Reliability Centered Maintenance
Reliability Centered Maintenance, (RCM), has its roots in the aviation industry in the late 1960s in conjunction with the introduction of the Boeing 747. The maintenance requirements from the Federal Aviation Administration resulted in a maintenance plan on which maintenance tasks were so extensive that the airlines probably would not have been able to operate the 747 with profit. As a result, the Federal Aviation Authority formed a Maintenance Steering Group, (MSG), consisting operator, manufacturer and regulator. They researched the maintenance area of particular importance to various systems, so that a logical and generally applicable approach could be used for developing maintenance strategies that could ensure maximum safety and reliability with minimization of cost. Further on, this resulted in MSG-2 and finally in the 1980s in MSG-3.
The RCM study has established that there where six failure related patterns, the most common failure pattern show a failure rate decreasing with age before going into a period of random failure pattern, and most uncommon pattern show a failure rate which is constant with age before going into a period of increasing failure rate. See all six kinds of patterns established in Figure 2.10.
Figure 2.10 Different pattern of failure rate (source, Kumar et al., 2000) 1) Bathtub curve; decreasing, constant and
gradually increasing failure rate
2) Low failure rate then item is new, quick increase to constant rate
3) Constant to gradually increasing failure rate
4) Constant failure rate
5) Gradually increasing failure rate 6) Decreasing to constant failure rate
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2.3.3 RCM Process
The process is used to identify system functions, the way these functions fail and the consequences of the failures and apply this information to develop an appropriate maintenance tasks to prevent system failures. Primarily, one can say that the objective of RCM is to preserve systems functions taking into account objectives as minimizing costs, safety and environmental goals, and finally meeting operational goals.
Additionally objectives of RCM can be described as follows:
• To eliminate ineffective preventive maintenance tasks
• To focus maintenance effort on failures that may affect health, safety, environment, economic, operation and any other business related consequences
• To increase system availability
• To ensure system achieves inherent level of reliability
• To achieve the above mentioned goals at minimum operation, maintenance and support costs
Figure 2.11 The RCM closed process (adapted from Kumar et al., 2000) RCM Decision Analysis System Design System Selection
Failure Consequences
Failure Mode, Effect and Analysis
Failure Mode, Characteristics
Failure and Maintenance
Records Maintenance
task evaluation Redesign
No preventive maintenance
Preventive maintenance
Maintenance Schedules
Implementation of Maintenance task
Functional Breakdown
FBD
The RCM process begins with failure mode, effect and analysis, (FMEA), which
identifies the systems failure modes in a systematic and structured manner. Every one
of these failure modes is then examined to determine the optimal maintenance task to
reduce or avoid the severity of each failure. In this process, most of the following
issues have been taken into account, namely cost, safety, and environmental and
operational consequences.
The RCM methodology varies within different industrial areas, but the basic steps are however quite common to all applications, including MSG-3. The RCM compromises the following steps:
1. System selection.
2. Perform Failure Modes and Effect Analysis.
3. RCM decision logic process. Identification of failure consequences.
4. Selection of maintenance tasks.
System selection are the first step where the areas which is assumed to benefit most from the analysis are specified, even though all areas would probably draw some benefit from RCM analysis. It is also necessary to identify the level of assembly at which the analysis should be conducted. Questions like; will an improvement in preventive maintenance reduce cost and improve reliability and safety? Does the current maintenance strategy include a large portion of time based maintenance that could be replaced with condition based? Is there a known design problem that is causing failures and results in high maintenance costs? These are common questions which occur during this phase of the process.
Failure Mode and Effects Analysis is a systematic approach to identify all possible ways in which failure of a system can occur together with its causes and thus the failures potential effect on the system. It is performed to find out how each item in a system is likely to fail and what happens if it does. The FMEA does additionally often include an evaluation of the failure criticality- and assessment of the severity of the failure effect and its probability of occurrence. This is in fact two steps which is called failure mode effect critically analysis (FMECA) when combined.
RCM Decision Logic Process analyses the consequences of each failure mode and identifies an applicable and maintenance task by using the principle that a maintenance task is worth doing if its deals successfully with the consequences of the failure mode which it is meant to prevent. For each failure mode, there are some questions to be asked:
1. Can the user detect the failure?
2. Does the failure mode have an affect on health of the user?
3. Does the failure mode have an affect on safety and the environment?
4. Is the cost of failure and its consequential damage greater than the cost of preventing the failure?
5. Does the failure mode have an affect on the operational performance?
6. Does the failure mode have an affect on the appearance?
The answers are in the simple “yes” and “no” format which make them easy to record on a RCM decision worksheet. The consequences of each failure are identified based on decision logic as illustrated in Figure 2.12.
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Figure 2.12 Identification of consequences of failures (source, Kumar et al., 2000)
2.4 Logistics
Logistics are defined as the process of planning, implementing and controlling the efficient and cost-effective flow and storage of materials, process inventory, finished parts and all related information from the point of origin to the point of consumption, for the purpose of conforming to customers’ requirements (Logistics World, web).
One may say that, it is the science of ensuring that the right products reach the right place, in the right quantity, at the right time, to satisfy the customers’ demands.
2.4.1 History
The origin of logistics can be found in the Second World War. It was important to achieve an effective support line to getting the necessary supplies and right equipment on to the front, this to enhance the fighting moral, as well as the possibilities to hold on to the captured land. These experiences were adopted by the civil market soon after the war.
The logistics theory evolved during the early 50s, and with the beginning of the computer age, it was implemented in the trade market, which contains of manufacturing and sales management. During this period, a new logistic process emerged. The new process involved all phases through a products life time, from its early development on to its disposal. This evolved to what we today know as Integrated Logistic Support.
2.5 Integrated Logistic Support
Integrated Logistics Support is explained as, the process of management, which facilitates the development and integration of the individual logistic support elements to specify, design, development, acquirement, test, field, and support systems. There are usually ten elements mentioned, and this is what it’s all about:
1. Maintenance planning 2. Supply support
3. Support and Test Equipment and Equipment support 4. Manpower and personnel
5. Training and training support 6. Technical data
7. Computer Resources support 8. Facilities
9. Handling, Storage, and Transportation 10. Design interface
These elements must be developed within the system engineering effort, and in coordination with each other. (Logistics World, web) Some compromises may be required between the elements in order to acquire a system that is affordable, operable, supportable, sustainable, transportable, and environmentally sound within the available resources. The planning for a system is normally contained in an Integrated Logistics Support Plan. The planning activities coincide with development of the system acquisition strategy, and the program will be tailored accordingly. The above mentioned elements are explained in detail, in numerical order (Wikipedia, 2001, web).
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2.5.1 Maintenance Planning
Maintenance planning begins early in the acquisition process with development of the maintenance concept. It is conducted to evolve and establish requirements and tasks to be accomplished for achieving, restoring, and maintaining the operational capability for the life of the system. Maintenance planning relies on something called, Level Of Repair Analysis, which is a function within the system acquisition process. The maintenance planning contains of:
1. Define the actions and support that is necessary to ensure that the system attains the specified system readiness objectives within minimum Life Cycle Cost.
2. Setting up specific criteria for repair, requirements, testability, reliability, and maintainability, support equipment requirements and manpower skills and facility requirements.
3. State specific maintenance tasks, to be performed on the system.
4. Define actions and support required for fielding and marketing the system.
5. Address warranty considerations.
6. The maintenance concept must ensure prudent use of manpower and resources. When formulating the maintenance concept, analysis of the proposed work environment on the health and safety of maintenance personnel must be considered.
7. Conduct a Level Of Repair Analysis, to optimize the support system, in terms of Life Cycle Costs, readiness objectives, design for disposal, maintenance task distribution, support equipment, and manpower and personnel
requirements.
8. Minimize the use of hazardous materials and the generation of waste.
2.5.2 Supply Support
Supply support, encompasses all management actions, procedures, and techniques used to determine the requirements to:
1. Acquire support items and spare parts.
2. Catalog the items.
3. Receive the items.
4. Store and warehouse the items.
5. Transfer the items to where they are needed.
6. Issue the items.
7. Dispose of secondary items.
8. Provide for initial support of the system.
9. Acquire, distribute, and replenish inventory.
2.5.3 Support and Test Equipment
Support and test equipment includes all required equipments, mobile and fixed, that is needed to perform all support functions, except that equipment which is an integral part of the system. This category also encompasses planning and acquisition of logistic support for the required equipments. It contains of:
1. Handling and maintenance equipment.
2. Tools (hand tools as well as power tools).
3. Metrology and measurement devices.
4. Calibration equipment.
5. Test equipment.
6. Automatic test equipment.
7. Support equipment for on- and off-equipment maintenance.
8. Special inspection equipment and depot maintenance plant equipment, which includes all equipment and tools required to assemble, disassemble, test, maintain, and support the production and/or depot repair of end items or components.
2.5.4 Manpower and Personnel
Manpower and personnel involves the identification and acquisition of skilled personnel with grades required to operate and maintain a system over its whole lifetime. Manpower requirements are developed and personnel assignments are made to meet the support demands throughout the life cycle of the system. Manpower requirements are based on related Integrated Logistics Support elements and other considerations. Human factors engineering or behavioral research is frequently applied to ensure a good man-machine interface. Manpower requirements are found on accomplishing the logistics support mission in the most efficient and economical way. This element includes the requirements during the planning and decision process to optimize numbers, skills, and positions. This area considers:
1. Man-machine and environmental interface.
2. Special skills.
3. Human factors considerations during the planning and decision process.
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2.5.5 Training and Training Devices
Training and training devices support encompasses the processes, procedures, techniques, training devices, and equipment used to train personnel to operate and support a system. This element defines qualitative and quantitative requirements for the training of operating and support personnel throughout the life cycle of the system. It includes requirements for:
1. Factory training.
2. Instructor and key personnel training.
3. New equipment training team.
4. Resident training.
5. Sustain trainings.
6. User training.
7. Disposal and safe procedures training.
Embedded training devices, features, and components are designed and built into a specific system to provide training or assistance in the use of the system. The design, development, delivery, installation, and logistic support of required embedded training features, mockups, simulators, and training aids are also included.
2.5.6 Technical Data
Data and Technical Publications consist of scientific or technical information that is necessary to translate system requirements into discrete engineering and logistic support documentation. Technical data is used in the development of repair manuals, maintenance manuals, user manuals, and all other documents that are used to operate or support the system. Technical data includes, but may not be limited to:
1. Technical manuals.
2. Technical and supply bulletins.
3. Transportability guidance technical manuals.
4. Maintenance expenditure limits and calibration procedures.
5. Repair parts and tools lists.
6. Maintenance allocation charts.
7. Preventive maintenance instructions.
8. Drawings/specifications/technical data packages.
9. Software documentation.
10. Provisioning documentation.
11. Depot maintenance work requirements.
12. Identification lists.
13. Component lists.
14. Product support data.
15. Flight safety critical parts list for aircraft.
16. Lifting and tie down pamphlet/references.
17. Hazardous Material documentation.
2.5.7 Computer Resources Support
Computer Resources Support includes the facilities, hardware, software, documentation, manpower, and personnel needed to operate and support computer systems and the software within those systems. Computer resources include both stand-alone and embedded systems. This element is usually planned, developed, implemented, and monitored by a Computer Resources Working Group, or Computer Resources Integrated Product Team, that documents the approach and tracks progress via a Computer Resources Life-Cycle Management Plan. Developers will need to ensure that planning actions and strategies contained in the Integrated Logistics Support Plan are complementary and that computer resources support for the operational software, and Automatic Test Equipment software, support software is available where and when it is needed.
2.5.8 Handling, Storage, and Transportation
This element includes resources and procedures to ensure that all equipment and support items are preserved, packaged, packed, marked, handled, transported, and stored properly for short- and long-term requirements. It includes material-handling equipment and packaging, handling and storage requirements, and pre-positioning of material and parts. It also includes preservation and packaging level requirements and storage requirements (for example, sensitive, proprietary, and controlled items). This element includes planning and programming the details associated with movement of the system in its shipping configuration to the ultimate destination via transportation modes and networks available and authorized for use. It further encompasses establishment of critical engineering design parameters and constraints (e.g., width, length, height, component and system rating, and weight) that must be considered during system development. Customs requirements, air shipping requirements, rail shipping requirements, container considerations, special movement precautions, mobility, and transportation asset impact of the shipping mode or the contract shipper must be carefully assessed. This planning must consider:
1. System constraints (such as design specifications, item configuration, and safety precautions for hazardous material).
2. Special security requirements.
3. Geographic and environmental restrictions.
4. Special handling equipment and procedures.
5. Impact on spare or repair parts storage requirements.
6. Emerging new technologies, methods, or procedures and resource-intensive procedures.
7. Environmental impacts and constraints.
These factors are measured by Mean Logistic Delay Time (MLDT), which also includes waiting time for personnel and parts.
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2.5.9 Facilities
The Facilities logistics element is composed of a variety of planning activities, all of which are directed toward ensuring that all required permanent or semi permanent operating and support facilities are available concurrently with system fielding.
Planning must be comprehensive and include the need for new construction as well as modifications to existing facilities. Facility construction can take from 5 to 7 years from concept formulation to user occupancy. It also includes studies to define and establish impacts on life cycle cost, funding requirements, facility locations and improvements, space requirements, environmental impacts, duration or frequency of use, safety and health standards requirements, and security restrictions. Also included are any utility requirements, for both fixed and mobile facilities, with accent on limiting requirements of insufficient or unique resources.
2.5.10 Design Interface
Design interface is the relationship of logistics-related design parameters of the system to its projected or actual support resource requirements. These design parameters are expressed in operational terms rather than as inherent values and specifically relate to system requirements and support costs of the system. Programs such as "design for testability" and "design for disposal" must be considered during a system design. The basic items that need to be considered as part of design interface include:
1. Reliability requirements.
2. Maintainability requirements.
3. Standardization requirements.
4. Interoperability requirements.
5. Safety requirements.
6. Security requirements.
7. Usability requirements.
8. Environmental requirements.
9. Privacy requirements, particularly for computer systems.
10. Legal requirements.
2.6 Aviation logistics
Aviation logistics does not differ from the basic logistics theory, although there is a
complex regulation demand that needs to be complied during the logistics process
development. This to meet the airworthiness directives set by national/international
regulating authorities.
2.7 Interviews
During our work, several cost and time-period related questions arose. We needed answers to these questions, otherwise we should not been able to calculate the time span, as well as costs related to unscheduled maintenance tasks. By time span, we mean the different time periods that an aircraft is standing on the ground because of unexpected failures, i.e. MDT, MTTF and MTTR. The costs are dedicated to the costs of labour, aircraft standing time (i.e. staff, passenger, etc) and spare parts, until the failure is rectified. Our questions were asked to, and answered by our sponsor Jan Viklund at LFT. Some questions were ATA-group related, and were thereby repeated through the most failure frequent ATA- groups. In our case, the ATA-25, 32, 72, 76 and 79 groups. By getting number and figures related to each question, we were able to calculate a mean value for every cost and time aspect for the regarding ATA-group.
This means that our figures are all fictive and in some manner even exaggerated.
What were the questions and what answers did we get? They are all listed in Table 2.1 below.
Questions Answers What is the cost when an
Aircraft is standing on the ground unintentionally?
An unexpected interruption in operation of a 160 pass aircraft costs approx.
100.000Sek / day (≈10.000$ / day).
What is the MTTR figure for ATA-group 25,32,72,76 and 79?
ATA-25 ≈1-3h, ATA-32 ≈1-8h, ATA-72 ≈16h ATA-76 ≈4-5days+ 8h ctr flight, ATA-79 ≈1-5h
What is the MDT figure for the regarding ATA-groups?
Add 20% to the MTTR-figure and you will have the approximate MDT- figure.
What is the cost for spare parts regarding to the ATA- groups?
ATA-25≈500-30000$, ATA-32≈60-80$/h(leasing account, brakes most freq. 1500$ ea), ATA-72 ≈(leasing account)
ATA-76 ≈ 150.000$, ATA-79 ≈100-500$
What is the cost of keeping spare parts in stock, like personnel and facilities?
This is a complex question. In the organizations closure you will regard the stock as an asset, not an expense. You might have a cost initially when parts are purchased but it will be converted in to an asset in the closure. Usually consumables are kept in stock, like filters, tires, relays, gaskets and bulbs.
How long does it take to get the parts, ASAP?
From the US you will get the part within 12hours.
From the EU you will get the part within 3hours.
Where is the bottleneck in the logistic system?
It is often the Toll that creates most of the delays and waiting time. The figures in the previous question is depending on if the consignment is stuck in a Toll random sample check or not.
What is the maintenance cost to repair or replace parts, approximately?
The labor and facility cost can be said to be 65$ per hour plus social fees of 32%, plus profit margin of 20%. This counts to approximately 103$ per hour.
What is the cost to repair or replace parts,
approximately?
For this question we had prepared a list of maintenance tasks which can be seen in Appendix I
Table 2.1 Questions and answers from interview
