1 1.0 why we have to do maintenance perfect systems can be designed on paper but perfect systems...

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1.0 Why We Have to Do Maintenance

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.

The Role of the Engineer

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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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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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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.

The Role of the Mechanic

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100%

ENTROPY

PERFECTION

TIME

NATURAL DECAY OF SYSTEM

(Increasing Entropy)

ATTAINABLE LEVEL OF PERFECTION

This graph shows the level of perfection of a typical system :

100% perfection is at the very top of the y-axis.

The x-axis shows time.

The solid curve shows the level of perfection of a real world system.

Note that the curve turns downwards with time.

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1.0 Why We Have to Do MaintenanceWhen the system deteriorates to some lower level of perfection, it is necessary to perform some corrective action - adjusting, servicing, etc.

TIME

100%

ENTROPY

PERFECTION NATURAL DECAY OF SYSTEM

(Increasing Entropy)

ATTAINABLE LEVEL OF PERFECTION

System needs to be corrected when it reaches some lower level.

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1.0 Why We Have to Do MaintenanceIn 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 sscheduled 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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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 MaintenanceFailure 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 MaintenanceA. 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 (68 %).

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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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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 Maintenance1. 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 Maintenance2. 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 Maintenance3. 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.

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 of failures ?

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2.0

Development of Maintenance

Programs

1.0 Why We Have to Do MaintenanceWhat is a component and what is a system ? A collection of components organized to accomplish a specific function or set of functions.

An assembly of various components designed to function as a whole.

A collection of interacting subsystems designed to satisfy a set of requirements.

So a system is composed of components, or smaller subsystems, and all of it is designed to provide one or more functions.

A component is the smallest part – you cannot subdivide a component.

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2.0 Development of Maintenance ProgramsIntroduction

We have seen in the first lecture that components and systems fail in different ways and at different rates.

This results in a requirement for unscheduled maintenance that is somewhat erratic and uncertain.

There are periods high workloads and periods with no work – these have to be managed to smooth out the workload and

stabilize the manpower requirements.

The maintenance programs currently in use in commercial aviation were developed by the aviation industry using two diffeent approaches :

the process-oriented approach, and

the task-oriented approach

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2.0 Development of Maintenance ProgramsThe Process-Oriented Approach

This uses three primary maintenance processes to accomplish the scheduled maintenance actions :

1. Hard-Time (HT)Used for components or systems that have definite life limits. Item is

removed at a predetermined interval, usually specified in either flight hours or flight cycles.

2. On-Condition (OC)Used for components or systems that have detectable wear out

periods. Item will be checked at specific intervals (in hours, cycles, or calendar time) to determine its remaining serviceability.

3. Condition Monitoring (CM)Used to monitor systems and components that cannot utilize either HT

or OC processes. Involves monitoring of failure rates, removal rates, etc. to facilitate maintenance planning

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2.0 Development of Maintenance Programs

HT and OC processes apply to items in categories A, B and C :

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 %).

HT – definite life limits

OC – detectable wear out periods

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CM items are operated to failure and failure rates are tracked to aid in failure prediction or failure prevention efforts.

These are “operate to failure” items in categories D, E and F :

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 (68 %).

CM – no definite wear out period.

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2.0 Development of Maintenance ProgramsThe Maintenance Steering Group (MSG) Approach

The modern approach to aircraft maintenance started with the Boeing Company in 1968.

It coincided with the introduction of the Boeing 747 – then the largest commercial airplane.

Six industry working groups analysed aircraft –

1. structures

2. mechanical systems

3. engines and auxiliary power plants

4. electrical and avionics systems

5. flight controls and hydraulics

6. “zonal” configurations.

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This analysis provided them with information on failure modes, failure effects and failure causes.

The approach was called “bottom up” because it looked at the components as the most likely causes of equipment

malfunction.

The purpose of the analysis was to determine which of the three processes (HT, OC or CM) would be required to repair the item and return it to service.

This resulted in the a generalized maintenance process called MSG-2 which could be applied to any aircraft, not just 747s.

The following table summarizes the steps involved.

Note that the process is sligtly different for (a) systems and components, (b) structures and (c) engines :

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Process-Oriented Maintenance

The Hard-Time Process

Hard time is a failure prevention process.

It is applied to items –

● having a direct adverse effect on safety

● subject to reliability degredation but having no possible maintenance check (e.g. rubber

components)

The item has to be removed from the vehicle and either –

● completely overhauled, or

● partial overhauled, or

● discarded

… before exceeding a pre specified life time.

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2.0 Development of Maintenance ProgramsThis life time or interval between each removal of the item

may be specified in terms of –

● calendar time

● engine or aircraft check intervals

● operating cycles

● flight hours

If a particular component fails at X hours of operation, ideally the component would be replaced at the last scheduled

maintenance period prior to the accumulation of X hours.

This would give the airline the maximum service hours from the component and the component would never fail in

service.

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The On-Condition (OC) Process

With on-condition, the item is not removed periodically. Instead, it is subject to periodic inspections or tests.

The objective of the inspections or tests is to determine whether or not the item can continue in service.

If an item fails an OC check, only then is it removed for overhaul, repair, or replacement.

OC items are restricted to component / equipment / systems on which checks and tests can be applied without having to

remove the item.

These OC checks must be performed within time limits (intervals) prescribed for each OC check.

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The Condition Monitoring (CM) Process

Unlike HT and OC processes, CM does not really monitor the condition of a component.

CM systems consist of data collection and data analysis procedures.

For example, a CM process may collect data on –

unscheduled removals of equipment from aircraft (i.e. due to some failure), maintenance log entries, pilot reports, workshop findings, sampling inspections, mechanical reliability reports, and other sources of maintenance data

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The CM process is applied when neither the hard time nor the on-condition process can be applied.

CM is not a failure prevention process as are HT and OC.

CM components have to be operated to failure, and replacement of CM items is an unscheduled maintenance action.

Since CM items are operated to failure, these items must comply with the following conditions :

1. A CM item has no direct, adverse effect on safety when it fails.

2. A CM item must not have any “hidden function” (i.e. that cannot be observed by flight crew).

3. A CM item must be included in a CM program.

What you need to know for the exam ! Can you explain what is meant by a component and system ?

What are the two main approaches developed by the aviation industry for developing maintenance programmes and what maintenance

processes do they use ?

Can you explain MSG-2 ?

Can you explain Process-Oriented Maintenance ?

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393SYS 393SYS Airport Engineering Airport Engineering

PracticePractice

Lecture 3Lecture 3Development of Development of

MaintenanceMaintenanceProgramsPrograms

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2.0 Development of Maintenance ProgramsTask Oriented Maintenance

These procedures are referred to as MSG-3.

MSG-3 is a modification of and an improvement on the MSG-2 approach.

MSG-2 was a bottom-up approach – involves monitoring systems at their component level to detect failure conditions.

MSG-3 is a “top-down” approach which is used to identify suitable scheduled maintenance tasks to prevent failures and maintain the reliability of the system.

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MSG-3 asks - how does a particular failure affect the operation of the aircraft ?

It does not matter whether a system, subsystem, or component fails.

What matters is how the failure affects the aircraft operation.

Failures are assigned one of two basic categories –

Safety, or

Economic

The following diagram is a simplified version of the first step in the MSG-3 logic process :

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There are three categories of tasks developed by the MSG-3 approach :

(a) airframe systems tasks

(b) structural item tasks

(c) zonal tasks

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2.0 Development of Maintenance ProgramsA. Maintenance Tasks for Airframe SystemsDecision analysis results obtained using MSG-3 assigns a combination of the

following eight tasks –

1. Lubrication – applying oil and grease to reduce friction and wear.

2. Servicing – attending to basic needs of components and/or systems.

3. Inspection – Examination of an item and comparison against a standard.

4. Functional Check – Measurements are made to determine if each function of an item performs within specified limits.

5. Operational Check – Determines if an item is fulfilling it’s intended purpose. Does not require measurements of tolerances.

6. Visual Check – Observation used to determine is fulfilling its intended purpose. Does not require measurements of tolerances.

7. Restoration – Returns an item to a specified standard. Varies from simple cleaning to complete overhaul.

8. Discard – Removes an item from service at a specified life limit.

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2.0 Development of Maintenance ProgramsB. Maintenace Tasks for Structural Items

Airplanes are subjected to three sources of structural deterioration –

1. Environmental Deterioration: Deterioration of an item’s strength or resistance to failure as a result of interaction with climate or the environment.

2. Acidental Damage : Deterioration of an item caused by –

impact with some object which is not part of the airplane,

errors in manufacturing

damage during operation

damage during maintenance

3. Fatigue Damage : The formation of cracks due to cyclic loading.

What you need to know for the exam ! Can you briefly describe the main characteristics of the MSG-3

approach to maintenance and how it differs from MSG-2 ?

Given the simplified logic diagram for MSG-3, could you trace a particular sequence of questions and arrive at a particular result ?

Could you identify or briefly describe some or all of the maintenance tasks identified in the three categories of tasks developed by the MSG-3 approach ?

Given a simple schematic diagram of a commercial aircraft, could you explain what zonal maintenance refers to ?

If you were given a sample of one of the main certifications required for commercial aircraft, could you identify the main items of information they must include ?

Can you identify and briefly describe 5 different types of maintenance documentation provided by aircraft manufacturers ?

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PracticePractice

Lecture 4Lecture 4Development of Development of

MaintenanceMaintenanceProgramsPrograms

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4.0 Documentation For Maintenance

Types of Documentation

Main types of documentation –

1. Manufacturer’s Documentation

2. Regulatory Documentation

3. Airline Generated Documentation

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Regulatory Documentation

In the United States, the FAA (Federal Aviation Administration) issues many documents on aircraft maintenance. The most significant ones are –

● Federal Aviation Regulations (FARs)

● Advisory Circulars (ACs)

● Airworthiness Directives (ADs)

● Notice Of Proposed Rule Making (NPRM)This last one is issued by the FAA whenever the FAA intends to change a FAR. It is issued in advance of the change to give aviation industry plenty of time to study and comment on the proposed change.

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Federal Aviation Regulations (FARs)

These are the laws of the United States which relate to all aspects of aviation including private, commercial and experimental aircraft; airports; navigational aids; air traffic control; training of pilots, air traffic controllers, mechanics etc.

Example FAR

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Advisory Circulars (ACs)

Designed to help airline operators meet the requirements of FARs.

An AC often states that something is “a means, but not the only means” of complying with a regulation.

Airworthiness Directives (ADs)

The airworthiness directives are very important regulations issued by the FAA to correct an unsafe condition that exists in an –

● aircraft● aircraft engine● propeller,● aircraft appliance

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Aircraft owners or operators are required to maintain aircraft in compliance with all ADs.

Typically an AD will include –

a) a description of the unsafe condition

b) the item to which the AD applies

c) the corrective action required

d) date of compliance

e) where to get additional information

f) information on alternative methods of compliance (if applicable)

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Effective, November 12, 2002 Airworthiness Directives;

Boeing Model 737 Series Airplanes

Applicability: All Model 737 series airplanes; certificated in any category.

To prevent an uncommanded rudder hardover event and consequent loss of control of the airplane due to inherent failure modes, including single-jam modes, and certain latent failure or jams combined with a second failure or jam; accomplish the following: ….

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Airline Generated Documentation

Operations Specification (“Ops Spec”)

• Written by the airline in accordance with strict FAA requirements.

• The Ops Spec is required for each aircraft type flown by the airline.

• It details the airline’s maintenance, inspection, and operations programs.

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Technical Policies and Procedures Manual (TPPM)

The airline’s Maintenance & Engineering (M&E) operations manual.

Defines exactly how all M&E functions and activities will be carried out.

A detailed document – may be several volumes.

Personnel in M&E must be trained on the TPPM.

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Inspection Manual (IM)

Usually a chapter in the TPPM.

Contents relate to -

a) mechanic inspection tasks

b) QC inspector tasks

c) special inspections (hard landings, bird strikes, etc.)

d) airlines required inspection item (RII) program

e) paperwork and forms to carry out these functions

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Quality Assurance Manual (QAM)

This could be either

a special manual for QA auditors only, or

a separate chapter in the TPPM.

Defines procedures used in the annual QA audits conducted on the M&E units, suppliers and outside contractors.

Includes formats for forms and reports to be used in the QA procedures.

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Minimum Equipment List (MEL)

The aircraft manufacturer produces the Master Minimum Equipment List (MMEL).

This includes all aircraft configurations available for a particular model of aircraft (e.g. Boeing 747-100, 747-200, … etc.).

For some models of aircraft, customers can choose –

engines from three different manufacturers

various auxiliary systems as buyer options

.. much of this is not applicable to some operators.

The airline operator is therefore required to customize the MMEL for their aircraft engine / airframe configurations.

The customized MMEL becomes the MEL.

Copies of this MEL must be carried in each aircraft for flight crew reference.

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Task Cards

Certain tasks defined in the AMM for - removal/installation, testing, servicing and similar maintenance - are reproduced on separate cards or sheets so that the mechanic can perform a particular maintenance task without having to carry the entire maintenance manual around

with them.

These tasks may call for the mechanic to open panels, set certain circuit breakers “in’ or “out”, turn other equipment “on” or “off”, etc., before they can do the maintenance task, and then they must reverse these steps when they have completed the maintenance task.

However, much of the work done during an aircraft check involves a combination of several tasks applied in the same area of the aircraft.

To avoid repeating the same tasks and unnecessary opening and closing of the same panels, most airlines write their own task cards to combine these maintenance activities.

What you need to know for the exam ! Can you identify the four main types of regulatory documentation

issued by the FAA in the United States ?

Explain, briefly, the purpose of the following : Federal Aviation Regulations (FARs), Advisor Circulars, Airworthiness Directives and Notices Of Proposed Rule Making (NPRMs).

Provide a brief description of the different types of documentation generated by airlines.

What are the five objectives of an aviation maintenance programme ?

Explain, briefly, the nature of the different areas relating to aviation maintenance which contribute to the five objectives of an aviation maintenance programme.

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PracticePractice

Lecture 5Lecture 5Engineering & Engineering &

ProductionProduction

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6.0 Engineering

Engineering Department Functions

Each model of aircraft has an initial maintenance program developed by an Industry Working Group.

The initial maintenance program is a generalized program and must be tailored to the individual airline operator’s

requirements from the very beginning.

The manufacturer produces the FAA approved MRB report and a maintenance planning document.

It is the responsibility of the engineering department of the airline to package these tasks into workable units based on factors such as –

time, space, personnel, fleet schedules, and overall airline capabilities.

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6.0 Engineering

Aircraft Maintenance Checks (from http://en.wikipedia.org/wiki/C_Check)

Aircraft maintenance checks are periodic checks that have to be done on all aircraft after a certain amount of time or

usage.

Airlines casually refer to these checks as one of the following: A check, B check, C check, or D check.

A and B checks are lighter checks, while C and D are considered heavier checks.

A Check — This is performed approximately every month.

This check is usually done overnight at an airport gate.

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6.0 Engineering

The actual occurrence of this check varies by aircraft type, the cycle count (takeoff and landing is considered an aircraft

"cycle"), or the number of hours flown since the last check.

The occurrence can be delayed by the airline if certain predetermined conditions are met.

B Check — This is performed approximately every 3 months.

This check is also usually done overnight at an airport gate. A similar occurrence schedule applies to the B check as to the A check.

C Check — This is performed approximately every 12-18 months.

This maintenance check puts the aircraft out of service and requires plenty of space - usually at a hangar at a maintenance base.

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6.0 Engineering

The schedule of occurrence has many factors and components as has been described, and thus varies by aircraft category and type.

D Check — This is the heaviest check for the airplane.

This check occurs approximately every 4-5 years.

This is the check that, more or less, takes the entire airplane apart for inspection.

This requires even more space and time than all other checks, and must be performed at a maintenance base.

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6.0 Engineering

The A and B checks are no problem for airlines because they do not take the aircraft out of service.

Large airlines have a large fleet of aircraft - enough for the airline to schedule people and facilities for regular “C” checks – e.g. one airplane per week or month.

In small airlines, there are not enough airplanes or manpower for the regular scheduling of “C” checks.

To solve this problem, the “C” check is divided into parts, called “phases”, and each phase is conducted separately.

For example, a C check could be divided into four phases – C1, C2, C3 and C4 – each one carried out every 3 months

until the entire C check is performed.

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6.0 Engineering

Alternatively, a C check could be divided into 12 packages with one package completed every month together with the A check.

In both cases, the manpower utilization is more constant throughout the year.

The airline “engineering department” is responsible for selecting the tasks to be done, for packaging the tasks into workable packages, and for ensuring that all task time limits are met.

The “Production, Planning and Control department” is then responsible for scheduling the checks.

The tasks to be performed by the maintenance unit of the airline at any of these checks can be quite detailed.

To endure that they are carried out correctly, tasks cards are issued to the mechanics.

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7.0 Production Planning and Control

Introduction

The “Production Planning and Control” (PP&C) activity within an airline is one of the key organizations within M&E.

It is actually the “heart” of the maintenance organization.

The title implies two functions – planning and control.

Actually, PP&C has three primary functions – forecasting, planning, and control.

Forecasting includes the estimated maintenance workload for the long term and short term based on the existing fleet and business plans, and on any known changes in these for the

forecast period.

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Planning involves the scheduling of upcoming maintenance and includes the planning and scheduling of all manpower,

parts, facilities, and time frame requirements.

Control allows adjustment of the plan and keeps (or attempts to keep) the check on schedule.

There are several methods of adjusting the plan, including –

deferral of maintenance to a later check,

addition of personnel to complete the work,

outsourcing the work to a contractor

Feedback from a check allows PP&C to adjust the planning effort for future checks.

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Without planning, action would be impulsive and produce unpredictable results.

The diagram below illustrates how work in expended on a typical project with and without proper planning :

WO

RK

LO

AD

TIME

Planning Performance

Start Planning

Start Check

End Check

Planned Check

Unplanned Check

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The preliminary planning consists of the development of a maintenance program and its schedule.

With proper planning, once the check has begun, the work progresses smoothly (dashed yellow line).

Without preliminary planning, the effort swells as the work progresses, mostly due to unexpected events and delays (solid red line).

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Example for “C” Check

Normally, a C check requires about 4 or 5 days.

A new airline operator started to consult the maintenance manual for the C check one week before the check was due.

Without adequate preplanning, the C check took 4 weeks to perform !

The goals of PP&C are -

to maximize the M&E contribution to the airline

to plan and organize work prior to execution

to adjust plans and schedules to meet changing requirements

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Production Planning

The goal of M&E is to deliver airworthy vehicles to the flight department in time to meet the flight schedule, with all

maintenance activities completed or properly deferred.

The airline engineering department will have developed the maintenance plan from the MRB (Maintenance Review Board report) or Ops Specs document and divided the work into

appropriate work packages indentifying –

the tasks to be done

the intervals at which they will be done, and

the manpower requirements for each task.

The check package schedule for a typical mid-sized airline is shown in the following table …

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7.0 Production Planning and ControlMaintenance Tasks for “Less-than-A-Check” Intervals

Certain items in the MRB report must be checked daily, bi-weekly, and weekly.

The scheduling is the responsibility of PP&C.

These checks can be scheduled overnight, or at certain turn-around times, or may be included in the “A” checks.

This is up to the airline and is usually determined by local conditions and man-power availability.

Airlines can experience problems if they defer these tasks day after day due to heavy work loads.

The deadline for completion gets nearer and nearer and, finally, the airline has to take the aircraft out of service for several hours to get the work done without exceeding FAA time limits. These delays can be costly.

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7.0 Production Planning and ControlMultiple Checks

Some MRB items are done at intervals like every other; every third check. This is true for A & C checks. That means that different “A” check have different task set and require different amount of time depending on where that “A” check is in the maintenance cycle.

The following table shows a typical aircraft “A” check and “C” check schedule :

From “Aviation Maintenance Management”, H.A.Kinniston, McGraw-Hill 2004.

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7.0 Production Planning and ControlEvery “C” check includes all the “A” check items.

The “C” check items can also be scheduled for longer intervals.

A chart similar to the one on the previous slide can be drawn for multiple “C” checks.

Phased Checks

These are different from multiple checks.

An “A” check may be split into two phases, each one performed on successive nights to minimize maintenance crew needs and down time.

The right side of an aircraft might be done on the first phase (called an “A1” check and the left side on the second phase (the “A2” check).

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7.0 Production, Planning and ControlA “C” check may be broken down into four parts (C1, C2, C3,

and C4) and performed every 3 months or so depending on the full “C” check cycle.

The “C” check can also be divided into 12 parts, with one part being completed each month (C1, C2, … C12).

The table on the following slide shows a typical airline estimate for the man-hours planned for a “C: check on the Airbus A300B4.

The check will consist of three categories of task – routine, variable routine, and non-routine.

Routine tasks are identified in the MRB report document.

Variable routine tasks vary from one check to another and from one aircraft to another.

Nonroutine tasks are generated by the accomplishment of other, routine tasks.

What you need to know for the exam ! Why do all airlines require a maintenance department, regardless of whether or

not they actually do the maintenance themselves ?

Explain the difference between mechanics and engineers in terms of their role in aircraft maintenance.

What is the MRB report, who is responsible for producing it and what are airlines required to do with it ?

Explain, briefly, what is meant be aircraft A, B, C and D checks.

What approach is adopted by small airlines in conducting “C” checks ?

What issues need to be addressed in evaluating new (used) aircraft for an airline fleet ?

Explain, briefly, the function of Production, Planning and Control department within an airline ?

What is the relationship between Engineering and PP&C in an airline ?

Explain the purpose of Production Planning.

Explain “Multiple Checks” and “Phased Checks” and also the difference between the two.

Apr 18, 2023 393SYS 70

Aviation reliability:Programs & calculation

Kinnison, Chapter 19

Reliability Program (for maintenance)

A set of rules and practices for managing and controlling a maintenance program. The main function is to monitor the performance of the vehicles and their associated equipment and call attention to any need for corrective action.

Additional functions: Monitor the effectiveness of those corrective actions Provide data to justify adjusting the maintenance interval or

maintenance program procedure as appropriate

Apr 18, 2023 393SYS 71

Maintenance programs have four types of reliability

Statistical reliability Historical reliability Event-oriented reliability Dispatch reliability

Apr 18, 2023 393SYS 72

Statistical reliability Based upon collection and analysis of

‘events’ such as failure, removal, and repair rates of systems or components.

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Historical reliability

Comparison of current event rates with those of past experience. Commonly used when new equipment is introduced and no established statistic is available.

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Events like bird strikes, hard landing, in-flight shutdowns (IFSD), lighting strikes or other accidents that do not occur on a regular basis and therefore produce no useable statistical or historical data. In ETOPS, FAA designated certain events to be tracked as ‘event-oriented reliability program’. Each occurrence of the events must be investigated to determinate the cause to prevent recurrence.

IFSD causes; for example: due to flameout, internal failure, crew-initiated shutoff, foreign object ingestion, icing, inability to obtain and/or control desired thrust.

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Event-oriented reliability

Blade penetrated fuselage above window

Dispatch reliability

Measurement of an airline operation respect to on-time departure. It receives considerable attention from regulatory authorities(e.g. FAA), airlines and passengers. Actually, it is just a special form of the event-oriented reliability approach.

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Elements of a Reliability Program

1. Data collection2. Problem area alerting3. Data display4. Data analysis5. Corrective actions6. Follow-up analysis7. Monthly report

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Data Collection: allows operator to compare present performance with the past, typical data type are:

1. Flight time and cycle for each aircraft2. Cancellations and delays over 15 minutes3. Unscheduled component removals4. Unscheduled engine removals5. In-flight shutdowns of engines6. Pilot reports or logbook write-ups7. Cabin logbook write-up8. Component failures (shop maintenance)9. Maintenance check package findings10. Critical failures

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Problem detection: alerting systems

alerting systems for quick identify areas where performance is significantly different from normal so that possible problems can be investigated. Standards for event rates are set according to past performance.

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Reliability :

Basic Calculation & Application

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DISCRETE FAILURE FUNCTION

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f(t), the failure density function over a time interval [t1, t2] and is defined as the ratio of the number of failures occurring in the interval to the size of the original population, divided by the length of the time interval:

Where n(t) is the number of the fault survivors at time t

The f(t) can measure the overall speed at which failures are occurring

f tn t n t N

t t( )

[ ( ) ( )] /

( )

1 2

2 1

DISCRETE HAZARD FUNCTION

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Z(t), the failure rate, or the Hazard function is the probability that a failure occurs in some time interval [t1, t2], given that the system has survived up to time t. It is the ratio of the number of failures occurring in the interval to the size of the original population, divided by the length of the time interval:

)(

)(/)]()([)(

12

121

tt

tntntntZ

The Z(t) can measure the instantaneous speed of failure

PROBABILITY OF SUCCESS

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F(t) is the probability of failure

(= Cumulative Distribution)

R(t) is the probability of success

(= Reliability)

F(t) + R(t) = 1

DISCRETE FUNCTION EXAMPLE

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Failure data for 10 hypothetical electrical components

Failure Number Operating time, h

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8/1 *10

= 80

20/2 *10

= 100

Intervalt1 to t2

Failure density/hrf(t)

Hazard rate/hrZ(t) F(t) R(t) MTTF

Overall MTTF

0-8

8-20

20-34

[10-9]/10 / (8-0)= 1//10/8= 0.0125

[9-8]/10/(20-8)= 1/10/12= 0.0083

[8-7]/10/(34-20)= 1/10/14 = 0.0071

[9-8]/9/(20-8)= 1/9/12= 0.093

[8-7]/8/(34-20)= 1/8/14= 0.089

1/10

2/10

3/10

9/10

8/10

7/10

8*10

=80

12 *9

=108

14*8

=112

34/3 *10

= 113

)(

)(/)]()([)(

12

121

tt

tntntntZ

[10-9]/10 / (8-0)= 1//10/8= 0.0125

f tn t n t N

t t( )

[ ( ) ( )] /

( )

1 2

2 1

Implementing Fault Tolerance

Hardware - Triple-Modular Redundancy (TMR) Hardware unit is replicated three (or more) times Output is compared from three units If one unit fails, its output is ignored Space Shuttle is a classic example

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Machine 1Machine 1

Machine 2Machine 2

Machine 3Machine 3

OutputComparator

OutputComparator

Implementing Fault Tolerance (2)

Using Software N-Version programming

Have multiple teams build different versions of the software and then execute them in parallel

Assumes teams are unlikely to make the same mistakes

Not necessarily a valid assumption, if teams all work from the same specification

…

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N-Version Programming

Version 2

Version 1

Version 3

Outputcomparator

N-versions

Agreedresult

Faultmanager

Input

Commonly used approach in railway signaling, aircraft systems & reactor protection system

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EVENT DIAGRAM

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A

B

C

D E

Event diagram for AND-OR configuration

.AND. CONFIGURATION

Rs = Reliability of the system.

= R1XR2 where R1 & R2 are the reliability of components C1 & C2.

Rs with n components arranged in logical .AND.

then,

Rs = R1 X R2 X … X Rn Apr 18, 2023 393SYS 90

Rs

R1 R2

.OR. CONFIGURATION

Rs = Reliability of the system.R1, R2, ..Rn = Reliability of component

1, 2, ..n.in this case, it is easier to calculate by

the probability of failure FFs = (1 - Rs)F1 = (1 - R1); F2 = (1 - R2)

Fs = F1 * F2

= (1 - R1) * (1 - R2)

Rs = 1 - Fs = 1 - [(1 - R1) * (1 - R2) * … (1 - Rn)]

for n components arranged in logical .OR.

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Rs

R1

R2

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Reliability Acronyms

MTBF - Mean Time Between Failures MTTF - Mean Time To Failure MTTR - Mean Time To Repair MTBF = MFFT + MTTR Many people consider it to be far more

useful than measuring fault rate per LOC

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AVAILABILITY

Where MTTF = Mean Time To Fail MTTR = Mean Time To Repair

MTBF = MTTF + MTTRWhere MTBF = Mean Time Between Failure

Time (up)A(T) = ------------------------------------------

Time(up) + Time(down)

MTTF MTTF= -------------------------- = --------------

MTTF + MTTR MTBF

Failure rate and Reliability during the random failure period

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Rs = e –t/

= e -t

where: Rs = Probability of failure free operation for period >=t e = 2.718 t = specified period of failure free operation = mean time between failure (MTBF) = failure rate = 1/

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Quality Assurance & ISO9000 Quality Standard


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ISO 9001:2000 Exclusionref. 1.2 & 4.2.2 a

Module E

Permissible exclusions

Module D


Module H


Sub-clause 7.1: planning of product realization

Sub-clause 7.2.3: customer communication

Sub-clause 7.3: design and development

Sub-clause 7.4 purchasing

Sub-clause 7.5.1: control of production and service provision

Sub-clause 7.5.2: validation of processes for production and service provision

Sub-clause 7.5.3: Identification and Traceability

Sub-clause 7.3: design and

development

NO exclusions permitted

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Example of ISO 9001:2000 Exclusion:

Customer property controlled by a bank.

Situation:

A bank provides a variety of services to its customers and chooses to implement a QMS only for its Internet banking services. For this service the bank has claimed conformity to ISO 9001:2000. The bank clearly states in its Quality Manual which services are covered by the QMS. The bank applies all the requirements of ISO 9001:2000 for the realization of its Internet banking services, with the exception of sub-clause 7.5.4 Customer property. The bank does not feel that it has possession of any customer property as part of its Internet banking services and has stated this in the justification for the exclusion of sub-clause 7.5.4 Customer property from its QMS.

Issue(s):

Can the bank exclude sub-clause 7.5.4 Customer property from its QMS and claim conformity to ISO 9001:2000?

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Example of ISO 9001:2000 Exclusion:

Customer property controlled by a bank. (conti.)

Issue(s):

Can the bank exclude sub-clause 7.5.4 Customer property from its QMS and claim conformity to ISO 9001:2000?

Analysis and Conclusion:

The decision made by the bank to exclude sub-clause 7.5.4 Customer property was not justified because the bank does receive information from its customers such as personal and confidential data. ISO 9001, 7.5.4 Customer property requires an organization to exercise care with customer property while it is under the organizations control or being used by the organization. In this situation, the bank’s customers provide important information in confidence when using the service, which constitutes “Customer property”. Therefore the bank has to address the requirements for customer property in its QMS.

Requirement of Quality Assurance (QA)For each type of aircraft flown, airlines must generate the Operations

Specifications (Ops Specs) that establish the maintenance and inspection programs to be used to keep the aircraft in an airworthy condition. It is referred as the Continuous Airworthiness Maintenance Program (CAMP) section in the Ops Specs.

In addition, Federal Aviation Regulation (FAR) 121.373 Continuing Analysis and Surveillance System (CASS) provides an additional requirement as follows: “Each certification holder shall establish and maintain a system for the continuing analysis and surveillance of the performance and effectiveness of its inspection program and the program covering other maintenance, preventive maintenance, and alternations and for the correction of any deficiency in those programs, regardless of whether those programs are carried out by the certificate holder or another person.”

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Functions of QA

1. Administration and management of QA and CASS activities

2. Conduct QA audit3. Maintenance of technical records4. Liaison with the regulatory authority

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Quality Management System & Quality Manuals

It is a generic approach evolved throughout the years. It is an efficient way to organize an Quality System and is specified in ISO9001:2000 4.2.1 & 5.3

Quality Manual

QualityProgramme

Procedures/Codes of Practice

Corporate manual, Company level Doc.: State & Define policy & objective commitment. Organization Interaction of ProcessDepartmental proc., Quality Plan:Say what you do;Do what you say;What, Where, When, Who, Why.Detail of how to do specific task, work instructions, wall reference charts, forms, spec sheets, blueprints, …

Apr 18, 2023 393SYS 102

Entities on ISO9001: 1994 vs. 2000ref.: c 3

PurchaserCustomer

subcontractorSupplier

Contract

Product / Service

Supply Chain

SupplierOrganization:

shall do, should do,May do …

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ISO 9000 language of Standardsref.: ISO 9000 Introduction and Support Package: Guidance on the Terminology used in ISO 9001:2000 and ISO 9004:2000

“shall” or “ shall not” is used to express a provision that is binding between two or more parties, means ‘it is necessary’, ‘ it is required to’

“should” or “should not” express a recommendation among several possibilities without mentioning or excluding others.

“may” or “need not” indicate a course of action permissible within the limits of ISO 9001

“can” or “cannot” refers to the ability of the user of the standard.

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Quality Audit and ISO9000 Quality Standard


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ISO 9000 Audit

First Party Auditing Audits carried out by the Organization on its

own systems (e.g. Internal Audit by QA unit) Second Party Auditing

Audits carried out by the Customer on the Organization prior to or after contract placement.(e.g. by Government QA unit)

Third Party Auditing Audits carried out by independent accredited

organizations

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Purpose of quality audit

1st Party audit: To confirm compliance with ‘the rules’. To ensure that the rules are implemented in an effective

manner. To identify ‘the system’ is applicable to the overall

objectives of the organization 2nd Party audit:

To decide whether future contracts can be awarded; or To determine whether existing contractual performance is

satisfactory. 3rd Party audit:

Mainly for the purpose of granting or to confirm continuation of (for ongoing surveillance audit) ISO9001 certification.

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Sample Audit Checklist

ISO 9001:2000 c6.2 PROVIDE QUALITY PERSONNEL Y N

Q 6.2.1 USE COMPETENT PERSONNEL

1 Does your organization ensure that the people who influence product quality are competent?

…

2 Do you ensure that personnel have the right education?

3 Do you ensure that personnel have the right training?

4 Do you ensure that personnel have the right skills?

5 Do you ensure that personnel have the right experience?

…

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Quality Audit Tool - Check List

Check List is a collection of simple questions. It help to focus the mind on the task in hand and ensure that full coverage is obtain. However, audits based on them can be very shallow, taking little account of the specific circumstance of what is being audit

Check List can be made from: Based on recognized quality standards Developed around departments operating system Based on company standards and procedures Based on product/customer requirements

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Translate IS0 9001 Standard into High Level Requirements

6.2 Human resources6.2.1 GeneralPersonnel performing work affecting product quality shall be

competent on the basis of appropriate education, training, skills and experience.

High Level Requirements: Personnel performing work affecting product quality shall be

competent Personnel performing work affecting product quality shall be

competent on the basis of appropriate education. Personnel performing work affecting product quality shall be

competent on the basis of appropriate training. Personnel performing work affecting product quality shall be

competent on the basis of appropriate skills. Personnel performing work affecting product quality shall be

competent on the basis of appropriate experience.

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Creating check-list (Based on ISO

9001:2000 6.2.1)High Level Organization Requirements: Check List items:

Personnel performing work affecting product quality shall be competent

Does your organization ensure that the people who influence product quality are competent?

Personnel shall be competent on the basis of appropriate education.

Do you ensure that personnel have the right education?

Personnel shall be competent on the basis of appropriate training.

Do you ensure that personnel have the right training?

Personnel shall be competent on the basis of appropriate skills.

Do you ensure that personnel have the right skills?

Personnel shall be competent on the basis of appropriate experience.

Do you ensure that personnel have the right experience?

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Non conformance report - example

ISO9001 Standard & Clause Number :ISO 9001: 2000,C 4.2.3dNon Compliance: Drawing DT132 Issue “E” in use in workshop area,

latest issue according to central register is “F”.

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The ‘What’ and the ‘How’The ‘Whats’ in The ‘Whats’ in ISO9001:2000-cISO9001:2000-c7.5.5

Preservation of product: The organization shall preserve the conformity of product during internal processing and delivery to the intended destination. This preservation shall include identification, handling, packaging, storage and protection. Preservation shall also apply to the constituent parts of a product.

The ‘Hows’ in Organization:

Chickens travel to the market in stacked, tightly packed crates. Dirt and contamination can spread. Temperature fluctuation and delays during transport may cause problems…

How does the shipper avoid the spread of dirt and contamination during shipment?

How does the chicken truck assure that temperature limit do not exceed?

1 1.0 why we have to do maintenance perfect systems can be designed on paper but perfect systems...

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