Aviation maintenance: safety and human factors [614284]

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Aviation maintenance: safety and human factors

BEng Final Project

Author: Vlad Dumitru Morar

Supervisor: Ș.l. dr. ing. Silviu Zancu

Session: July 2017

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Anti-Plagiarism Declaration

I the undersigned Vlad Dumitru Morar , student: [anonimizat], Faculty of
Aerospace Engineering declare herewith and certify that this final project is the result of my own,
original, individual work. All the external sources of information used were quoted and inc luded in the
References. All the figures, diagrams, and tables taken from external sources include a reference to the
source.

Date: _________ Signature: __________________________

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Acknowledgements

Firstly, I would like to express my sincere gratitude and consideration to my advisor Prof. Silviu Zancu for
the immense support offered for my project. He have guided me with patience, motivation and have
offered me his knowledge that have helped me get through the difficult moments. I special thank him
for the support and close cooperation that he have offered.
I would like to thank to all my teachers of the Faculty of Aerospace Engineering who have contribute to
my educational f ormation.
Last but not the l east, I would like to thank to my family for the support and help offered to me in this
period.

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Contents
Anti-Plagiarism Declaration ………………………….. ………………………….. ………………………….. ……………… 2
Acknowledgements ………………………….. ………………………….. ………………………….. ………………………….. . 3
Contents ………………………….. ………………………….. ………………………….. ………………………….. ………………… 4
Glossary of acronyms ………………………….. ………………………….. ………………………….. ……………………….. 6
Glossary of terms ………………………….. ………………………….. ………………………….. ………………………….. ….. 7
Summary in English ………………………….. ………………………….. ………………………….. ………………………….. 8
Summary in Romanian ………………………….. ………………………….. ………………………….. ……………………… 9
1. Introduction ………………………….. ………………………….. ………………………….. ………………………….. ….. 10
2. Aviation Maintenance ………………………….. ………………………….. ………………………….. ……………….. 11
2.1. History of maintenance ………………………….. ………………………….. ………………………….. ………… 11
2.2. Organizational Structure ………………………….. ………………………….. ………………………….. …… 12
2.2.1. EASA ………………………….. ………………………….. ………………………….. ………………………….. ………. 13
2.2.2. PART 145 ………………………….. ………………………….. ………………………….. ………………………….. … 14
2.3. Maintenance classification ………………………….. ………………………….. ………………………….. .. 14
2.3.1. Line Maintenance ………………………….. ………………………….. ………………………….. ……….. 16
2.3.2. Base Maintenance ………………………….. ………………………….. ………………………….. ………. 18
2.3.3. Component Maintenance ………………………….. ………………………….. ……………………….. 21
3. Human Factors ………………………….. ………………………….. ………………………….. ……………………… 24
3.1 Human performance ………………………….. ………………………….. ………………………….. ………………. 25
3.1.1 The Dirty Dozen ………………………….. ………………………….. ………………………….. ……………….. 25
3.1.2. Memory ………………………….. ………………………….. ………………………….. ………………………….. . 29
3.1.3. Human sensory capabilities ………………………….. ………………………….. ………………………. 30
3.1.4 Stressor factors ………………………….. ………………………….. ………………………….. ……………….. 31
3.2 Human Error ………………………….. ………………………….. ………………………….. ………………………….. . 33
3.2.1. Types of human errors ………………………….. ………………………….. ………………………….. …… 34
3.2.2 Human Error prediction models ………………………….. ………………………….. ………………….. 34
3.2.3 SHELL model ………………………….. ………………………….. ………………………….. ……………………. 39
4. Safety ………………………….. ………………………….. ………………………….. ………………………….. …………….. 41
4.1 Introduction ………………………….. ………………………….. ………………………….. ………………………….. … 41
4.2 The evolution of safety ………………………….. ………………………….. ………………………….. ………. 42
4.3 Swiss -cheese Model ………………………….. ………………………….. ………………………….. …………… 42

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4.4 Aviation Culture ………………………….. ………………………….. ………………………….. ………………….. 43
4.4.1 Safety culture ………………………….. ………………………….. ………………………….. …………………… 44
4.4.2. Safety Management Systems (SMS) ………………………….. ………………………….. ………….. 45
4.5 Tools handling ………………………….. ………………………….. ………………………….. ……………………. 46
4.5.1 Tools handing procedures ………………………….. ………………………….. ………………………….. 47
4.6 Material Handling ………………………….. ………………………….. ………………………….. ……………….. 49
4.7 Accidents/Incidents cause by maintenance errors ………………………….. ………………………. 57
5. Conclusions ………………………….. ………………………….. ………………………….. ………………………….. ….. 62
References ………………………….. ………………………….. ………………………….. ………………………….. …………… 63

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Glossary of acronyms

ICAO – International Civil Aviation Organization
ATA – Air Transport Association
FAA – U.S Federal Aviation Administration
AMC – Acceptable Means of Compliance
GM – Guidance material
EASA – European Aviation Safety Agency
MOA – Maintenance Organization Approval
MOE – Maintenance organization exposition
NDT – non-destructive testing
MS – Maintenance Schedule
MPD – Maintenance Planning Document
FC – Flight cycles
FH – Flight Hours
CRS – Certificate of Release to Service
CMM – Component Maintenance Manuals
LMCM – Line Maintenance Ce rtifying Mechanic
LMCT – Line Maintenance Certifying Technician
BMCE – Base Maintenance Certifying Engineer
AMT – Aircraft Maintenance Technician
AOG – Aircraft on Ground
CofC – Certificate of Conformance
ECAM – Electronic Centralized Aircraft Monitor
SMS – Safety Management Systems
MRO – Maintenance, repair and overhaul
SRM – Structure Repair Manual

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Glossary of terms

Accident – an unfortunate incident that happens unexpectedly and unintentionally, typically resulting in
damage or injury.
Incident – an instance of something happening; an event or occurrence.
Airworthiness – the measure of an aircraft's suitability for safe flight.
Human factors – the study of how humans behave physically and psychologically in relation to particular
environments, produc ts, or services
Stress – a state of mental or emotional strain or tension resulting from adverse or demanding
circumstances.
Performance – the action or process of performing a task or function.
Human performance – Accomplishment of a task in accordance wi th agreed upon standards of accuracy,
completeness, and efficiency.
Safety – the condition of being protected from or unlikely to cause danger, risk, or injury.
Error – the state or condition of being wrong in conduct or judgment.
Human error – the making of anerror as a natural result of being human
Maintenance – the process of preserving a condition or situation or the state of being preserved.
Stressors – an activity, event, or other stimulus that causes stress .
Hazard – a potential source of danger.
Defenses – specific mitigating actions, preventive controls or recovery measures put in place to prevent
the realization of a hazard or its escalation into an undesirable consequence .

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Summary in English

Air transport is the safest method of traveling. This is ensured by the rul es and regulations set in
place . Different than other domains, in aviation a mistake can cost hundreds of lives and produce high
costs. Therefore, aviation is strictly regulated and monitored by authorities at global level.
We often read that statistically the human factor, rather than mechanical or electrical failures of
the airplane, are the primarily cause of accidents. As in maintenance, a single mistake like for getting a
screwdriver inside a component assembly, not installing a seal or not tightening a screw enough or
tightening it too much can result in an accident w ith a high number of fatalities, clear procedures have to
be established by maintenance organizations. Therefore, even though human errors, by nature, are
inevitable the rate of errors must be reduced to minimum.
In the first part I am generally talking about the maintenance at organizational and functional
levels. I am making reference to organizational operations and procedures that must be established in
order to ensure the proper environment and conditions for mainte nance personnel to be able perform in a
safe manner.
I want to bring awareness that maintenance is not only about the direct work on the aircraft like
installing or removing components but also the background of it. In order to be able to install an
compon ent you must first have the part in a serviceable condition and the tools required to perform the
job. In my vision, the aircraft maintenance is divided in two sections, the so called ―production ‖ which is
the effective work on the aircraft and supporting activities that are the component maintenance,
warehousing of the parts and tools and the administrative section.
In the second part of the project I am talking about th e way in which human factors affect the
safety of maintenance and implicitly the safe operation of the aircraft in flight .
Last part exemplifies human errors and consequently their effect that can stay latent even 20
years before acting.
To conclude, I can say that in such industry as aviation the human factors play a very important
role in the safety of aircraft maintenance.

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Summary in Romanian

Transportul aerian este cea mai sigura metoda de a cala tori. Acest lucru este asigurat de regu lile si
regulatiile puse in aplicare. Spre deosebire de alte domenii, o greseala in aviate poate costa sute de vieti si
produce costuri uriase. Asadar, aviatia este strict regulate si monitorizata de autoritati la nivel global.
Citim des ca statistic, factorul uman ,mai degraba decat defectiuni de tip mecanic sau electric al
avionului, este sursa primara cauzatoare de accidente. In mentenance, o singura greseala ca a uita o
surubelnita intr -un ansamblu de component, a nu instala o garniture, a nu strange sau a astrange prea mult
un surub poate rezulta intr -un accident cu un numar mare de fata lititi, procedure clare trebuie stabilite de
catre organizatiile de mentenanta. Astfel, chiar daca greselile umane sunt, prin natura lor, inevitabile,
trebuie reduse la minimum.
In prima parte a proiectului vorbesc general despre mentenanta la niveluri org anizationale si
functionale. Fac referinta la operatiunile si procedurile care trebuie stabilite pentru asigurarea unui mediu
si a unor conditii adecvate pentru personalul mententnatei sa isi desfasoare activititatea intr -o maniera
prudenta.
Vreau sa aduc la constiinta ca mentenanta nu este doar lucrul efectuat direct la avion cum ar fi
instalarea sau demontarea componentelor dar si munda din spatele acestora. Pentru a putea instala o
componenta trebuie intai sa ai component intr -o stare functionalitate si trebuie sa ai uneltele necesare
pentru a efectua operatiunea. In viziunea mea, mentenanta este impartite in doua categorii, asa numita
―producere‖ care este munca efectiva efectuata la avion si activitatile de support cum ar fi mentenanta
componentelor, d epozitarea componentelor si a unelteor si partea administrative.
In partea a doua a lucrarii vorbesc despre felul in care factorii umani afecteaza siguratanta
mentenantii si implicit siguratan zborului.
In ultima p arte am exemplificat cateva erori si efect ul lor care poate sta latent chiar si pentru 20
de ani.
Pentru a incheia, pot sa spun ca intr -o industrie ca aviatia factorul uman joaca un rol foarte
important in sguranta mentenante avionului.

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1. Introduction

―It is an unequivocal fact that whenever men and women are involved in an activity, human error
will occur at some point‖
Joe Amadi -Echendu
One of the most important components in aviation safety is maintenance. When it is not done
properly it can contribute to a significant accidents and inciden ts. Eventually all mechanical components
or equipment will fail and the scope of maintenance is not only to recognize but also to cope with the
failure in the most safe and effective way.
The essential purpose of aircraft maintenance is to either to repai r and return a defective system
or component to serviceability either to maintain the aircraft system, component or structure in an
airworthy condition.
The overall goal of aviation maintenance human factors research is to identify and optimize the
factor s that affect human performance in maintenance and inspection. Maintenance and inspection
procedures are largely dependent on humans and although no one intends for errors to happen,
psychology informs us that by our nature humans are prone to error and it is inevitable that mistakes will
be made from time to time.
In comparison to other safety threads in aviation, the human errors in aviation maintenance are
very difficult to detect. Such examples of maintenance errors are parts being wrongly installed, wr ong
troubleshooting, missing parts, necessary checks not being performed properly or tools lost during a job.
Especially i n line maintenance s uch mistakes has a higher chance of happening due to the harsh
environmental conditions and high stress level. The tasks have to be carried out outdoor in any weather
condition and being time pressured.

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2. Aviation Maintenance

Aircraft maintenance industry also known as Maintenance, Repair and Ov erhaul (MRO) may be
defined as a ll actions which have the objective of retaining or restoring an item in or to a state in
which it can perform its required function. The actions include the combination of all technical and
corresponding administrative, man agerial and supervision actions.
In order to ensure the sa fe and correction functioning of an aircraft during the flight, the
maintenance is highly regulated. National regulations are coordinated under international standards,
maintained by bodies such as the International Civil Aviation Organization (ICAO). The maintenance
tasks, personnel and inspections are all tightly regulated and staff must be licensed for the tasks they
carry out.
Aviation maintenance refers to the overhaul, repair, inspection or modification of an aircraft or
aircraft component.
The reaso n for which aircraft maintenance is highly regulated is that because the smallest mistake
have the potential to result in an aircraft crash that would lead to a high number of life losses and
huge money loss. The International Civil Aviation Organization (ICAO) sets global standards which
are then implemented by national and regional bodies around the world.
Local airworthiness authorities include:
 Agência Nacional de Aviação Civil (ANAC) Brazil
 Civil Aviation Administration of China (CAAC) China
 Civil Aviation Authority (United Kingdom) (CAA) United Kingdom
 Civil Aviation Safety Authority (CASA) Australia
 Directorate General of Civil Aviation (India) (DGCA) India
 European Aviation Safety Agency (EASA) Europe
 Federal Aviation Administration (FAA) United States
 Transport Canada (TC) Canada

2.1. History of maintenance

“In the early days of aviation, there was a great deal of experimentation and a high death rate.”
Elon Musk
In the early days of aviation, maintenance progra ms were developed by mechanics directl y. The
programs were simple and without any kind of analytical basis. Once airline companies have been
established the implementation of worldwide regulation and procedures was mandatory.

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By the 1950 when large jet airplanes have entered the commercial mar kets the airplane
manufacturer became the source of maintenance program development. The underlying concept was to
overhaul every component at a given time.
In 1960, the industry formed a task force to investigate the capabilities of preventive
maintenance . The findings of the task force led to a new type of maintenance called on -condition
maintenance.
In 1968 the handbook ―Maintenance Evaluation and Program Development ― , also referred to as
― MSG -1‖ was developed for Boing 747 by the Air Transport Assoc iation (ATA) Maintenance Steering
Group (MSG), a group of airframe manufacturers, airlines U.S Federal Aviation Administration (FAA)
representatives, and suppliers.
The experience gained during MSG -1 program is the base of the maintenance programs today. It
used decision logic to develop scheduled maintenance.
In 1970, the document ―Airline/Manufacturer Maintenance Program Planning‖ or ―MSG -2‖ was
developed. It was process oriented and analyzed failure modes from the part level up. Its philosophy was
based on the theory that all airplanes and their component reach a period when they should be ―zero
timed‖ or ―overhauled‖ and restored to new condition. When applied to a particular aircraft type the
MSG -2 logic would produce a list of Maintenance Significant Items (MSIs), to each of which one or more
process categories would be applied, i.e. ―hard time‖ , ―on -condition‖ or/and ―reliability control‖.
In 1978, the United Airlines, commissioned by the Department of Defense, developed a
methodology for designing m aintenance programs based on tested and proven airline practices. This new
methodology was the basis for MSG -3, the current industry standard. The resultant MSG -3 system is
based on the basic philosophies of MSG -1 and MSG -2, but prescribes a different appr oach in the
assignment of maintenance requirements. This methodology has a task -oriented approach to maintenance
that analyzes system failures modes from a system level , or top down. Maintenance tasks are performed
for safety, operational or economic reas ons. They involve both preventive maintenance and failure
finding tasks.
Since 1980, a number of 11 revisions 1980 (Original Issue), 1988 (Revision1), 1993 (Revision 2),
2001.1, 2002.1, 2003.1, 2005.1, 2007.1, 2009.1, 2011.1, 2013.1, 2015.1 have been made.
2.2. Organizational Structure

Maintenance, repair and overhaul (MRO) in the aeronautical industry is a complex process that
has strict and precise requirements defined by airworthiness authorities to guarantee the safety of
passengers and aircrew.
Every organization that is performing maintenance activities in the European territory must be
approved by The European Aviation Safety Agency (EASA) .

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2.2.1 . EASA

The European Aviation Safety Agency (EASA) is an agency of the European Union (EU) with
regulatory and executive tasks in the field of civilian aviation safety . Based in Cologne , Germany, the
EASA was created on 15 July 2002, and it reached full functionality in 2008, taking over functions of
the Joint Aviation Authorities (JAA). European Free Trade Association (EFTA) countries have been
granted participation in the agency.
There are 32 member states (28 EU states + 4 non EU states –Switzerland, Norway , Iceland and
Liechtenstein ) belong to EASA.
The agency’s responsibilities include:
 Implement and monitoring safety rules including inspections in the member states
 Type certification of aircrafts and components as well as the approval of organizations involved
in the design, manufacturing and mainte nance aeronautical product.
 Authorization of third country (non EU) operators
 Safety analysis and research

Regulation Structure

Each Part to each implementing regulation has its own Acceptable Means of Compliance and
Guidance material (AMC/GM). These AMC and GM are amended along with the amendments of the
regulations. They are s o called ―soft law‖ (non -binding rules) and put down in form of EASA Decisions.

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2.2.2 . PART 145

All maintenance and release to service (CRS) of large aircraft (>5.7 t) and/or aircraft for
commercial air transport and components thereof must be done by a PART 145 approved organization.
Commission regulation (EU) No 1321/2014; ANNEX II describes the requirements which are
needed to obtain and keep the PART -145 Maintena nce Organization Approval (MOA)

Technical Requirements
 Facilities
 Environmental conditions
 Equipment, tools
 Components, materials
Organizational Requirements
 Production Planning
 Maintenance organization expo sition (MOE)
 Quality system
 Approved maintenance data
Personnel Requirements
 Maintenance records
 Personnel Requirements
 Accountable manager
 Quality manager
 Maintenance manager
 Staff to plan, perform, supervise, inspect and quality monitor the maintenance organization
 Certifying staff (Part -66)
 NDT personnel (non -destructive testing)
2.3. Maintenance classification

Generally speaking, there are two types of maintenance in use:
 Preventive or scheduled maintenance, where equipment or facilities are inspected, maintained and
protected before break down or other problems occur.
 Corrective maintenance where equipment is repaired or replaced after wear, malfunction or break
down.

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Preventive maintenance (PM) has the following meanings:
The care and servicing by per sonnel for the purpose of maintaining equipment in satisfactory
operating condition by providing for systematic inspection, detection, and correction of incipient failures
either before they occur or before they develop into major defects.
This type of mai ntenance is also known as time -driven or interval -based maintenance and is
carried out without any regard to equipment condition.
Preventive maintenance consists of regularly scheduled inspection, cleaning, adjustments,
calibration, parts replacement, lubrication, and repair of components, equipment, and systems. Preventive
maintenance schedules regular inspections and maintenance at predefined intervals i n order to lower
failures for susceptible items and equipment.
It is important to note that, depending on the interval set, preventive maintenance can lead to a
significant increase in inspections and routine maintenance. However, it can help to reduce the severity
and frequency of unplanned equipment failures for parts with set, age -related wear patterns.
Preventive maintenance tends to follow planned guidelines from time -to-time to prevent
equipment and machinery breakdown
The work carried out on equipmen t in order to avoid its breakdown or malfunction. It is a regular
and routine action taken on equipment in order to prevent its breakdown.
Maintenance, including tests, measurements, adjustments, parts replacement, and cleaning,
performed specifically to prevent faults from occurring.
The primary goal of maintenance is to avoid or mitigate the consequences of failure of
equipment. This may be by preventing the failure before it actually occurs which Planned Maintenance
and Condition Based Maintenance help to achieve. It is designed to preserve and restore equipment
reliability by replacing worn components before they actually fail. In addition, workers can record
equipment deterioration so they know to replace or repair worn parts before they cause system f ailure.
The ideal machine maintenance program would prevent any unnecessary and costly repairs.
Machine maintenance for various equipment and facilities is quite nuanced. For instance,
maintaining certain equipment may include a "preventive maintenance che cklist" which includes small
checks which can significantly extend service life. Furthermore, other considerations such as weather and
equipment are taken into account
Corrective maintenance is a maintenance task performed to identify, isolate, and rectify a fault
so that the failed equipment, machine, or system can be restored to an operational condition within the
tolerances or limits established for in -service operations.
It can be subdivi ded into "immediate corrective maintenance" (in which work starts immediately
after a failure) and "deferred corrective maintenance" (in which work is delayed in conformance to a
given set of maintenance rules).
Additionally , we can also talk about operat ional maintenance.

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Operational maintenance is the care and minor maintenance of equipment using procedures that
do not require detailed technical knowledge of the equipment’s or system’s function and design. This
category of operational maintenance normall y consists of inspecting, cleaning, servicing, preserving,
lubricating, and adjusting, as required. Such maintenance may also include minor parts replacement that
does not require the person performing the work to have highly technical skills or to perform internal
alignment.
As the term implies, operational maintenance, is performed by the operator of the equipment. Its
purpose is threefold:
 to make the operator aware of the state of readiness of the equipment;
 to reduce the delays that would occur if a qu alified technician had to be called every time a
simple adjustment were needed;
 to release technicians for more complicated work
Some operational maintenance responsibilities can be as simple as inspecting the machine to spot
any changes or issues. This al lows the operator to detect a potential danger, such as loose fasteners or
debris that could contribute to an accident. Basic cleaning, including removing debris or excess grease
from a machine, is also considered part of operational maintenance.
Depending on the type of equipment in use, operators may also be responsible for replacing worn
out filters or cartridges, or removing and replacing a worn belt, cutting tool, or grinding stone.
Operational maintenance may entail keeping machinery well lubricated t o reduce the risk of friction or
failure. Many basic machine adjustments needed during the course of operation also fall within this
category of preventative maintenance.
Depending on place and time allocated, the maintenance is also divided in three categ ories:
 Line Maintenance
 Base Maintenance
 Component Maintenance
2.3.1. Line Maintenance

Line Maintenance generally refers to the maintenance performed at the gate ,launch area or
hardstand area. The level of work is limited to what can be disassembled and reassembled during a short
period of a shift or two. A limiting factor is also the level of ground support equipment availability, such
as electrical carts, hydraulic mules, work stands and lifts.
The work performed to the aircraft in this maintenance segm ent is minor. Most aircraft (although
there are exceptions, like many business jets for example) require line maintenance tasks to be performed
quite frequently. In many aircraft types, typical line maintenance tasks would include a daily check
(performed anywhere from every 24 to every 48 hours) and a weekly check (every 7 -8 days). Apart from
that, there may be several OOP (out of phase) maintenance tasks which can be considered to be line
maintenance and carried out by a line maintenance provider.

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Line Ma intenance Environment
The physical environment is obvious. It includes ranges of temperature, humidity, lighting, noise
control, cleanliness, and workplace design. Companies must acknowledge these conditions and cooperate
with the workforce to either accommodate or change the physical environment. 3 t

Line Maintenance Capability
First of all, for some aircraft, the scope of line maintenance is specified in the MPD or MS
(Maintenance Planning Document or Maintenance Schedule). Those documents may either bluntly tell
you that line maintenance is for every check up to and including the 500 HR A -check, for instance.
The capability of aircraft line maintenance is provided in AMC.145.A.10, together with a list of
activities which may be considered as line ma intenance. We say ―may‖ because it is not possible to
establish an exact border of what tasks are considered line maintenance and what are considered base
maintenance as the general applicability is different from case to case.
It is the responsibility of the line maintenance organization to demonstrate to the competent
authority their capability to carry out certain jobs as per their scope of approval.
Capability Criteria
The following general criteria apply to each line maintenance provider as per their a uthorization:
A. Trouble shooting / Defect rectification – Are those unscheduled tasks that are required to be
carried out in order to ensure the safety operation of the aircraft during the flight.
B. Minor scheduled maintenance – are those scheduled tasks that do not exceed the weekly
check as per the specified MS or MPD.
C. Scheduled tasks that exceed the weekly check. In this case the a clear limit must be identified
and must be approved by the competent authority and are expressed in the following ways:
 Up-to an d excluding X Check – (i.e. X= 1A, 2A, 3A, 4A) for a MPD
 Up to and excluding X FH / Y FC / Z calendar days for a MPD where progressive
tasks intervals are defined in terms of FH/FC/calendar time (i.e. X=3000FH ,
Y=1500FC , Z = 12 months )
Exception from th is are the tasks that are meant to support an unserviceable aircraft. This means
that the tasks can be performed only in order to restore the capability of flying of an aircraft in case of an
AOG resulted from an unscheduled event (i.e. engine bird strike – engine replacement )
Engine replacement example on line maintenance
An example of such unscheduled task occurred in August , 2016 when an aircraft had a bird
strike hit directly the engine when taking off with full thrust from Otopeni Airport. The aircr aft have
return to the stand and the technicians have concluded that the engine is unserviceable and therefore the
aircraft was unworthy to fly, being grounded.

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2.3.2. Base Maintenance

Base maintenance, also known as ―hangar‖ or ―heavy‖ maintenance refers to the maintenance
performed in a hangar. This means mainly heavy checks such as C and D checks. During those checks
major and minor aircraft systems are being evaluated together with complex and time consuming tasks
such as corrosion prevention, structura l work, replacement of major components or interior
refurbishment .
Activity considered to be base maintenance
When any of the following task is required to be carried out (regardless if contained in a
scheduled maintenance check or arising from a defect re ctification/AOG situation), a base maintenance
scope of approval is needed to accomplish the following:
 High number of different type of tasks to be carried out, even if taken singularly
those tasks may still fall under the definition of line maintenance , for example a
combination of routine task cards, non -routine task cards issued following defects
discovered during the check, out of phase tasks, deferred items from previous
maintenance, minor repairs, minor modifications, component replacement, etc. . Su ch
case is clearly requiring a base maintenance production planning support and/or base
maintenance release to service process (category C c/s supported by B1/B2 support

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staff) in order to ensure that all the maintenance ordered has been carried out before
issuing the CRS.
 Any scheduled maintenance task (i.e. routine task from the MP) which requires extensive
disassembly of the aircraft and/or extensive in depth inspection;
 Major repairs and/or major modifications
 Trouble Shooting and/or Defect Rectificatio n requiring special ground support usually
relevant to base maintenance like special equipment, structured production planning,
complex and lengthy maintenance.
 A scheduled maintenance event, which in the planning phase has been already identified
as signi ficant in terms of duration and/or man -hours (i.e. an A/C down time above 72
hours and/or four shifts whichever is less).
 A work package requiring a complex team composition in terms of high Number &
Categories (avionic, structure, cabin, NDT qualificatio n and skills, ….) of staff involved
per shift.
The management of the event by B1 and B2 support staff and the release by a C certifying staff.
Base Maintenance Environment
The maintenance performed in this segment mandatory requires and is total dependen t on the
hangar. An aircraft hangar provides a substantial space in which to perform aircraft maintenance.
Operating at all hours and whatever the weather, these large, specialized spaces require effective, large –
scale lighting solutions. Depending on the aircraft and the level of maintenance performed, there are
special requirements for the hangar.
Replacement of any major component where the related maintenance procedures clearly address
the need of an hangar environment requiring specia l ground support equipment and/or structured
production planning and/or complex and lengthy maintenance, such as for example a full landing gear
change, change of two engines, etc.;
Depending on the size of the hangar, it can be classified as:
 Size S: Sp an less than 30 meters
 Size M: Spann between 30 and 60 meters
 Size L: Span between 60 and 90 meters
 Size XL: Span between 90 and 120 meters.
 Size XXL: Span bigger than 12 0meters
Large aircrafts as the Airbus A380, Boeing 747 or Antonov 225 requi re hangars of size XXL as
they are most complex to erect.
A hangar facility with all the necessary hardware equipment do to Boeing 747 maintenance
checks up to D -Checks or HMV ( heavy maintenance visit) could be worth up to 50 million dollars
depending on the airpor t location and country. It may house a labor force of a few hundred workers
depending of operation and utilization of hangar space.

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Lightening level
Aircraft maintenance hangars must have high lighting levels. When repairing aircraft, precision
work cannot be performed appropriately under low illumination levels. Excessive illumination could
result in glare Similarly, glare can impact negatively on lighting levels. Inspection of aircraft takes place
in an environment where specular reflection from airplane structures can cause glare so that low
brightness luminaries should be installed. Often additional task lighting will be necessary when internal
work, or shadowed parts around the aircraft, result in low illumination levels.
Fire protection
Due to their nature, hangars are very hard to be kept fire proof. Due to the large quantities of
liquid jet fuel that are present, mixture of chemicals and maintenance activities the fire potential risk is
very high. Large aircraft wings and fuselages can create obstr uctions to both fire detection and fire
suppression. Large hangars require fire suppression systems requirements that are hard to bet met. The
fire detection system must function over unusual heights and distances. Sensitivity is needed for fast
response, but this factor must be balanced against protection from nuisance alarms. There are a number of
fire suppression options, most of which involve fire pumps, foam systems and sprinkler systems with
large design areas. Zoning and distances from equipment room s to discharge points can also create design
complications.
Types of checks performed
Aircraft maintenance checks are periodic inspections that have to be done on all
commercial/civil aircraft after a certain amount of time or usage. Airlines and airworthiness authorities
casually refer to the detailed inspections as "checks", commonly one of the following: A check, B check,
C check, or D check. A and B checks are lighter checks which are usually performed on the line
maintenance, meanwhile C check and D checks are considered heavier checks that are always being
performed in a hangar
A check
This is performed approximately every 250 flight hours or 200 –300 cycles. It needs about 20 –50
man-hours and is usually performed overnight at an airport gate. The actual occurrence of this check
varies by aircraft type, the cycle count (takeoff & landing is considered an airplane "cycle"), or the
number of hours flown since the last check. The occurren ce can be delayed by the airline if certain
predetermined conditions are met.
B check
This is performed approximately every 6 months. It needs about 120 -150 man -hours, depending
on the aircraft, and is usually completed within 1 –3 days at an airport hangar. A similar occurrence
schedule applies to the B check as to the A check. However, B checks may also be incorporated into
successive A checks, i.e.: Checks A -1 through A -10 complete all the B check items .

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C check
This is performed approximately e very 20 –24 months or a specific amount of actual flight hours
or as defined by the manufacturer. This maintenance check is much more extensive than a B check,
requiring a large majority of the aircraft's components to be inspected. This check puts the airc raft out of
service and until it is completed, the aircraft must not leave the maintenance site. It also requires more
space than A and B checks. It is, therefore, usually carried out in a hangar at a maintenance base. The
time needed to complete such a ch eck is generally 1 –2 weeks and the effort involved can require up to
6,000 man – hours. 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 by far the most compre hensive and demanding check for an airplane. It is also known as a
"heavy maintenance visit" (HMV). This check occurs approximately every 6 years. It is a check that,
more or less, takes the entire airplane apart for inspection and overhaul. Also, if requi red, the paint may
need to be completely removed for further inspection on the fuselage metal skin. Such a check can usually
demand up to 50,000 man -hours and it can generally take up to 2 months to complete, depending on the
aircraft and the number of tec hnicians involved. It also requires the most space of all maintenance checks,
and as such must be performed at a suitable maintenance base. Given the elevated requirements of this
check and the tremendous effort involved in it, it is also by far the most e xpensive maintenance check of
all, with total costs for a single visit ending up well within the million -dollar range. Because of the nature
and the cost of such a check, most airlines, especially those with a large fleet have to plan D checks for
their ai rcraft years in advance. Often, older aircraft being phased out of a particular airline's fleet are
either stored or scrapped upon reaching their next D check, due to the high costs involved in comparison
to the aircraft's value. On average, a commercial a ircraft undergoes 2 –3 D checks before being retired.

2.3.3. Component Maintenance

Aircraft components have high costs of manufacturing and therefore all across the globe there are
many component repair shops. Component repair shops go hand in hand with preventive maintenance as
most of the times the parts are being replaced due to preventive reasons rather than failures.
A workshop that can support all airplane components maintenance services can cost up to 30
million dollars, including the purchasing of electronic automatic test equipment, heat treatment ovens,
integrated chemical waste disposal plant and other expensive machinery, processes and equipment.
Component maintenance shops divide in many categories, as following:
 Avionic shops
 Mechanical compo nent shops
 NDT shops
 Composite shops

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 Emergency Equipment shops
 Nitrogen and Oxygen shop
 Sheet Metal Shop
 Wheel and Brake Shop
Avionic Shops
The term of ―avionics‖ comes from the journalist Philip J. Klass and it stands for ―aviation
electronics ―. The cockpit of an aircraft is a typical location for avionic equipment . The maintenance
performed in this segments includes components such as communication , navigation , monitoring , flight
control systems, collision -avoidance systems, cockpit data recorders , weather systems and radars, aircraft
management systems, entertainment systems, batteries , etc.
Mechanical Components shop
Due to high operating temperature and high friction mechanical components often require
maintenance. Flight control linkage actuat ion, check valves, high pressure shutoff valves, Engine bleed
valves, starter control valve, Pressure/flow regulator valve, fan assembly, aircraft window, are among the
types the shop carries maintenance on.
NDT shops
Non-destructive Test is used during ai rplane, engine and component inspections to detect surface
or internal discontinuities and defect of parts without any deterioration of the geometric configuration or
property of parts.
Composite shops
The function of the composite shop is to repair composite laminates, cored flight control surfaces,
radomes, cabin interior items,
Emergency Equipment shops
Emergency Equipment Shop maintains repairs and overhauls the airplane escape slides, life rafts,
life vests , seat belts, oxygen masks, etc.
Nitroge n and Oxygen shop
The Nitrogen & Oxygen Shop charges aircraft nitrogen and oxygen bottles
Sheet metal shop
This shop is designated to perform sheet metal and tube repair works, modification and
fabrication for airplane maintenance, modification services and other applicable industrial sheet metal
works according to appropriate approvals and the applicable aircraft Structure Repair Manuals (SRM) and
Component Maintenance Manuals (CMM)

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Wheel and Brake Shop
The Wheel and Brake Shop function is to change tires, repair and overhaul wheels, heat stack
change and overhaul of metal brake components .

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3. Human Factors

―We cannot change the human condition, but we can change the conditions under which
humans work‖
James Reason
The human element is the most flexible, adaptable and valuable part of the aviation system but it is
also the most vulnerable to the external factors that can lead to tragedies. The commercial aviation
industry have realized that human error is the main cause of accidents and incidents analyzing models
have been created with the scope of preventing aviation mishaps.
Human factors are issues affecting how people do their jobs. They are the social and personal skills,
such as communication and decision making which complement our technical skills. These are important
for safe and efficient aviation .
The study of human factors involves applying scientific knowledge about the human body and mind to
help understand human capabilities and limitations. Human factors knowledge can be used to reduce the
likelihood of errors and build more error tolerant and more resilient systems. We Need discipline in the
workplace to ensure everyone complies with rules, regulations, procedures, processes and safe practices.
Discipline involves training of the mind and sel f-control. It is a mind -set about forming and sticking to
certain beneficial habits and routines. It has to be taught during the introduction course when new staff is
employed. An undisciplined worker could make a lot of mistakes; some could be serious eno ugh to cause
danger to his life, and in a supply chain process, to jeopardize the safety of others and the well -being of
the organization
The overall goal of Aviation Maintenance human factors research is to identify and optimize the
factors that affect hu man performance in maintenance and inspection.
Part 66
In Europe, every maintenance personnel must comply with EASA Part 66 regulations.
The part -66 is based on the old JAR system and the level of training required is followed the
ATA104 system.
There are 3 levels of authorization:
 Category A
 Category B1 / B2
 Category C

Category A
Also name Line Maintenance Certifying Mechanic (LMCM) it is a basic license with a task
training that depends on the complexity of the task performed that together with the Company
Certification Authorization can issue certificates or release to service minor scheduled line

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maintenance and simple defect rectification within the limits of the task for which he have received
training on. The personnel having a Category A licens e can only perform tasks for which the
authorization company within he is licensed has the authority as per the approved Part -145.
Category B1 / B2
Also called Line Maintenance Certifying Technician (LMCT) Category B1 refers to mechanical
tasks andB2 refer s to Avionics. Category B1/B2 together with the type training and the Company
Certification Authorization permit the mechanic to issue certificates of release to service for aircraft
structure, power plant and mechanical or electrical systems. Replacement of avionic Line Replaceable
Units (LRU), requiring simple tests to prove their serviceability should also be included in the
privileges of technician holding a B1 or B2 license.
Category A is also included in this license.
Category C
Also called Base Maint enance Certifying Engineer (BMCE) together with the type training and
the company authorization can issue certificate of release to service for the activities for which the
company is authorized in the Part -145 approved.
3.1 Human performance

As technolo gy advances and systems become more complex, we sometimes forget that even the
most sophisticated processes still rely on human judgment and skills. We can redesign tools, select better
materials and develop better processes but we cannot design better peo ple.
It is very important for maintenance personnel to obtain some basic knowledge about human
performance and the limitations of their short -term memory, how fatigue affects performance and other
facts about human strengths and weaknesses. For this reason s, human factors training for maintenance
personnel is now required by the International Civil Aviation Organization (ICAO).
3.1.1 The Dirty Dozen

Transport Canada have identified twelve human factors that degrade people’s ability to perform
effectively and safely, which could lead to maintenance errors. These were adopted by the aviation
industry as a straight forward means to discuss human error in maintenance. It is important to know these
identified human factors known as ―dirty dozen‖, how to recogni ze their symptoms, and most
importantly, know how to avoid or contain erro rs produced by the dirty dozen.
The dirty dozen factors are:
 Lack of Communication:
 Complacency
 Lack of knowledge

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 Distraction
 Lack of Teamwork
 Fatigue
 Lack of Resources
 Pressure
 Lack of Assertiveness
 Stress
 Lack of Awareness
 Norms

 Lack of Communication
Lack of communication is a key human factor that can result in suboptimal, incorrect, or faulty
maintenance. Communication occurs between the AMT and many people (i.e. , management, pilots, parts
suppliers, aircraft servicers). Each exchange holds the potential for misunderstanding or omission. But
communication between AMTs may be the most important of all. Lack of communication between
technicians could lead to a maint enance error and result in an aircraft accident. This is especially true
during procedures where more than one technician performs the work on the aircraft. It is critical that
accurate, complete information be exchanged to ensure that all work is complete d without any step being
omitted .
 Complacency
Complacency is a human factor in aviation maintenance that typically develops over time. As a technician
gains knowledge and experience, a sense of self satisfaction and false confidence may occur. A repetitive
task, especially an inspection item, may be overlooked or skipped because the technician has performed
the task a number of times without ever finding a fault. The false assumption that inspection of the item is
not important may be made. However, even if rare, a fault may exist.
 Lack of knowledge
The regulatory requirements for training and qualification can be comprehensive, and
organizations are forced to strictly enforce these requirements. However, lack of on -the-job experience
and specific knowledge can lead workers into misjudging situations and making unsafe decisions. Aircraft
systems are so complex and integrated that it is nearly impossible to perform many tasks without
substantial technical training, current relevant experience and adequate refe rence documents
 Distraction
A distraction while performing maintenance on an aircraft may disrupt the procedure. When work
resumes, it is possible that the technician skips over a detail that needs attention. It is estimated that 15%
of maintenance relat ed er rors are caused by distraction

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 Lack of Teamwork
A lack of teamwork may also contribute to errors in aircraft maintenance. Closely related to lack
of communication, teamwork is required in aviation maintenance in many instances. Sharing of
knowledge between technicians, coordinating maintenance functions, turning work over from shift to
shift, and working with flight personnel to troubleshoot and test aircraft are all are executed better in an
atmosphere of teamwork. Often associated with improved saf ety in the workplace, teamwork involves
everyone understanding and agreeing on actions to be taken.
 Fatigue
Fatigue is a major human factor that has contributed to many maintenance errors resulting in
accidents. Fatigue can be mental or physical in nature . Emotional fatigue also exists and effects mental
and physical performance. A person is said to be fatigued when a reduction or impairment in any of the
following occurs: cognitive ability, decision -making, reaction time, coordination, speed, strength, an d
balance. Fatigue reduces alertness and often reduces a person’s ability to focus and hold attention on the
task being performed.
Symptoms of fatigue may also include short -term memory problems, channeled concentration on
unimportant issues while neglecti ng other factors that may be more important, and failure to maintain a
situational overview. A fatigued person may be easily distracted or may be nearly impossible to distract.
He or she may experience abnormal mood swings. Fatigue results in an increase i n mistakes, poor
judgment, and poor decisions or perhaps no decisions at all. A fatigued person may also lower his or her
standards.
Tiredness is a symptom of fatigue. However, sometimes a fatigued person may feel wide awake
and engaged in a task. The prim ary cause of fatigue is a lack of sleep. Good restful sleep, free from drugs
or alcohol is a human necessity to prevent fatigue. Fatigue can also be caused by stress and overworking.
A person’s mental and physical state also naturally cycles through variou s levels of performance each
day. Variables such as body temperature, blood pressure, heart rate, blood chemistry, alertness, and
attention rise and fall in a pattern daily. This is known as one’s circadian rhythm.
 Lack of Resources
A lack of resources ca n interfere with one’s ability to complete a task because there is a lack of
supply and support. Low quality products also affect one’s ability to complete a task. Aviation
maintenance demands proper tools and parts to maintain a fleet of aircraft. Any lac k of resources to safely
carry out a maintenance task can cause both non -fatal and fatal accidents. For example, if an aircraft is
dispatched without a functioning system that is typically not needed for flight but suddenly becomes
needed, this could creat e a problem.
Within an organization, making sure that personnel have the correct tools for the job is just as
important as having the proper parts when they are needed. Having the correct tools means not having to
improvise. The cost of improvising can be very steep. The right tools to do the job need to be used at all
times, and if they are broken, out of calibration, or missing, they need to be repaired, calibrated, or
returned as soon as possible.

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Technical documentation is another critical resource that can lead to problems in aviation
maintenance. When trying to find out more about the task at hand or how to troubleshoot and repair a
system, often the information needed cannot be found because the manuals or diagrams are not available.
When the prop er resources are available for the task at hand, there is a much higher probability
that maintenance will do a better, more efficient job and higher likelihood that the job will be done
correctly the first time
 Pressure
Aviation maintenance tasks require individuals to perform in an environment with constant
pressure to do things better and faster without making mistakes and letting things fall through the cracks.
Unfortunately, these types of job pressures can affect the capabilities of maintenance worker s to get the
job done right . Airlines have strict financial guidelines, as well as tight flight schedules, that force
mechanics to be under pressure to identify and repair mechanical problems quickly so that the airline
industry can keep moving. Most impor tant, aircraft mechanics are responsible for the overall safety of
everyone who uses flying as a mode of transportation.
Organizations must be aware of the time pressures that are put on aircraft mechanics and help
them manage all of the tasks that need t o be completed so that all repairs, while done in a timely manner,
are completed correctly with safety being the ultimate goal. Sacrificing quality and safety for the sake of
time should not be tolerated or accepted. Likewise, AMTs need to recognize on the ir own when time
pressures are clouding their judgments and causing them to make unnecessary mistakes. Self -induced
pressures are those occasions where one takes ownership of a situation that was not of their doing.
 Lack of Assertiveness
Assertiveness is the ability to express your feelings, opinions, beliefs, and needs in a positive,
productive manner and should not be confused with being aggressive . It is important for AMTs to be
assertive when it pertains to aviation repair rather than choosing or not b eing allowed to voice their
concerns and opinions. The direct result of not being assertive could ultimately cost people their lives
 Stress
Aviation maintenance is a stres sful task due to many factors. Aircraft must be functional and
flying in order for ai rlines to make money, which means that maintenance must be done within a short
timeframe to avoid flight delays and cancellations. Fast -paced technology that is always changing can add
stress to technicians. This demands that AMTs stay trained on the lates t equipment. Other stressors
include working in dark, tight spaces, lack of resources to get the repair done correctly, and long hours.
The ultimate stress of aviation maintenance is knowing that the work they do, if not done correctly, could
result in tra gedy .
 Lack of Awareness
Lack of awareness is defined as a failure to recognize all the consequences of a n action or lack of
foresight. In aviation maintenance, it is not unusual to perform the same maintenance tasks repeatedly.

Page 29
After completing the same t ask multiple times, it is easy for technicians to become less vigilant and
develop a lack of awareness for what they are doing and what is around them. Each time a task is
completed it must be treated as if it were the first time.
 Norms
Norms is short for ―normal,‖ or the way things are normally done. They are unwritten rules that
are followed or tolerated by most organizations. Negative norms can detract from the established safety
standard and cause an accident to occur. Norms are usually developed to so lve problems that have
ambiguous solutions. When faced with an ambiguous situation, an individual may use another’s behavior
as a frame of reference around which to form his or her own reactions. As this process continues, group
norms develop and stabilize . Newcomers to the situation are then accepted into the group based on
adherence to norms. Very rarely do newcomers initiate change in a group with established norms.
3.1.2. Memory

Memory is critical to our ability to act consistently and to learn new things. Without memory, we
could not capture a "stream" of information reaching our senses, or draw on past experience and apply
this knowledge when making decisions. Memory depends on three processes: • Registration – the input of
information into memory
Memory depends on three processes:
• Registration – the input of information into memory
• Storage – the retention of information
• Retrieval – the recovery of stored information
It is possible to distinguish between three forms of memory:
• Ultra short -term memory (or sensory storage)
• Short term memory (often referred to as working memory)
• Long term memory

Ultra short term memory
Physical stimuli are received via the sensory receptors (i.e., eyes, ears, etc.) and stored for a very
brief period of time in sensory stores (sensory memory).Visual information is stored for up to half a
second in iconic memory and sounds are stored for slightly longer (up to 2 seconds) in echoic memory.
This enables us to remember a sentence as a sentence , rather than merely as an unconnected string of
isolated words, or a film as a motion picture, rather than as a series of disjointed images.
Short term memory

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Short term memory receives a proportion of the information received into sensory stores, and
allows us to store information long enough to use it (hence the idea of "working memory"). It can store
only a relatively small amount of information at one time, items of information, for a short duration,
typically 10 to 20 seconds. The duration of short term memory can be extended through rehearsal (mental
repetition of the information) or encoding the information in some meaningful manner (e.g., associating it
with something as in the example above). Memory can be considered to be the storage and retenti on of
information, experiences and knowledge, as well as the ability to retrieve this information.
Long term memory
The capacity of long -term memory appears to be unlimited. It is used to store information that is
not currently being used, including:
• Knowledge of the physical world and objects within it and how these behave
• Personal experiences
• Beliefs about people, social norms, values, etc.
• Motor programs, problem solving skills, and plans for achieving various activities
• Abilities, such as language comprehension
Information in long -term memory can be divided into two types: semantic and episodic. Semantic
memory refers to our store of general factual knowledge about the world such as concepts, rules, and
one's own language, etc. It is infor mation that is not tied to where and when the knowledge was originally
acquired. Episodic memory refers to memory of specific events, such as our past experiences (including
people, events and objects). We can usually place these things within a certain co ntext. It is believed that
episodic memory is heavily influenced by a person's expectations of what should have happened, thus two
people's recollection of the same event can differ
3.1.3. Human sensory capabilities

In maintainability work, there is a need for an understanding of human sensory capacities as they
apply to areas such as parts identification, noise, and color coding. The five major senses possessed by
humans are sight, taste, smell, touch, and hearing. Hum ans can sense items such as pressure, vibration,
temperature, linear motion, and acceleration (shock).
Touch
This complements human ability to interpret visual and auditory stimuli. In maintainability work the
touch sensor may be used to relieve eyes and ears of part of the load. For example, its application could
be the recognition of control knob shapes with or without using other sensors. The use of the touch sensor
in technical work is not new; it has been used for many centuries by craft workers for d etecting surfac e
irregularities and roughness. The detection accuracy of surface irregularities dramatically improves when
the worker moves an intermediate piece of paper or thin cloth over the object surface rather than simply
using his or her bare fingers .

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Sight
This is another sensor that plays an important role in maintainability work. Sight is stimulated by
electromagnetic radiation of certain wavelengths, often known as the visible segment of the
electromagnetic spectrum. In daylight, the human eye is very sensitive to greenish -yellow light and it sees
differently from different angles.
Some of the important factors concerning color with respect to the human eye are as follows:
 Normally, the eye can perceive all colors when looking straight ahead. Howe ver, with an
increase in viewing angle, color perception decreases significantly.
 In poorly lit areas or at night, it may be impossible to determine the color of a small point
source of light (e.g., a small warning light) at a distance. In fact, the light colors will appear to
be white.
 The color reversal phenomenon may occur when one is staring, for example, at a green or red
light and then glances away. In such situations, the signal to the brain may reverse the color.
Some useful guidelines for designer s and others are to choose colors in such a way that color -weak people
do not get confused, use red filters with a wavelength greater than 6,500 Å, and avoid placing too much
reliance on color when critical tasks are to be performed by fatigued personnel
Hearing
This sensor can also be an important factor in maintainability work, as excessive noise may lead to
problems including reduction in the workers’ efficiency, adverse effects on tasks, need for intense
concentration or a high degree of muscular coordina tion, and loss in hearing if exposed for long periods.
In order to reduce the effects of noise, some useful guidelines related to maintainability are as follows [3]:
 Protect maintenance personnel by issuing protective devices where noise reduction is not
possible.
 Incorporate into the equipment appropriate acoustical design and mufflers and other sound –
proofing devices in areas where maintenance tasks must be performed in the presence of
extreme noise.
 Keep noise levels below 85 dB in areas where the presen ce of maintenance persons is
necessary.
 Prevent unprotected repair personnel from entering areas with sound levels more than 150 dB
3.1.4 Stressor factors

Everyone handles stress differently and particular situations can bring about different degrees of
difficulty for different people. For example, working under a strict timeline can be a stressor for one
person and normal for another. The causes of stress are referred to as stressors. They are categorized as
physical, psychological, and physiological str essors. Following, is a list of each and how they may affect
maintenance.

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Physical Stressors
Physical stressors add to the personnel’s workload and make it uncomfortable for him or her in their
work environment. People cope with stress in many different ways. Specialists say that the first step is to
identify stressors and the symptoms that occur after exposure to those stressors. Other recommendations
involve development or maintenance of a healthy lifestyle with adequate rest and exercise, a healthy diet,
limited consumption of alcoholic drinks, and avoidance of tobacco products.
 Temperature – high temperatures in the hanger increase perspiration and heart rate causing the
body to overheat. Low temperatures can cause the body to feel cold, weak, and drowsy.
 Noise – hangers that have high noise levels (due to aircraft taking off and landing close by)
can make it difficult for maintenance personnel to focus and concentrate.
 Lighting -poor lighting within a work space makes it difficult to read technical data and
manuals. Likewise, working inside an aircraft with poor lighting increases the propens ity to
miss something or to repair something incorrectly.
 Confined spaces -small work spaces make it very difficult to perform tasks as technicians are
often contorted into unusual positions for a long period of time.
Psychological Stressors
Psychological stressors relate to emotional factors, such as a death or illness in the family, business
worries, poor interpersonal relationships with family, co -workers, supervisors, and financial worries.
 Work -related stressors —over anxiousness can hinder performanc e and speed while
conducting maintenance if there is any apprehension about how to do a repair or concerns
about getting it done on time.
 Financial problems -impending bankruptcy, recession, loans, and mortgages are a few
examples of financial problems th at can create stressors.
 Marital problems -divorce and strained relationships can interfere with one’s ability to
perform their job correctly.
 Interpersonal problems -problems with superiors and colleagues due to miscommunication or
perceived competition and backstabbing can cause a hostile work environment.
Physiological Stressors
Physiological stressors include fatigue, poor physical condition, hunger, and disease.
 Poor physical condition – trying to work when ill or not feeling well can force the bod y to use
more energy fighting the illness and less energy to perform vital tasks.
 Proper meals -not eating enough, or foods lacking the proper nutrition, can result in low
energy and induce symptoms like headaches and shaking.
 Lack of sleep – fatigued, t he maintainer is unable to perform to standard for long periods of
time and can become sloppy with repairs and miss important mistakes.
 Conflicting shift schedules — -he effect of changing sleep patterns on the body’s circadian
cycle can lead to a degradat ion of performance

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3.2 Human Error

Maintenance errors contribute to a significant proportion of worldwide commercial aircraft accidents
and incidents and these occurrences are costly; yet until recently, little was known of the nature of
maintenance errors and the factors that promote them.
In approximately 75 -80% cases of aviation mishaps the main cause were human performance
deficiencies. Human error is rather the starting point than the end in the investigation and prevention of
aviation mishaps. One of the most popular models for analyzin g the human factor and its role in
aeronautical activity is the SHELL model. The components of this model are: software, hardware,
environment, liveware. The system need for human factors is determined by their impact in two major
areas: system efficiency and the health of operational staff. The most important applications of human
factors are in the field of preventing and managing human errors through education in the human factors.
However the term "human error" does not help in the prevention and invest igation of aviation even ts.
Although it shows us where the system failed it does not tell us anything about the causes that led to the
failure. Also the term "human error" hides the latent factors that should be revealed in order to prevent
aviation accide nts. For example, errors attributed to individuals may actually be caused by design flaws,
poor training, incorrect procedures or operating manuals. In the modern approach to aviation safety,
human error is not the end but rather the starting point in inve stigating and preventing aviation events.
There are many causes of human errors. Some of the common ones are poor training or skills of
personnel, inadequate work tools, poor motivation of personnel, poorly written product and equipment
operating and maint enance procedures, complex tasks, poor work layout, poor equipment and product
design, and poor job environment ( for example poor lighting, crowded work space, high noise level, high
or low temperature, etc.)
Human errors may be classified into many categor ies as following:
 Handling errors
 Maintenance errors
 Assembly errors
 Operator errors
 Design errors
 Installation errors
 Inspection errors
Operator errors are the result of operator mistakes, and the causes of their occurrence include poor
environment, compl ex tasks, lack of proper procedures, operator carelessness, and poor personnel
selection and training.
Maintenance errors occur in field environments because of oversights by maintenance personnel.
Some examples of maintenance errors are repairing a failed item incorrectly, calibrating equipment
incorrectly, and applying the wrong grease at appropriate points on the equipment.

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Assembly errors are the result of human mistakes during product assembly. Some of the causes of
assembly errors are poor illuminati on, poor blueprints and other related material, poorly designed work
layout, and poor communication of related information.
Installation errors occur for various reasons including failure to install equipment or items per the
manufacturer’s specification a nd using the incorrect installation instructions or blueprints.
Design errors are the result of inadequate design. Some of the causes of their occurrence are failure
to ensure the effectiveness of person –machine interactions, failure to implement human nee ds in the
design, and assigning inappropriate functions to humans.
Inspection errors are the result of less than 100% accuracy of inspection personnel. One typical
example of inspection errors is accepting and rejecting out -of-tolerance and in -tolerance c omponents and
items, respectively.
Handling errors occur because of improper transportation or storage facilities.
3.2.1 . Types of human errors

Nearly all adverse events involve a combination of these two sets of factors, active failures and latent
conditions .
Active failures are the unsafe acts committed by people who are in direct contact with the patient or
system. They take a variety of forms: slips, lapses, fumbles, mistakes, and procedural violations. Active
failures have a direct and usually s hort lived impact on the integrity of the defenses
Latent conditions are the inevitable ―resident pathogens‖ within the system. They arise from decisions
made by designers, builders, procedure writers, and top level management. Such decisions may be
mistaken, but they need not be. All such strategic decisions have the potential for introducing pathogens
into the system. Latent conditions have two kinds of adverse effect: they can translate into error
provoking conditions within the local workplace (fo r example, time pressure, understaffing, inadequate
equipment, fatigue, and inexperience) and they can create long lasting holes or weaknesses in the
defenses (untrustworthy alarms and indicators, unworkable procedures, design and construction
deficiencies , etc.). Latent conditions —as the term suggests —may lie dormant within the system for many
years before they combine with active failures and local triggers to create an accident opportunity. Unlike
active failures, whose specific forms are often hard to f oresee, latent conditions can be identified and
remedied before an adverse event occurs. Understanding this leads to proactive rather than reactive risk
management.
3.2.2 Human Error prediction models

Many mathematical models can be used to predict the oc currence of human error in maintenance . I
am going to present two of them.
Model 1 – The probability of error In maintenance

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This mathematical model can be used to predict the probability of a maintenance person making an
error. The model state –space diagram is shown in diagram below

MT performing his MT making an error
task correctly 0 1

Figure 1. State –space diagram of a maintenance person performing a t ime-continuous
task.
The numerals in boxes denote system states. The following assumptions are associated with this
mode:
 The Maintenance technician (MT) is performing a time -continuous task.
 The rate of error made by the Maintenance Technician (MT) is con stant.
 The errors occur independently.
The following symbols are associated with Figure
 is the constant maintenance error rate
 ( ) is the probability that the maintenance technician is performing his or her task
correctly at time t.
 ( ) is the probability that the maintenance person has committed an error at time t.

Using the Markov Method, we can write down the following equations for Figure 1.
( )
+ ( ) (1.1)
( )
– ( ) (1.2)
At time t=0, ( ) ( )
Solving Equation 1 and Equation 2 , using Laplace Transformation, we get
( ) (1.3)
( ) (1.4)
The maintenance technician’s reliability is given by
( ) ( ) (1.5)
Where ( ) is the MT reliability at time t.

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Mean time to maintenance error (MTTME) is given by the formula
MTTME= ∫ ( )
(1.6)
=∫
(1.7)
=
(1.8)
Model 1 example
A maintenance person is performing some time -continuous tasks and his or her error rate is 0.008
errors per hour. Calculate his or her reliability during a 7 -hour mission.
( ) ( )( )
= 0.9455
Model 2 – The prediction of an error in fluctuating environment
This mathematical model can be used to predict the probability of the maintenance person making an
error in a fluctuating environment (i.e., normal or stressful). The model state –space diagram is shown in
figure below.

Figure 2

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The numerals in boxes and circles denote system states. The following assumptions are associated with
this model
 The maintenance person is performing time -continuous tasks in fluctuating environments.
 The rates of errors made by the maintenance person in fl uctuating environment are different and
constant.
 The errors occur independently.

The following symbols are associated with Figure 2

 ( ) is the probability of the maintenance person being in state i at time t for i=0 ((maintenance
person performing tasks correctly in a normal environment); i=1 (maintenance person made an
error in a normal); i=2 (maintenance person performing tasks correctly in a stressful environment)
and i=3 ( maintenance person made an error in a stressful environment).
 is the constant error rate of the maintenance person when working in a normal environment.
 is the constant error rate of the maintenance person when working in a stressful environment.
 is the constant transition rate from a stressful environment to a normal environment
 is the constant transition rate from a normal environment to a stressful environment.
Using Markov method, we can write down the following equations for figure 2:
( )
( ) ( ) ( ) (2.1)
( ) ( ) ( ) (2.2)
( )
( ) ( ) ( ) (2.3)
( )
( ) (2.4)
At time t=0, ( ) ( ) ( ) ( )
Solving equation 2.1 to Equation 2.4 using Laplace transformation, we get the following state probability
equations:
(t) ( ) [( ) ( ) ] (2.5)
Where:
( )
(2.6)
( )
(2.7)
( )
(2.8)

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(2.9)
( ) (2.10)
( ) (2.11)

Where

(2.12)

( )
(2.13)
( ) (2.14)
( ) (2.15)
( ) ( ) (2.16)
( ) [( )( )] (2.17)

(2.18)
The maintenance person’s reliability is given by:
( ) ( ) ( ) (2.19)
The mean time to maintenance error (MTTME) is given by:
MTTME=∫ ( )
(2.20)
( )
(2.21)
Example for model 2
Assume that a maintenance person is performing maintenance tasks in a fluctuating environment, normal
and stressful. The constant error rates of the maintenance person under normal and stressful conditions are
0.002 errors per hour and 0.007 errors per hour, respectively . The value of the transition from a normal
environment to a stressful environment is 0.04 per hour, and conversely, 0.01 per hour.
MTTME=( )
( )

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( )( ) ( )( )
=181.53 hours
Thus, the mean time to mai ntenance error is 181.53 hours.

3.2.3 SHELL model

For a better understanding of human factors, a gradual approach needs to be taken with the help of the
"SHELL" theoretical model. The figure below illustrates this model using squares representing differe nt
elements of human factors. The model name comes from the initials of the components: Software
(procedures, symbols), Hardware (machines, aircraft), Environment (environment, the context in which
L-H-S system works) and Liveware (the human). This model has only a didactic value and aims to
facilitate a better understanding of human factors.

Liveware . The center of this model is the human, the most sensitive and flexible system component.
People, however, are subject to considerable variation in performance and its limitations, most of which
are predictable in general terms. Liveware is the core compo nent of the model; all the other components
should be adapted to "fit" with it .
Liveware -Hardware. This interface is most often considered when talking about man – machine
system: designing seats depending on human body characteristics, designing displays depending on the
characteristics of sensory information processing, or the cockpit controls with proper control.
Liveware -software. This interface is about man and proc edures, manuals and checklists and computer
software.
Liveware -environment. This interfa ce was among the first addressed. Initially, the first steps were to
adapt human to the environment (helmets, flight suits, oxygen masks). After that the trend reversed to
adapt the environment to humans by introducing pressurization and air conditioning, soundproofing. This
includes also perceptual illusions generated by environment, but also aspects of political and economic
constraints.

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Liveware -Liveware. This interface is about interpersonal relationships. Traditionally, education,
training and perfo rmance evaluation was done individually for each person. In this interface we are
concerned with leadership, cooperation between team members, teamwork and interpersonal interactions.
Also the scope of this interface is staff/ management relationships, wor k climate, organizational climate
and the pressures of the organization th at may affect human performance.

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4. Safety

― The way to be safe is never be secure.‖
Thomas Fuller

4.1 Introduction

Safety is crucial in the airline industry. In the past, most rese arch were being focus on the decrease of
mechanical failures and pilots errors. Recently, researchers are devoting more attention to the contribution
of maintenance to accidents and incidents.
Aviation safety depends on minimizing error in all facets of t he system. While the role of flight deck
human error has received much emphasis, recently more attention has been directed toward reducing
human error in maintenance and inspection. Aviation maintenance and inspection tasks are part of a
complex organizati on, where individuals perform varied tasks in an environment with time pressures,
sparse feedback, and sometimes difficult ambient conditions. These situational characteristics, in
combination with generic human erring tendencies, result in varied forms of error.
The aim of all the procedures and regulations is to ensure the airworthiness of the aircraft in terms of
safety. Different than in any other industrial segment, a mistake in aviation can lead to disasters, not only
in economic terms but also in human lives.
The most severe result in accidents and loss of life.. While errors resulting in accidents are most
salient, maintenance and inspection errors have other important consequences ( for example air turn -backs,
delays in aircraft availability, gate returns, and diversions to alternate a irports) which impede productivity
and efficiency of airline operations, and inconvenience the flying public
The safety and reliability of aircraft maintenance operations depend as much upon people as they do
on the technical systems of aircraft, parts, t ools and equipment. Nevertheless, accident and incident
reports continue to show that aircraft maintenance engineers sometimes make errors, that aircraft
maintenance organizations sometimes fail to organize and monitor their work effectively, and that thes e
failures can have disastrous consequences. Furthermore, even when things do not go radically wrong, the
evidence suggests that on a routine day -to-day basis, the systems which should ensure that work is
accomplished to the highest possible standard are n ot functioning effectively.
Because maintenance is carried out in all sectors and workplaces and involves a wide range of tasks,
it is associated with a great variety of hazards. It often involves unusual work, non -routine tasks and is
often performed in e xceptional conditions, such as working in confined spaces. During maintenance
activity employees often need to be in close contact with processes and moving machinery. Working
under time pressure is also typical for maintenance operations, especially when shutdowns or high –
priority repairs are involved. There are four items that merit particular attention because of the severity of

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the harm that could be involved, and because they are commonly encountered during plant and building
maintenance .
4.2 The evolution of safety

The history of the progress in aviation safety can be divided into three eras.
The technical era
from the early 1900s until the late 1960s. Aviation emerged as a form of mass transportation in which
identified safety deficiencies wer e initially related to technical factors and technological failures. The
focus of safety endeavors was therefore placed on the investigation and improvement of technical factors.
By the 1950s, technological improvements led to a gradual decline in the freq uency of accidents, and
safety processes were broadened to encompass regulatory compliance and oversight.
The human factors era
from the early 1970s until the mid -1990s. In the early 1970s, the frequency of aviation accidents was
significantly reduced due to major technological advances and enhancements to safety regulations.
Aviation became a safer mode of transportation, and the focus of safety endeavors was extended to
include human factors issues including the man/machine interface. This led to a search for safety
information beyond that which was generated by the earlier accident investigation process. Despite the
investment of resources in error mitigation, human performance continued to be cited as a recurring factor
in accidents The application of hu man factors science tended to focus on the individual, without fully
considering the operational and organizational context. It was not until the early 1990s that it was first
acknowledged that individuals operate in a complex environment, which includes m ultiple factors having
the potential to affect behavior.
The organizational era
from the mid -1990s to the present day. During the organizational era safety began to be viewed from
a systemic perspective, which was to encompass organizational factors in add ition to human and technical
factors. As a result, the notion of the ―organizational accident‖ was introduced, considering the impact of
organizational culture and policies on the effectiveness of safety risk controls. Additionally, traditional
data collec tion and analysis efforts, which had been limited to the use of data collected through
investigation of accidents and serious incidents, were supplemented with a new proactive approach to
safety. This new approach is based on routine collection and analysi s of data using proactive as well as
reactive methodologies to monitor known safety risks and detect emerging safety issues. These
enhancements formulated the rationale for moving towards a safety management approach.
4.3 Swiss -cheese Model

The Swiss -Cheese‖ Model, developed by Professor James Reason, illustrates that accidents involve
successive breaches of multiple system defenses. These breaches can be triggered by a number of
enabling factors such as equipment failures or operational errors. Since the Swiss -Cheese Model contends

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that complex systems such as aviation are extremely well defended by layers of defenses, single -point
failures are rarely consequential in such systems. Breaches in safety defenses can be a delayed
consequence of decis ions made at the highest levels of the system, which may remain dormant until their
effects or damaging potential are activated by specific operational circumstances. Under such specific
circumstances, human failures or active failures at the operational l evel act to breach the system’s
inherent safety defenses. The Reason Model proposes that all accidents include a combination of both
active and latent conditions.
The figure below shows how the Swiss -Cheese Model assists in understanding the interplay of
organizational and managerial factors in accident causation. It illustrates that various defenses are built
into the aviation system to protect against fluctuations in human performance or decisions at all levels of
the system. While these defenses act to p rotect against the safety risks, breaches that penetrate all
defensive barriers may potentially result in a catastrophic situation. Additionally, Reason’s Model
represents how latent conditions are ever present within the system prior to the accident and c an manifest
through local triggering factors.

Swiss -cheese model figure

4.4 Aviation Culture

Aviation safety goes beyond geographic boundaries or culture. The aviation industry has managed to
standardize the industry according to aircraft types, nationalities and countries. However, identifying that

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people react differently to similar situations i s not difficult. Interaction within the industry is affected by
cultural differences and differences in backgrounds. Culture affects how individuals deal with various
situations. Companies are affected by cultural influences at all levels of the organizati on.
4.4.1 Safety culture

―Aviation Safety Is Learned Behavior ‖
J. J. Baker
Safety culture is an ensemble of beliefs, attitude, perceptions and values that the employees share
related to safety in the workplace.
An organization's culture is defined by wh at the people do. The decisions people makes reflects the
values of the organization. The following are four critical elements of safety culture, these activities
would make up an "informed culture" – one in which those who manage and operate the systems h ave
current knowledge about the human, technical, organizational and environmental factors that determine
the sa fety of the system as a whole.
Aviation organizations are increasingly introducing safety management systems (SMS) that go
beyond legal compliance with rules and regulations, and instead emphasize continual improvement
through the identification of hazards and the management of risk. The activities involved in managing the
risk of maintenance error can be appropriately included within the SMS approach. Key activities include
internal incident reporting and investigation systems, human factors awareness for maintenance
personnel, and the continual identification and treatment of uncontrolled risks.
Just culture
Creating a just culture that could be as well called a trusting culture is the first and the most important
step in creating a safe culture. No one is going to confess his mistakes in a blaming and punitive culture.
Trusting is an essential element in creating a reporting culture and h ence a learning culture.
In a just culture errors and unsafe acts will not be punished if the error was unintentional. However,
those who act recklessly or take deliberate and unjustifiable risks will still be subject to disciplinary
action. It hinges critically on a collectively agreed and clearly understood distinction being drawn
between acceptable and unacceptable behavior. A clear line should be drawn between errors and
procedure violations. Meanwhile errors are largely unintended, most violations are intentional.
Reporting culture
Instead of hiding, people are encouraged to report their errors or near errors concerning safety. When
the safety concerns are being analyzed there can be taken proper corrective actions in order to find a
solution or to prevent such cases from happening in the futur e.
There is a high number of psychological and organizational barriers to be overcome in order to create
a reporting culture. The first is the natural disinclination to confess your colleagues mista kes as the
society tends to ridicule the act of turning someone. The second is the suspicion that such reports might

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go on the record and count against us in the future. Another barrier is related to the learning culture as
unless there is a strong learnin g culture thinking that the time you took to investigate and report the issue
may be in vain as no actions will be taken.
Flexible Culture
Aircraft maintenance is an environment where things tend to change a lot. Therefore, the people must
be capable of ad apting effectively to the changing demands. It happens that an aircraft maintenance
technician changes the working place during the s ame shift or has to over 12 hours due to unscheduled
events.
Learning Culture
A learning culture is a collection of organiz ational conventions, values, practices and processes.
These conventions encourage employees and organizations develop knowledge and competence. An
organization with a learning culture encourages continuous learning and believes that systems influence
each other. Without an efficient reporting culture that collect and analyze the safety -related information
the organization is not able to adopt the most appropriate learning mode.
4.4.2. Safety Management Systems (SMS)

Effective SMS implementation by the pro duct or service provider as well as effective SMS oversight
by the State are both dependent upon a clear, mutual understanding of errors and violations and the
differentiation between the two. The difference between errors and violations lies in intent. Wh ile an error
is unintentional, a violation is a deliberate act or omission to deviate from established procedures,
protocols, norms or practices.
Errors
As indicated previously, an error is defined as ―an action or inaction by an operational person that
leads to deviations from organizational or the operational person’s intentions or expectations‖. In the
context of an SMS, both the State and the product or service provider must understand and expect that
humans will commit errors regardless of the level of technology used, the level of training or the
existence of regulations, processes and procedures
Errors can be divided into the two following categories:
 Slips and lapses are failures in the execution of the intended action. Slips are actions that do
not go as planned, while lapses are memory failures. For example, operating the flap lever
instead of the (intended) gear lever is a slip. Forgetting a checklist item is a lapse.
 Mistakes are failures in the plan of action. Even if execution of the plan were correct, it
would not have been possible to achieve the intended outcome
Safety strategies must be put into place to control or eliminate errors. The strategies to control errors
leverage the basic defenses within the aviation system. These include the fo llowing:

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 Reduction strategies provide direct intervention to reduce or eliminate the factors contributing
to the error. Examples of reduction strategies include improvement of ergonomic factors and
reduction of environmental distractions.
 Capturing strateg ies assume the error will be made. The intent is to ―capture‖ the error
before any adverse consequences of the error are felt. Capturing strategies are different from
reduction strategies in that they utilize checklists and other procedural interventions r ather
than directly eliminating the error
 Tolerance strategies refer to the ability of a system to accept that an error will be made but
without experiencing serious consequences. The incorporation of redundant systems or
multiple inspection processes are examples of measures that increase system tolerance to
errors.
Since the performance of personnel is generally influenced by organizational, regulatory and
environmental factors, safety risk management must include consideration of organizational policies,
processes and procedures related to communication, scheduling of personnel, allocation of resources and
budgeting constraints that may contribute to the incidence of errors.
Violations
A violation is defined as ―a deliberate act of wilful misconduct or om ission resulting in a deviation
from established regulations, procedures, norms or practices‖. Nonetheless, non -compliance is not
necessarily the result of a violation because deviations from regulatory requirements or operating
procedures may be the resul t of an error. To further complicate the issue, while violations are intentional
acts, they are not always acts of malicious intent. Individuals may knowingly deviate from norms, in the
belief that the violation facilitates mission achievement without crea ting adverse consequences.
Violations of this type can be categorized as follows:
 Situational violations are committed in response to factors experienced in a specific context,
such as time pressure or high workload.
 Routine violations become the normal wa y of doing business within a work group. Such
violations are committed in response to situations in which compliance with established
procedures makes task completion difficult.
 Organizationally induced violations may be considered as an extension of routi ne violations.
This type of violation tends to occur when an organization attempts to meet increased output
demands by ignoring or stretching its safety defenses .
4.5 Tools handling

The working environment in line maintenance is filled with hazards, from toxic chemicals and h igh
sound levels to the use of tools that mishandled can cause great danger to both the user an d the
component or the aircraft .

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4.5.1 Tools handing procedures

In terms of general safety rules, both for the integrity of the aircraft and for the safety of the user there
are clearly established procedures created by each aviation organization. They all must contain, as basic
procedure, the following:
Tools must be kept in good condition according to the manufacturer’s recommendation or according
to the internal quality procedure of the company.
The right tool must be used for the respective required job. This requirement affect line maintenance
as very often the maintenance in this segment is performed at the gate , being under time -pressure. Not
having the correct tool in the close proximity or due to unavailability of the tool could result in a job
being performed incorrectly.
Each tool , before performing the job , must be inspected and made sure the tool is not damaged or
out of cal ibration.
Every tool must be used in accordance to the manufacturers’ instructions. Wrong use of the tool or
improvisations could lead in jobs performed incorrectly.
In addition to general guidelines, there are specific rules associate d with each class of tools used
Hand tools

Hand tools can include anything from hammers to safety wire pliers. The greatest hazard posed by hand
tools result from the misuse or improper maintenance. If a chisel is used as a screwdriver, the tip of the
chisel may break and fly off, injuring the user or other employees. If a wooden handle of a tool such as a
rubber mallet is loose, splintered, or cracked, the head may fly off and cause injury. If the jaws of a
wrench are sprung, the wrench might slip. If impact tools like rivet sets have mushroomed heads, the
heads might shatter on impact, sending sharp fragments flying toward the user.
Iron or steel hand tools may produce sparks that can be an ignition source around flammable
substances. When working in a fire hazard area, spar k-resistant tools made of non -ferrous materials
should be used. This includes areas where flammable gases and highly volatile liquids are stored or used.
Power tools
Power tools include electric and pneumatic tools. In order to avoid injuries or damages both to the
aircraft or the tools each maintenance organization have its own procedures on how to operate and handle
different tools. There are general procedures applying for both categories and special procedures
depending on the category of tools. Basic procedures should include:
 Disconnect tools when not using them, before servicing and cleaning them, and when
changing accessories such as blades, bits, and cutters.
 When possible, secure work with clamps or a vise, freeing both hands to operate the tool.

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 To avoid accidental starting, do not hold fingers on the switch button while carrying a
plugged -in tool. Follow instructions on the user's manual for lubricating and changing
accessories.
 Be sure to keep good footing and maintain good balance when operati ng power tools.
 Wear proper apparel for the task. Loose clothing, ties, or jewelry can become caught in
moving parts.
 Keep cords and hoses away from heat, oil, and sharp edges.
Electric tools
When specifically talking about electric tools, the most serious hazards associated with electric tools
are electrical burns and shock. Shock can lead to injuries or even heart failure. Under certain conditions,
even a small amount of electric current can result in heart fibrillation and death. An electric shock can
also cause the user to fall off of a ladder or other elevated work surface and be injured due to the fall.
In order to protect yourself from shock and burns, electric tools must have a three -wire cord with a
ground and be plugged into a grounded receptacle, be double insulated, or be powered by a low -voltage
isolation transformer. W henever using an adapter to accommodate a three -wire plug into a two -hole
receptacle, the adapter wire must be attached to a known ground. The third prong must never be removed
from the plug.
Double -insulated tools provide potection against electrical shock without third -wire grounding. On
double -insulated tools, an internal layer of protective insulation completely isolates the external housing
of the tool.
The following general sa fety guidelines are applicable to working with electric tools:
• Operate electric tools within their design limitations.
• Store electric tools in a dry place when not in use.
• Do not use electric tools in damp or wet locations unless they are approved fo r that purpose.
• Ensure that cords from electric tools do not present a tripping hazard.
Pneumatic tools

There are several safety issues concerning pneumatic tools. Foremost of them is the danger of getting hit
by one of the tool's attachments (such as a rivet set).
Pneumatic tools must be checked to see that the tools are fastened securely to the air hose to prevent
them from becoming disconnected.
If an air hose is more than 1/2 inch in diameter, a safety excess flow valve must be installed at the
sourc e of the air supply to reduce pressure in case of hose failure.
The same general precautions should be taken with an air hose that are recommended for electric
cords because the hose is subject to the same kind of damage and can also cause tripping hazards .

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When using a rivet gun or air hammer, a safety clip or retainer should be installed in order to prevent
the set from being ejected during operation. Rivet guns should never pointed against anyone.

4.6 Material Handling

Basically materials can be classified into four categories:
Consumables: are materials identified as raw material, composites and chemicals such as metal
extrusion and profiles, plastic extrusions, wires, cables and protection, seals and strips, insulation
material, tapes, hoses ,fl uids for surface treatment ( cleaning/ pickling agents, primers, paints), adhesives,
additional material for welding or plasma jet welding, other auxiliary or additive (lubricant, fuel)
Expendable : are components or parts )such as bolt, nut rivet, relay , switch ) for which
No authorized repair procedure exists, and/or
The cost of repair would exceed cost of its replacement.
Expandable items may have serial numbers but are usually considered to be consumed when issued
and are not recorded as returnable inv entory.
Rotables: are components which are serialized and tracked on a database. These are also parts that can
be rebuilt/overhauled and put back in stock to use again. Rotables are basically the opposite of
―expandable‖ (throw -away) parts.
Repairable – Parts which the technician can repair per approved authority. (ex. Wire harnesses )

Material Documentation
New aircraft components must hold an airworthiness release certificate issued by or on behalf of the
authority. Aircraft components without such certif icate when passed on cannot be accepted.
For new aircraft components, an EASA Form 1 must be available with an issue date as from 28
September 2015. Depending on the customer’s procedures it may be acceptable that the material received
to come with differe nt certificates like FAA FORM 8130 -3, TCCA FORM ONE, ANAC F -100-01.
A certificate of Conformance (CofC) is acceptable for aircraft components which are piece parts – not
assemblies thereof , of electrical, electromechanical or mechanic type. Such parts inc lude active and
passive electronic components, switches, fixtures, consumables .
A certificate of Conformance (CofC) is sufficient as a release for certain aircraft material whereas
there is no official form required. By this document, the supplier confirms that the component conforms
to the required specification (or standard ).

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The CofC must hold at least the following data:
 Issue’s name and address
 Date of issue
 Quantity supplied
 Designation of the material
 The serial number of the part if applicable
 Batch number if applica ble
 Shelf life limitations if applicable.

A statement that the component was produced accord to or conform to the specification. This
specification has to be explicitly stated. General references like ― applicable specifications ‖ should not be
acceptable.
Example of EASA Form 1:

Material warehousing procedures
The warehouse should be clean, well ventilated and maintained at an even dry temperature to
minimize the effects of condensation. In many instances the manufacturer will specify the temperature
and relative humidity in which the products should be stored
.

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Flammable materials
Material of flammable nature should be kept in an isolated room. Dangerous chemicals and
flammable solvents, even held in small quantities can fuel and lead to a dangerous fire. Flammable
substances are denoted by black pictogram of a flame against a yellow background as below.

Bonding and grounding of flammable liquids.
Static electricity can be generated when a fluid flows through a pipe or from an opening into a tank or
container. If an electrical static discharge is present during the transfer of flammable or combustible liquid
a spark could ignite the liquids and cause injury to the personnel or the aircraft.
To avoid the building of stati c electricity that may cause a spark and the ignition of flammable and
combustible liquids the bounding or grounding of the materials is required.
Radioactive Materials
Occasionally, radioactive materials are used in aircrafts due to their physical charact eristics, which are
applied for different technical solutions.
Rules in dealing with radioactive materials
Because of the harmful potential of radioactive components there are special rules in the handling of such
components. They shall not be stored toget her with other hazardous materials and be stored in separated
shelves or storage areas which are labeled accordingly. The parts must be kept at all times in their original
corresponding packing. In case of damage or damage suspicion of a radioactive part a distance of at least
one meter shall be kept from the part and corresponding authorities should be informed.

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They are marked with the sign.

Example of radioactive parts:

Emergency light of EXIT signs.

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Emergency light contains the radioactive material Tritium H3. It contains a small source of radiation
which is used as a light source. It is welded into the part. The B -radiation rays into a glass ampule, which
is covered internally with a fluorescent. Therefore it glows in a gree n-yellow color.

Smoke detectors :

The smoke detectors contain the radioactive material Americium 241. It is a ionization smoke detector:
through -flow of air is ionized and tested for its conductivity. Smoke can be detected due to its different
conductivity and will be displayed electrically. Americium 241 permanently radiates alpha radiation. The
housing (or shielding of the radioactive source) of this device functions as a protective shield against the
radiation thus it has to be undamaged.
Ignition exciter

Ignition exciters are installed on engines to create the ignition spark.

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They include the element Krypton -85 which has beta radiations. It is marked on the electrode with
―KR85‖. The radiation level is very weak thus it is not possible to measure the radiation outside the part.
Other radioactive materials found in aircraft parts are Uranium 238 which is used in balance weights as a
replacemen t for materials with high density like plumb for example, Strontium 90 that can be found in
anti-ice detectors and Kobalt 60 which can be found in receiver tubes.
Oxygen generators
Oxygen generators contain up to 600g of sodium chlorate and particularly P rotective Breathing
Equipment (PBE hoods) can contain up to 1,100g of potassium superoxide. These substances are
classified as highly reactive oxidizing substances of group I.
Aviation authority requirements require oxygen generators to be protected by a h ousing to prevent any
risk when they are handled incorrectly. However, an exothermic reaction with a considerable build -up of
heat can be trigger if oxygen generators are damaged by impact or shock. The reacted is accelerated by
contact with flammable orga nic substances like greases, oil or even with water in the case of potassium
superoxide which has a high risk of explosion.
The storing of oxygen generators is having strictly requirements like :
storing them separately from products of the same section. B uildings in which oxygen generators are
stored are not to be more than one store high. The walls are to be made of fire -resistant materials. If the
surface is 1600 square meters the ceiling has to be fire resistant as well.
The transportation of the oxygen generators must be done in their original containers. The transportation
by air, land or sea can only be done by appropriate tr ained and authorized personnel.

Electrostatic discharge sensitive devices (ESDS)
Units which are sensitive and prone to damages due to discharge of static electricity are called
Electrostatic Discharge Sensitive Devices (ESDS). The human body, all work surfaces, floors, personnel
clothing, Packaging materials are prime generators of electrostatic voltages. A person walking on airc raft
carpet, removing his shirt, rubbing his hair accumulates electrostatic charge of over 1000 volts. If such a
person touches an ESDS, the device gets damaged due to static discharge.
Most people cannot feel an electrostatic discharge below 3000 volts. A visible spark occurs normally
above 12000 volts. Therefore, the person may become charged during normal work an damage an ESDS
even without realizing it. The main objective of all electrostatic prevention method is to prevent static
charge accumulation. W rist straps are the primary method used to minimize charge generation on the
human body. They must be able to drain dis charge as rapidly as it is generated.
While installing, removing or storing ESDS special procedures and precautions should be taken as
following:
 Electrical power sources must be removed.

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 Wrist wrap should be worn around the bare wrist that in turn is connected to ground by means of
a wire and plug.
 If ESDS sub -assembly is removed from the aircraft, its connecting pins/leads must be shorted
together by means of wires, shorting clips, metal foil or conductive foam. Printed circuit board
connections must also be shorted as above to keep all components a t the same voltage potential.
For top level assemblies that are fully assembled with all covers as not normally considered
electrostatic sensitive. However , the connectors of such units should be properly blanked with
antistatic blanks and packed in prote ctive antistatic packing material.
 The ESDS components must be packed in conductive material which will ensure that the entire
package is maintained at the same electric potential.
 ESDS units should be processed on ESDS work benches.
Nickel cadmium and le ad acid batteries
This type of batteries can be stored for long periods without damage, in any state of charge, provide the
storage place is clean and dry and the battery is correctly filled.
Damage to the battery will occur if it is allowed to stand idle beyond the period for charging specified by
the manufacturer.
Compressed gas cylinders.
Stores which are used for storage of compressed gas cylinders should be well ventilated. The cylinders
should not be exposed to the direct rays of the sun and no covering should be used which is in direct
contact with the cylinders.
They should not be laid on damp ground or exposed to any conditions liable to cause corrosion.
Gas store cylinders should norm ally be fitted with a transportation/ storage cap over the shut -off valve to
prevent handling damage and contamination of parts which could cause a risk of explosion or fire.
Portable gas cylinder should be stored on racks and, where appropriate, control he ads and causes should
be protected against impact.
No heating is required in stores where compressed gas cylinders are kept unless specified by the
manufacturer.
Store rooms should be constructed of fireproof materials and the cylinders placed so as to be
easily removable in the event of fire. The store should be at distance from corrosive influences such as
battery rooms.
Oxygen and combustible gases such as acetylene should not be stored together. Acetylene
cylinders should be stored in the upright positi on.
Oxygen cylinders are generally rounded at the bottom, thereby making it unsafe to store in a n
upright position without suitable support. If cylinders are stacked horizontally special wedges should be
used to prevent the cylinder rolling and the stack of cylinders should not be more than four high.

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Breathing oxygen and welding oxygen should be segregated and properly labeled to avoid
confusion.
Electrical cables
Where electrical cables are stored in large reels it is necessary that the axis of the reel s is in a horizontal
positions . If stored with the axis vertical there is a possibility that the cable in the lowest side of the reel
will become crushed .
Pipes
Rigid pipes should be adequately supported during storage to prevent distortion. Flexible pipes should,
unless otherwise stated by the manuf acturer:
 Be suitably wrapped
 Be stored in a darkened room, maintained at a temperature of approximately 15 C.
In hot climates flexible pipes should be stored in cool places where air circulates feely as high
temperatures tend to accelerate surface hardening of the outer cover.
Flexible pipes should be stored in a completely unstressed condition and, where possible, should be
suspended vertically.
Pyrotechnics
Pyrotechnics should be stored in a dry, well ventilated building and kept at constant room temperature.
At the periods specified by the manufacture pyrotechnics should be examined for any sign of damp or
other external damage. With paper -cased items, such a signal cartridges , the effect of dam is usually
indicated by the softening or buling of the outer case and evidence of staining.
With metal -cased items, the effects of damp may often be indicated by traces of corrosion or tarnishing of
the case and/or staining of th e instructions label.
All pyrotechnics gradually deteriorate in time, although such deterioration will vary with factors such as
quality or type of composition and degree of protection afforded by the containers.
For this reason a proportion of the items s hould be proof -tested at regular intervals as specified by the
manufacturer.
Pyrotechnics have sever different lives that must be complied with, including:
 Shelf life
 Exposed life
 Total life.
Total life, regardless of proof testing, should not be exceeded.

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Rubber parts and components containing rubber
The following storage conditions are generally acceptable for a wide range of components
containing rubber in their construction or parts made of rubber. In many cases manufacturers make
special recommendation s and these should be observed.
The storage temperature should be controlled between 10C and 21C and sources of heat should
be at least one meter from the stored articled, unless screened, to minimize exposure to radiant heat.
Some special rubber materials may withstand a wider range of temperature satisfactorily. The
relative humidity in the store room should be about 75 percent. Very moist or very dry conditions should
be avoided.

Hydraulic and pneumatic system components
Hydraulic and pneumatic components generally have a nominal sever year shelf life which may
usually be extended for period of two years by inspections.
In many instances, hydraulic components are stored filled with hydraulic fluid which may leak
slightly from the component. It is therefore important to ensure that fluid will not come into contact with
other stored item. If the stored component is filled with a fluid other than that used in aircraft system,
such as DTD 5540, the component should be clearly labe led to ensure the removal of all traces of storage
fluid prior to installation in the hydraulic system.
Survival equipment.
Survival equipment should be stored in a room which can be maintained at temperature between 15c and
21c and which is free form stro ng light and any concentration of ozone.
The manufacturer’s instructions should be carefully followed when preparing survival equipment for
storage. These instructions normally include:
 Ensuring that the component is completely deflated
 Removing easily detachable components
 Fitting protection blanks or pads to inflation valves and other connections
 Dusting the component with chalk and folding it loosely
 Wrapping in waterproof paper
 Placing the equipment on a shelf above the floor
4.7 Accidents/Incidents cause by maintenance errors

In an effort to identify the most frequently occurring maintenance discrepancies, the United
Kingdom Civil Aviation Authority (CAA) conducted in -depth studies of maintenance sites on aviation
maintenance op erations. The following list is what they found to be the most common occurring
maintenance errors.

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 Incorrect installation of components.
 Fitting of wrong parts.
 Electrical wiring discrepancies to include crossing connections.
 Forgotten tools and parts.
 Failure to lubricate
 Failure to secure access panels, fairings, or cowlings.
 Fuel or oil caps and fuel panels not secured.
 Failure to remove lock pins.
Historically, twenty percent of all accidents are caused by a machine failure, and eighty percent of all
accidents are caused by human factors. Originally focusing on the pilot community, human factors
awareness has now spread into the training sphere of maintenance technicians. An in -depth review of an
aviation incident reveals time and again that a seri es of human errors (known also as a chain of events)
was allowed to build until the accident occurred. If the chain of events is broken at the maintenance level,
the likelihood of the accident occurring can be drastically decreased is a list of maintenance related
incidents/accidents and their causes

Lufthansa Airbus A320 Reversed wiring of flight controls

Incident
On March 20, 2001 a Lufthansa Airbus A320 almost crashed shortly after takeoff because of reversed
wiring in the captain's side stick flight control. Quick action by the co -pilot, whose side stick was not
faulty, prevented a crash.
Cause
The investigation has focused on maintenance on the captain's controls carried out by Lufthansa
Technik just before the flight. During the previous flight, a p roblem with one of the two elevator/aileron
computers (ELAC) had occurred. An electrical pin in the connector was found to be damaged and was
replaced. It has been confirmed that two pairs of pins inside the connector had accidentally been crossed
during t he repair. This changed the polarity in the side stick and the respective control channels
―bypassing‖ the control unit, which might have sensed the error and would have triggered a warning.
Clues might have been seen on the electronic centralized aircraft monitor (ECAM) screen during the
flight control checks, but often pilots only check for a deflection indication, not the direction. Before the
aircraft left the hangar, a flight control check was performed by the mechanic, but only using the first
officer ’s side stick.

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Alaska Airlines Flight 261 lack of lubrication
Accident
Alaska Airlines Flight 261, a McDonnell Douglas MD -83 aircraft, experienced a fatal accident on
January 31, 2000, in the Pacific Ocean. The two pilots, three cabin crewmembers, and 83 p assengers on
board were killed and the aircraft was destroyed.
Cause
The subsequent investigation by the National Transportation Safety Board (NTSB) determined that
inadequate maintenance led to excessive wear and catastrophic failure of a critical fligh t control system
during flight. The probable cause was stated to be ― loss of airplane pitch control resulting from the in –
flight failure of the horizontal stabilizer trim system jackscrew assembly’s acme nut threads. The thread
failure was caused by excess ive wear resulting from Alaska Airlines insufficient lubrication of the
jackscrew assembly.‖
Excalibur Airwaris Airbus A320 Side stick loss of spoiler control
Incident
August 26, 1993, an Excalibur Airways Airbus 320 took off from London -Gatwick Airport (LGW)
and exhibited an undemand roll to the right on takeoff, a condition which persisted until the aircraft
landed back at LGW 37 minutes later. Control of the aircraft required significant left side stick at all times
and the flight control system was de graded by the loss of spoiler control.
Cause
Technicians familiar with Boeing 757 flap change procedures lacked the knowledge required to
correctly lock out the spoilers on the Airbus during the flap change work that was done the day before the
flight. Tur nover to technicians on the next shift compounded the problem. No mention of incorrect spoiler
lockout procedure was given since it was assumed that the 320 was like the 757. The flap change was
operationally checked, but the spoiler remained locked out in correctly and was not detected by the flight
crew during standard functional checks. The lack of knowledge on Airbus procedures was considered a
primary cause of this incident.
Emery Worldwide Airlines DC -8-71F Maint Landing gear not extending
Incident
April 26, 2001, an Emery Worldwide Airlines DC -8-71F left main landing gear would not extend for
landing.
Cause
Probable cause was failure of maintenance to install the correct hydraulic landing gear extension
component and the failure of inspection to comply with post -maintenance test procedures. No injuries.
China Airlines Flight 611 Boeing 747 Broke in mid -air

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Accident
On May 25, 2002, China Airlines Flight 611 Boeing 747 broke into pieces in mid -air and crashed,
killing all 225 people on board.
Cause
The final investigation report found that the accident was the result of metal fatigue caused by
inadequate maintenance after a much earlier tail strike incident. On 7 February 1980, the aircraft was
flying from Stockholm Arlanda Airport to Taoyuan Internationa l Airport via King Abdul -Aziz
International Airport and Kai Tak International Airport. While landing in Hong Kong, part of the plane's
tail had scraped along the runway. The aircraft was depressurized, ferried back to Taiwan on the same
day, and a temporar y repair done the day after. The aircraft was depressurized, ferried back to Taiwan on
the same day, and a temporary repair done the day after. However, the permanent repair of the tail strike
was not carried out in accordance with the Boeing Structural Re pair Manual (SRM). According to the
manual, repairs could be made by replacing the entire affected skin or by cutting out the damaged portion
and installing a reinforcing doubler to restore the structural strength. These two acceptable options were
set asi de in favor of a third, improvised option, which entailed installing a doubler over the scratched skin.
This external doubler did not effectively cover the damaged area in its entirety, as scratches were found
at, and outside, the outer row of fasteners se curing the doubler. The doubler having been installed with
scratches remaining outside the rivets meant that there was no protection against the propagation of a
concealed crack in the area between the rivets and the perimeter of the doubler.
Colgan Air Be ech 1900D crash soon after take off
Accident
On August 26, 2003, a Colgan Air Beech 1900D crashed just after takeoff from Hyannis,
Massachusetts. Both pilots were killed.
Cause
The improper replacement of the forward elevator trim cable and subsequent inadequate functional
check of the maintenance performed that resulted in a reversal of the elevator trim system and a loss of
control in flight. Factors were the flight crew’s failure to follow the checklist procedures and the aircraft
manufacturer’s erro neous depiction of the elevator trim drum in the maintenance manual.
American Airlines Flight 1400 DC -9 engine fire
Accident
On September 28, 2007, American Airlines Flight 1400 DC -9 experienced an in -flight engine fire
during departure climb from Lambert St. Louis International Airport (STL). During the return to STL, the
nose landing gear failed to extend, and the flight crew executed a go -around, during which the crew
extended the nose gear using the emergency procedure. The flight crew conducted an emer gency landing,
and the 2 flight crewmembers, 3 flight attendants, and 138 passengers deplaned on the runway. No
occupant injuries were reported, but the airplane sustained substantial damage from the fire.

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CAUSE
American Airlines maintenance personnel’s us e of an inappropriate manual engine -start procedure,
which led to the uncommented opening of the left engine air turbine starter valve, and a subsequent left
engine fire.

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5. Conclusions

To conclude, it is important to point out that aviation maintenance safety is strictly related to the
human factors. It is clear that the responsibility of the safe operations in maintenance is in the hands of all
the operating organizations as it is everybody’s business. Only when everyone is aware of this a true safe
culture can be implemented. Many types of human errors are systematic and by following certain
predictable patterns they can be identified and measures can be taken to prevent them from repeating.
This is only possible only by studying and the factors that ac tion upon maintenance personnel. Without
an understanding of human behavior and the requirements that human personnel need in order to perform
in a safe manner aviation would have not made it this far in technology and number of passengers
transported eve ry year.
Therefore i t is worthwhile to spend time, effort and money in order to create the safe environment a
human person need in order to be able perform efficiently and safely.

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References

[1] Managing Maintenance Error – A practical Guide , James Reason; Alan Hobbs
[2] Essentials of airplane maintenance ; Michale Loong
[3] Maintainability, Maintenance, and Reliability for Engineers; B. S. Dhillon
[4] UK CAA Civil Aviation Publication 715 – An Introduction to Aircraft Maintenance Eng ineering
Human Factors for JAR 66
[5] Transport Canada – Human Performance Factors for Elementary Work and Servicing
[6] Civil Aviation Safety Authority https://www.casa.gov.au
[7] Federal Aviation Administration https://www.faa.gov
[8] European Aviation Safety Agency https://www.easa.europa.eu
[9] ICAO Circular 240 -AN/144 Human Factors Digest No7 – Investigation of Human Factors in
Accidents and Incidents
[10] Aviation Safety Council of Taiwan. "In-Flight Breakup Over the Taiwan Stra it Northeast of
Makung, Penghu Island, China Airlines Flight CI611, Boeing 747 -200, B -18255, May 25, 2002".
Aviation Occurrence Report Volume 1. 1 (ASC -AOR -05-02-001).
[11] "Aircraft Accident Report, Loss of Control and Impact with Pacific Ocean Alaska Ai rlines Flight
261 McDonnell Douglas MD -83, N963AS About 2.7 Miles [4.3 km] North of Anacapa Island, California,
January 31, 2000" National Transportation Safety Board. December 30, 2002. NTSB/AAR -02/01.
[12] In-Flight Left Engine Fire American Airlines Flight 1400 McDonnell Douglas DC -9-82,
N454AA St. Louis, Missouri September 28, 2007 Accident Report NTSB/AAR -09/03 PB2009 -910403
[13] Safety Management Manual (SMM) ; International Civil Aviation Organization , Third edition
2013