Appendix

Human Factors in Maintenance

B
Background
In the early 1980s, the aviation industry implemented crew resource manage-
ment (CRM) in an effort to detect and correct human errors made by flight
crews. The action was successful and is continuing. In the 1990s, it was deter-
mined that the same approach should be used to identify and correct errors in
maintenance activities that contributed to aircraft accidents and incidents. This
activity—human factors in maintenance (HFM)—has developed into the main-
tenance resource management (MRM) program. The FAA addresses this activ-
ity in Advisory Circular AC 120-72.1
While many people assume that human factors in maintenance refers to the
actions of mechanics, the MRM program admits to several major areas where
maintenance errors can occur. These areas are (a) equipment design and man-
ufacture; (b) manufacturers’ documentation and procedure writing; (c) airline
procedures and work areas; and (d) mechanic training and performance.
The airframe and equipment manufacturers have implemented HF programs
to improve design so maintenance can be performed more easily and to reduce
the number of possible errors that can be made. Improvements in maintenance
manuals and other documents are also under manufacturer’s scrutiny and cer-
tain academics are looking into the problem of human error. But the airlines also
have a responsibility to monitor the processes and procedures they employ and
to modify those with respect to human error reduction. The training organiza-
tion should modify courses to accommodate any changes necessary to meet the
HF aspect and is also required to develop and implement an HFM course. The
AC mentioned above provides guidelines on establishing such a course.

1
Federal Aviation Administration: AC 120-72, Maintenance Resource Management Training,
September 28, 2000.

255
256 Appendix B

In this appendix, we will first discuss human factors as a part of systems engi-
neering (see Appendix A); then we will address some of the other activities in HFM.

Basic Definitions
The term human factors is defined in the Handbook of Aeronautical and
Astronautical Engineering as follows:
Ergonomics [Human factors] is the scientific discipline concerned with the under-
standing of the interactions among humans and other elements of a system, and the
profession that applies theory principles, data and methods to design in order to opti-
mize human well-being and overall system performance. ...2

Another popular definition is rather brief but captures the essence of human
factors.
In capsule form, the nub of human factors can be considered as the process of
designing for human use.3

In the past, human factors has usually referred to physical characteristics of

people, such as size, strength, physical dexterity, and visual acuity. But there
are other human attributes that affect a system’s performance and the human’s
ability to use or maintain the system. Such human characteristics as a lack of
knowledge or understanding of how the system works can lead to improper use
or to inadequate troubleshooting or improper maintenance. Human forgetful-
ness or even a person’s attitude can affect how well the system works, or how
that person interfaces with the system. The human attributes of those people
who interface with systems in any way can have an effect on how well the
system performs through their ability or lack of ability. Design people may not
understand the needs of maintenance, trainers may not be able to communicate
the correct information to others, and operators may use the system improperly.

Human Factors and Systems Engineering

In Appendix A, we discussed systems engineering. We discussed systems bound-
aries, system elements, and the interfaces relative to the interaction of these
systems, subsystems, and components. Here, we take up the notion that the
human being—the user, operator, or mechanic, as well as all others (writers,
designers, teachers, etc.) who interface with the system—must be considered as
elements of the system. Likewise, these elements and interfaces must be
addressed during the design stage of the system.

2
Kesterson, Bryan P., William L. Rankin, Steven L. Sogg: Maintenance Human Factors, Section 18,
Part 8, The Handbook of Aeronautical and Astronautical Engineers, McGraw-Hill, New York, NY,
2001.
3
McCormick, Ernest J.: Human Factors in Engineering and Design, 4th ed., McGraw-Hill, New
York, NY, 1976, p. 4.
 Human Factors in Maintenance 257

The human interaction with systems makes it imperative that the users,
operators, and maintenance people be considered during the design, develop-
ment, and operational phases of the system’s life. During design and develop-
ment, the human requirements and interactions must be known or anticipated
at all levels of the system. This includes not just the equipment but also the man-
uals and the training program for that equipment. During the operational
phase, feedback from the field will dictate changes necessary for system improve-
ment relative to the operator, user, or mechanic in terms of local procedures, as
well as the manufacturer’s procedures, training, and design efforts. Lessons
learned during this operational period relative to human interaction with the
system can be used to advantage by the manufacturers in the development of
new systems or modification of existing ones.
Traditionally, the systems engineer needs to be familiar with a variety of
engineering disciplines to perform his or her job successfully. Adding human fac-
tors to the toolbox means adding one more discipline: human factors engineer-
ing. This involves not just the understanding of human characteristics but also
how these characteristics relate to the overall operation of the system. It requires
the systems engineer to understand the effects these humans can have on the
system operation whether the necessary interaction exists or not, whether the
response is correct or incorrect, and even if the response or interaction is absent
when it is required. It is necessary for the systems engineer to address these
effects as part of the basic system design. The effects of human presence are as
real as the presence of voltages and mechanical linkages. The human being is
an element of the system. When all the elements are working properly, the
system will work properly.

Goals of the System versus Goals of the User

Elwyn Edwards4 states that the effectiveness of a system is measured by the
extent to which the system goals are achieved. McCormick5 also mentions the
functional effectiveness of the system as one of the goals of design. In this
appendix, we integrate the philosophy of systems engineering, discussed in
Appendix A, with the philosophy of human factors. In doing that, we consider
the significant goals to be not the goals of the system but rather the goals that
the user of the system expects to achieve by employing the system.
We can no longer design for the sake of the system or for the sake of technol-
ogy. This new philosophy requires that we now design for the system applica-
tion. A system, whether a simple tape player or an exotic mode of transportation,
is just a tool. It is a tool used by people to accomplish some personal or work-
related goal. To make that tool,“user friendly,” we must design it to be usable by

4
Wiener, Earl L., David C. Nagel (eds.): Human Factors in Aviation, Academic Press, Harcourt
Brace Jovanovich, no date. From the introduction by Elwyn Edwards.
5
McCormick, Human Factors in Engineering and Design.
258 Appendix B

human beings. That means that the system not only has to perform some func-
tion efficiently, but it has to perform that function in the manner that the system
user wants it performed.
A system that achieves the design goals of a collection of mechanical and
electrical parts may represent engineering perfection, but if the device cannot
be used by people for some human purpose, it is just a collection of mechanical
and electrical parts; just another “contraption.”

Designing for the Human Interface

Whether we are talking about electrical or mechanical systems, about processes
or procedures to be carried out, or about forms we need to complete during
maintenance, the interface between these systems and the human users must
be addressed as any other system interface; and the system optimization efforts
we spoke of in Appendix A must be applied to make the total system—including
the user—work efficiently. The main difference, however, is that these humans,
unlike the other system elements, cannot be redesigned during the optimiza-
tion process for the improvement of the total system operation. Therefore, the
designers of these systems must adhere to several basic rules. The first of these
is to design the system to be compatible with human abilities, capabilities,
needs, and strengths. The second is to design these systems around human
failings and deficiencies so as to avoid possible human error.
The third rule is especially important in developing good, usable systems. For
any problem or condition that cannot be accommodated by the first two rules
above or one that is limited due to various constraints, such as design limits,
trade-offs, or budget requirements as discussed in Chap. 1 of this book, the
designers must provide the users, operators, and mechanics—as well as other
human elements involved—with sufficient education and training on the system
to resolve any human factors–related problems that could arise from improper
understanding of the design. These basic design rules for human interface with
systems are summarized in Table B-1.

Human Factors in Maintenance

In Appendix A, we extended the definition of systems to include more than just
the electromechanical components we normally consider. A system can also be
a checklist, a procedure, or a form to be filled out. Maintenance, of course, deals

TABLE B-1 Human Factors Design Guidelines

1. Design the system to be compatible with human abilities, capabilities, needs, and strengths.
2. Design the system to compensate for human failings and deficiencies to avoid human errors.
3. Provide the human elements of the system with sufficient education and training to resolve
any human factors–related problems that could not be alleviated by application of the first two
rules above.
 Human Factors in Maintenance 259

with all of these kinds of systems, and the human element is just as important
in each of these. How maintenance people perform is only part of the problem;
the facilities in which they work, the equipment they encounter, and the forms,
processes and procedures they use are all subject to human actions and, there-
fore, to human error. And the errors are not always due to the mechanic. There
are several areas in maintenance that contribute to the errors made by the
users, operators, or mechanics.

Human Factors Responsibilities

Human factors efforts are usually divided into three basic categories of activity:
(a) aircraft and component design, (b) maintenance product design, and
(c) maintenance program applications.6 Each of these is discussed below.

Aircraft and component design

The responsibility for this category rests with the manufacturers of airframes,
engines, and installed equipment. It deals with the task of designing for main-
tainability. This concerns the design of equipment that can be worked on for serv-
ice, inspection, adjustment, and removal/installation (R/I) efforts. These design
efforts must ensure that there is sufficient workspace to do the work required
and that there is also enough space to use the tools and test equipment that may
be needed. The manufacturer’s responsibility also includes consideration of the
weight and handling characteristics of the unit undergoing maintenance.
Equipment parameters must be within the physical limits of the workers
required for the particular task. If this cannot be accommodated, special han-
dling equipment must be developed to permit proper handling and to protect
both the equipment and the workers from harm. Design effort should also take
into account the number and skills of the workers required for a given task to
be completed with reasonable staffing requirements.
Whenever computer diagnosis is utilized, using built-in test equipment (BITE)
or other external systems, the equipment, processes, menus, and other task or
information selection methods must be designed for the mechanic’s ease of use
and understanding; that is, it should be user friendly. Results from such activ-
ities must be understandable and usable by the mechanic.

Maintenance product design

Maintenance personnel require auxiliary equipment and written material to per-
form the required maintenance on aircraft systems. Ground support equipment
(GSE), special tools and test equipment, and various forms of documentation must
be designed with the mechanics’ capabilities and limitations in mind, and these
products must be made available to the mechanics. Mechanics must be able to use

6Kesterson, Rankin, and Sogg: Maintenance Human Factors.

260 Appendix B

the GSE and tools effectively, so the design requirements discussed above for air-
frames, engines, and installed equipment must apply to these elements also.
Documentation, whether written by the manufacturer, the regulators, or the
airline, must be clear, understandable, and accurate (i.e., technically correct) for
the mechanic to effectively utilize the information. This written information
must also be accessible to the mechanics on the line, in the hangar, and in the
shops, as necessary. It must also be available to the training organization. The
user-friendly approach is also required for all these maintenance products.

Maintenance program applications

The basic maintenance program developed by the MSG process is based on the
needs of the equipment (i.e., design goals, safety, and reliability) and on the reg-
ulatory requirements (safety, airworthiness, etc.). When the airline receives
the aircraft and its initial maintenance program, that program is usually tai-
lored to the specific airline operation. This adjustment of tasks and task inter-
vals must also include human factors considerations. That is, the adjustment
of the program must be in line with the human capabilities and requirements
concerning work schedules, endurance, and skill makeup of the work crew to
avoid over work, fatigue, etc. The appropriate GSE, tools, and test equipment
must be provided to do the work, and the work force must be properly trained
on all aspects of the job: the actual maintenance work to be performed; the use
of GSE, tools, and test equipment; the use of built-in or external computer diag-
nostic equipment; and the basic human factors aspects of the job. These actions
are the responsibility of the airline itself.

Safety
Chapter 20 of this book discusses the safety and health issues related to main-
tenance. It does not take much deep thought to realize that safety is also a
human factors issue. Although the two fields relate to different aspects of the
maintenance activity, they are not mutually exclusive.

Summary
The manufacturers of airframes, engines, and installed equipment are doing
their part to reduce the chances of human error in maintenance, but they require
inputs from airline operators and third-party maintenance organizations.
Research from the academic community (behavior scientists, etc.) is also nec-
essary to advance the state of the art. Meanwhile, the airline operators and other
maintenance facilities are responsible for the actions of their mechanics and the
materials with which they work. In human factors, as well as in safety, the
work force at all levels must be constantly aware of problems and be ready to
effect solutions. Human factors is a way of life.