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·  Identify hazards, unsafe acts or conditions;
·  Identify “at risk” behaviors;
·  Collect the information in a readily available format for providing immediate feedback;
·  Train front-line people to implement and take responsibility for day-to-day operation of the
program.
The behavioral-based safety process allows an organization to create and maintain a positive safety
culture that continually reinforces safe behaviors over unsafe behaviors. This will ultimately result in a
reduction of risk. For further information concerning behavioral-based safety contact the FAA’s Office
of System Safety.
3.8 Models Used by System Safety for Analysis
The AMS system safety program uses models to describe a system under study. These models are known as
the 5M model and the SHEL model. While there are many other models available, these two recognize the
interrelationships and integration of the hardware, software, human, environment and procedures inherent in
FAA systems. FAA policy and the system safety approach is to identify and control the risks associated with
each element of a system on a individual, interface and system level.
The first step in performing safety risk management is describing the system under consideration. This
description should include at a minimum, the functions, general physical characteristics, and operations of
the system. Normally, detailed physical descriptions are not required unless the safety analysis is focused on
this area.
FAA System Safety Handbook, Chapter 3: Principles of System Safety
December 30, 2000
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16
Keep in mind that the reason for performing safety analyses is to identify hazards and risks and to
communicate that information to the audience. At a minimum, the safety assessment should describe the
system in sufficient detail that the projected audience can understand the safety risks.
A system description has both breadth and depth. The breadth of a system description refers to the system
boundaries. Bounding means limiting the system to those elements of the system model that affect or
interact with each other to accomplish the central mission(s) or function. Depth refers to the level of detail in
the description. In general, the level of detail in the description varies inversely with the breadth of the
system. For a system as broad as the National Airspace System (NAS) our description would be very
general in nature with little detail on individual components. On the other hand, a simple system, such as a
valve in a landing gear design, could include a lot of detail to support the assessment.
First, a definition of “system” is needed. This handbook and MIL-STD-882i (System Safety Program
Requirements) define a system as:
Graphically, this is represented by the 5M and SHEL models, which depict, in general, the types of
elements that should be considered within most systems.
5M model of System Engineering
• Msn - Mission: central
purpose or functions
• Man - Human element
• Mach - Machine: hardware
and software
• Media - Environment:
ambient and operational
environment
• Mgt- Management:
procedures, policies, and
regulations
Man Mach.
Msn
Mgt
Media
A composite at any level of complexity, of personnel, procedures, material, tools,
equipment, facilities, and software. The elements of this composite entity are used together
in the intended operation or support environment to perform a given task or achieve a
specific production, support, or mission requirement.
FAA System Safety Handbook, Chapter 3: Principles of System Safety
December 30, 2000
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17
Figure 3-6: The Five-M Model
Mission. The mission is the purpose or central function of the system. This is the reason that all the other
elements are brought together.
Man. This is the human element of a system. If a system requires humans for operation, maintenance, or
installation this element must be considered in the system description.
Machine. This is the hardware and software (including firmware) element of a system.
Management. Management includes the procedures, policy, and regulations involved in operating,
maintaining, installing, and decommissioning a system.
(1) Media. Media is the environment in which a system will be operated, maintained, and installed. This
environment includes operational and ambient conditions. Operational environment means the
conditions in which the mission or function is planned and executed. Operational conditions are those
involving things such as air traffic density, communication congestion, workload, etc. Part of the
　
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