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Reliability The ability of a system to perform its required functions under stated conditions for a specified
period of time. A reliable system is no total assurance of acceptable risk.
Requirements Statements describing essential, necessary or desired attributes.
Requirements
Specification
Specification that sets forth the requirements for a system or system component.
Risk Risk is an expression of possible loss over a specific period of time or number of operational
cycles. It may be indicated by the probability of an accident times the damage in dollars, lives,
FAA System Safety Handbook, Appendix A: Glossary
December 30, 2000
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CONCEPT or
TERM
DESCRIPTION
and / or operating units.
Hazard Probability and Severity are measurable and, when combined, give us risk.
Total risk is the sum of identified and unidentified risks.
Identified risk is that risk which has been determined through various analysis
techniques. The first task of system safety is to identify, within practical limitations, all
possible risks. This step precedes determine the significance of the risk (severity) and the
likelihood of its occurrence (hazard probability). The time and costs of analysis efforts, the
quality of the safety program, and the state of technology impact the number of risks identified.
Unidentified risk is the risk not yet identified. Some unidentified risks are subsequently
identified when a mishap occurs. Some risk is never known.
Unacceptable risk is that risk which cannot be tolerated by the managing activity. It is
a subset of identified risk that must be eliminated or controlled.
Acceptable risk is the part of identified risk that is allowed to persist without further
engineering or management action. Making this decision is a difficult yet necessary
responsibility of the managing activity. This decision is made with full knowledge that it is the
user who is exposed to this risk.
Residual risk is the risk left over after system safety efforts have been fully employed.
It is not necessarily the same as acceptable risk. Residual risk is the sum of acceptable risk and
unidentified risk. This is the total risk passed on to the user.
Eliminate
Control
Unacceptable Residual
Acceptable
Unidentified
Total Risk Residual Risk
Risk Analysis The development of qualitative and / or quantitative estimate of risk based on evaluation and
mathematical techniques.
Risk
Acceptance.
Accepting risk is a function of both risk assessment and risk management. Risk acceptance is
not a simple matter and the concept is difficult for some to accept. Several points must be kept
in mind.
(1) Risk is a fundamental reality.
FAA System Safety Handbook, Appendix A: Glossary
December 30, 2000
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CONCEPT or
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DESCRIPTION
(2) Risk management is a process of tradeoffs.
(3) Quantifying risk doesn't ensure safety.
(4) Risk is a matter of perspective.
On the surface, taking risks seems foolish and to be avoided. Everything we do, however,
involves risk. Defining acceptable risk is subjective and perceived risks are often as important
as actual risks. Risks imposed on us by others are generally considered to be less unacceptable
than those inherent in nature. There are dangers in every type of travel, but there are dangers in
staying home--40 percent of all fatal accidents occur there. There are dangers in eating most
food caused by pesticides, preservatives, natural fats, or just eating more than necessary. There
are breathing related dangers in industrial and urban areas. The results of air pollution leads to
the death of at least 10,000 Americans each year; inhaling natural radioactivity is believed to
kill a similar number; and many diseases are contracted by inhaling germs. 12,000 Americans
are killed each year in job related accidents, and probably 10 times that number die from job
related illness. There are dangers in exercising and dangers in not getting enough exercise.
Risk is an unavoidable part of our everyday lives.
We all accept risk, knowingly or unknowingly. In a FAA program, it is the ultimately the
responsibility of the MA to determine how much and what kind is to be accepted and what is
not. In the real word, making this decision is a trade-off process involving many inputs. As
tradeoffs are being considered and the design progresses, it may become evident that some of
the safety parameters are forcing higher program risk. From the program manager's
perspective, a relaxation of one or more of the established parameters may appear to be
advantageous when considering the broader perspective of cost and performance optimization.
The program manager has the authority and responsibility, in some circumstances, to make a
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