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resources or other remedies to help the contractor satisfactorily resolve the issue. If not, the
PM can add his position to the contractor information and forward the matter to a higher
decision level. A decision matrix very similar to a Risk Hazard Index called in this example a
Risk Hazard Level index can be used to establish which decisions fall under the PM and which
should be forwarded to a higher organizational level.
Risk Severity The harm expected should the hazardous event occur, (i.e., loss, consequence, adverse outcome,
damage, fatality, system loss, degradation, loss of function, injury) considering the risk
associated with the hazardous event under evaluation.
See chapter three for specific definitions of severity.
Severity ranges should be sized so that events within each category are of comparable severity.
Equating the severity of event and conditions, which can cause one fatality with those, which
can cause 100 or 1,000 does not make sense. The potential problems associated with sizing of
the severity ranges grow as the size of the system grows. Program managers need to be
provided with risk information that has the fidelity to distinguish the hazardous events that meet
general criteria.
Severity range thresholds for each severity category should be comparable when considering
personal, system, or facility losses. For example, events or conditions that could cause the loss
of an entire aircraft or facility would be categorized by MIL-STD-882 as catastrophic. Loss of
a single crewman, mechanic, or passenger would also fall in the catastrophic category. Severe
injuries, such as total loss of sight of a mechanic, and system damage of several million dollars
are not normally considered to have equal value, even though both are in the critical category.
If the RHI ranking criteria use risk as a function of severity and probability, quantitative scales
or qualitative scales based on quantitative logic should be used. If the concept that the expected
losses (or risk) associated with a hazardous event or condition may be estimated by multiplying
the expected severity of the accident by the probability of the accident, then some sort of
quantitative basis is necessary. Failure to provide a quantitative basis for the scales can cause
significant confusion and dissipation of safety resources when an arbitrary risk ranking scale is
used.
Develop the severity values using order of magnitude ranges. There are several advantages to
separating severity categories by orders of magnitude ranges: They include:
Limiting the likelihood of misuse of the analysis.
Avoiding meaningless hair-splitting arguments.
Simplifying severity assessment during PHAs without impacting usefulness.
FAA System Safety Handbook, Appendix A: Glossary
December 30, 2000
A -13
CONCEPT or
TERM
DESCRIPTION
Quantify the threshold values for the probability ranges. Quantification reduces confusion
associated with strictly qualitative definitions. Although it is impossible to quantify the ranges
in 882(C) due to its extremely broad application, developing quantified probability ranges for
specific systems is a relatively easy task to accomplish.
The probability of occurrence should refer to the probability of an accident/consequence as
opposed to the probability of an individual hazard/basic event occurring. The typical accident
sequence is much more complicated than a single line of erect dominos where tipping the first
domino (hazard) triggers a clearly predictable reaction.
Develop the probability values using order of magnitude ranges.
Reaction Time Human response movement time plus response initiation time.
Root Cause The contributory events, initiating events, which started the adverse event flow are considered
root causes. Should these causes be eliminated the hazardous event would not have occurred. It
should be noted that accidents are the result of many contributors, both unsafe acts and /or
unsafe conditions; also see Contributory Hazards, Hazard.
Safe Freedom from all forms of harm. Nothing is safe. Generals term denoting an acceptable level of
risk of, relative freedom from, and low probability of harm. The associated risks that have been
identified have been accepted provided that all identified controls are implemented and
enforced.
Safety or Safe. Freedom from those conditions that can cause death, injury, occupational
illness, or damage to or loss of equipment or property, or damage to the environment.
Note that absolute safety is not possible because complete freedom from all hazardous
conditions is not possible. Therefore, safety is a relative term that implies a level of risk that is
both perceived and accepted. Thus the emphasis in SSPs as reflected in the definitions above is
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