Hazards and Risk
A hazard is defined in FAA Order 8040.4 as a "Condition, event, or
circumstance that could lead to or contribute to an unplanned or
undesireable event." Seldom does a single hazard cause an accident.
More often, an accident occurs as the result of a sequence of causes.
A hazard analysis will consider system state, for example operating
environment, as well as failures or malfunctions.
While in some cases safety risk can be eliminated, in most cases a
certain degree of safety risk must be accepted. In order to quantify
expected accident costs before the fact, the potential consequences of
an accident, and the probability of occurrence must be considered.
Assessment of risk is made by combining the severity of consequence
with the likelihood of occurrence in a matrix. Risks that fall into
the "unacceptable" category (e.g., high severity and high probability)
must be mitigated by some means to reduce the level of safety risk.
When software is involved in a system, the development of that
software is often governed by DO-178B. The severity of consequence
identified by the hazard analysis establishes the criticality level of
the software. Software criticality levels range from A to E,
corresponding to severities of Catastrophic to No Safety Effect.
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