Unformatted text preview: roject. Even if the documented lessons
learned from previous projects are not applicable at the
system level, there may be valuable data applicable at
the subsystem or component level.
FMECAs, FMEAs, Digraphs, and Fault Trees. Failure
Modes, Effects, and Criticality Analysis (FMECA),
Failure Modes and Effects Analysis (FMEA), digraphs,
and fault trees are specialized techniques for safety
(and/or hazard) risk identification and characterization.
These techniques focus on the hardware components
that make up the system. According to MIL-STD-1629A,
FMECA is "an ongoing procedure by which each
potential failure in a system is analyzed to determine the
results or effects thereof on the system, and to classify
each potential failure mode according to its severity."
Failures are generally classified into four seventy
• Category I—Catastrophic failure (possible death
or system loss)
Category II—Critical failure (possible major injury or system damage)
Category III—Major failure (possible minor
injury or mission effectiveness degradation)
Category IV — Minor failure (requires system
maintenance, but does not pose a hazard to
personnel or mission effectiveness). A complete FMECA also includes an estimate of
the probability of each potential failure. These probabilities are usually based, at first, on subjective
judgment or experience factors from similar kinds of
hardware components, but may be refined from
reliability data as the system development progresses.
An FMEA is similar to an FMECA, but typically there is
less emphasis on the severity classification portion of
Digraph analysis is an aid in determining fault
tolerance, propagation, and reliability in large,
interconnected systems. Digraphs exhibit a network
structure and resemble a schematic diagram. The
digraph technique permits the integration of data from a
number of individual FMECAs/FMEAs, and can be
translated into fault trees, described in Section 6.2, if
quantitative probability e...
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