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Probable Remote Extremely Remote Extremely Improbable
Failure condition
severity classification
FAR Minor Major Catastrophic
JAR Minor Major Hazardous Catastrophic
Effect on aircraft
occupants
FAR
• Does not significantly reduce
airplane safety (Slight decrease
in safety margins)
• Conditions which prevent
continued safe flight and
landing
• Crew actions well within
capabilities (Slight increase in
crew workload)
• Some inconvenience to
occupants
• Reduce capability of airplane or crew to cope with adverse
operating conditions
• Significant reduction in safety margins
• Significant increase in crew workload
Severe Cases:
• Large reduction in safety margins
• Higher workload or physical distress on crew -
can't be relied upon to perform tasks accurately
• Adverse effects on occupants
JAR • Nuisance • Operating
limitations
• Emergency
procedures
• Multiple deaths,
usually with loss
of aircraft
Frequent
• Large reduction in safety
margins
• Crew extended because of
workload or environmental
conditions
• Serious or fatal injury to
small number of occupants
• Significant reduction in
safety margins
• Difficulty for crew to cope
with adverse conditions
• Passenger injuries
Table 3-6 Most Severe Consequence Used for Classification
FAA System Safety Handbook, Chapter 3: Principles of System Safety
December 30, 2000
3-
9
3.4 Comparative Safety Assessment
Selection of some alternate design elements, e.g., operational parameters and/or architecture components
or configuration in lieu of others implies recognition on the part of management that one set of
alternatives will result in either more or less risk of an accident. The risk management concept
emphasizes the identification of the change in risk with a change in alternative solutions. Safety
Comparative Safety Assessment is made more complicated considering that a lesser safety risk may not
be the optimum choice from a mission assurance standpoint. Recognition of this is the keystone of safety
risk management. These factors make system safety a decision making tool. It must be recognized,
however, that selection of the greater safety risk alternative carries with it the responsibility of assuring
inclusion of adequate warnings, personnel protective systems, and procedural controls. Safety
Comparative Safety Assessment is also a planning tool. It requires planning for the development of
safety operating procedures and test programs to resolve uncertainty when safety risk cannot be
completely controlled by design. It provides a control system to track and measure progress towards the
resolution of uncertainty and to measure the reduction of safety risk.
Assessment of risk is made by combining the severity of consequence with the likelihood of occurrence in
a matrix. Risk acceptance criteria to be used in the FAA AMS process are shown in Figure 3-3 and
Figure 3-4.
Likelihood
Severity
Probable
A
Major
3
Catastrophic
1
Hazardous
2
Minor
4
No Safety
Effect
5
Remote
B
Extremely
Remote
C
Extremely
Improbable
D
High Risk
Medium Risk
Low Risk
FAA System Safety Handbook, Chapter 3: Principles of System Safety
December 30, 2000
3-
10
Figure 3-3: Risk Acceptability Matrix
High Risk --Unacceptable. Tracking in the FAA
Hazard Tracking System is required
until the risk is reduced and accepted.
Medium -- Acceptable with review by the appropriate
management authority. Tracking in the FAA
Hazard Tracking System is required until
the risk is accepted.
Low -- Low risk is acceptable without review.
No further tracking of the hazard
is required.
Figure 3-4: Risk Acceptance Criteria
An example based on MIL-STD-882C is shown in Figure 3-5. The matrix may be referred to as a Hazard
Risk Index (HRI), a Risk Rating Factor (RRF), or other terminology, but in all cases, it is the criteria used
by management to determine acceptability of risk.
The Comparative Safety Assessment Matrix of Figure 3-5 illustrates an acceptance criteria methodology.
Region R1 on the matrix is an area of high risk and may be considered unacceptable by the managing
authority. Region R2 may be acceptable with management review of controls and/or mitigations, and R3
may be acceptable with management review. R4 is a low risk region that is usually acceptable without
　
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