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participants who have extensive experience in their respective fields, and the adoption of a standard risk
classification scheme should ensure that the outcome will be an informed judgement.
Once the assessment of likelihood has been completed for all the identified hazards, the results, including
the rationale for the classification chosen, should be recorded in the hazard log.
STEP 5: EVALUATION OF RISK
Since the acceptability of a risk is dependent on both its likelihood and the severity of its consequences, the
criteria used to judge acceptability will always be two-dimensional. Acceptability is therefore usually based
on comparison with a severity/probability matrix.
Table 13-5 shows an example of a matrix for the assessment of acceptability of risk in ATS. This was taken
from the U.K. CAA CAP 670, Air Traffic Services Safety Requirements, and adapted from the risk
classification scheme in JAR-25.
As discussed in Chapter 6, there is a zone between acceptable and unacceptable risk where the decision
concerning acceptability is not clear-cut. These latter risks form a third category, where the risk may be
tolerable if it is reduced to a level as low as reasonably practicable (ALARP). Where a risk is classed as
ALARP, mitigation measures will always have been attempted, and those mitigation measures classed as
feasible will have been implemented.
In Table 13-5, the risks that fall in the middle are marked “Review”. Risks in this category are not
automatically classed as tolerable. Every case must be reviewed on its merits, taking into account the
benefits that will result from implementation of the proposed changes as well as the risk.
13-10 Safety Management Manual (SMM)
Table 13-5. Risk classification scheme
Probability of Occurrence
Extremely
improbable
Extremely
remote Remote
Reasonably
probable Frequent
Catastrophic Review Unacceptable Unacceptable Unacceptable Unacceptable
Hazardous Review Review Unacceptable Unacceptable Unacceptable
Major Acceptable Review Review Review Review
Severity
Minor Acceptable Acceptable Acceptable Acceptable Review
STEP 6: MITIGATION OF RISK
As noted in Step 5, if the risk does not meet the predetermined acceptability criteria, an attempt should
always be made to reduce it to a level which is acceptable, or if this is not possible, to a level as low as
reasonably practicable, using appropriate mitigation procedures.
The identification of appropriate risk mitigation measures requires a good understanding of the hazard and
the factors contributing to its occurrence, since any mechanism that will be effective in reducing risk will
have to modify one or more of these factors.
Risk mitigation measures may work through reducing the probability of occurrence, or the severity of the
consequences, or both. Achieving the desired level of risk reduction may require the implementation of more
than one mitigation measure.
The possible approaches to risk mitigation include:
a) revision of the system design;
b) modification of operational procedures;
c) changes to staffing arrangements; and
d) training of personnel to deal with the hazard.
The earlier in the system life cycle that hazards are identified, the easier it is to change the system design if
necessary. As the system nears implementation, changing the design becomes more difficult and costly.
This could reduce the available mitigation options for those hazards which are not identified until a late stage
of the project.
The effectiveness of any proposed risk mitigation measures must be assessed by first examining closely
whether the implementation of the mitigation measures might introduce any new hazards, then repeating
Steps 3, 4 and 5 to evaluate the acceptability of the risk with the proposed mitigation measures in place.
Once the system is implemented, particular attention should be paid, when evaluating the results of safety
performance monitoring, to verifying that the mitigation measures are working as intended. For further
guidance on risk mitigation, refer to Chapter 6.
Chapter 13. Safety Assessments 13-11
STEP 7: DEVELOPMENT OF SAFETY ASSESSMENT DOCUMENTATION
The purpose of the safety assessment documentation is to provide a permanent record of the final results of
the safety assessment, and the arguments and evidence demonstrating that the risks associated with the
implementation of the proposed system or change have been eliminated, or have been adequately
controlled and reduced to a tolerable level.
Note.— This presentation of the arguments and evidence to demonstrate safety is referred to in many
references on safety management as a safety case. The term safety argument is also sometimes used with
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