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Safety Case

The purpose of the Safety Case is to set out a risk assessment process, detail all the perceived failure mechanisms, list the operational hazards such failures may give rise to, any mitigations that have been put in place, and then summarize all this in an accident sequence table. This table starts with each hazard, notes the consequences and how these might arise, what the risk is, and then by referring to the appropriate mitigations in place (if any) establishes the residual risk in each case. Every effort must be made to step through all possible scenarios to ensure as complete a risk assessment as possible is in place. Tables 19.9 and 19.10 set out the probability and severity definitions we use, while Tables 19.11 and 19.12 define our risk classification matrix and risk classes.

Table 19.9 Risk probability definitions (figures refer to flight hours).

Probability

Occurrence

Frequent

Likely to occur many times

Several times a month

Probability 10-2

Likely

Probable; likely to occur often

Monthly

Probability 10-3

Infrequent

Likely to occur several times

Annually

Probability 10-4

Seldom

Likely to occur at some time

Once in several years

Probability 10-5

Unlikely

Improbable; may exceptionally occur

Unlikely

Probability 10-6 or less

Table 19.10 Accident severity definitions.

Severity

Definition

Catastrophic

Aircraft/vehicle destroyed; multiple deaths

Major

Major damage/serious incident; loss of life or multiple severe injuries/occupational illnesses

Moderate

Moderate damage/significant incident; several severe injuries/occupational illnesses

Minor

Minor damage/minor incident; a single severe injury/occupational illness, and/or minor injuries/occupational illnesses

Negligible

Little consequence; a single minor injury/occupational illness

Table 19.11 Risk classification matrix.

Severity

Probability

Frequent

Likely

Infrequent

Seldom

Unlikely

Catastrophic

A

A

A

B

C

Major

A

A

B

C

C

Moderate

A

B

B

C

C

Minor

B

B

C

C

D

Negligible

B

C

C

D

D

Table 19.12 Risk class definitions.

Risk class

Occurrence

A

Intolerable/extremely high

Requires urgent action/risk reduction

B

Undesirable/high

Requires action, and should only be accepted when risk reduction is impracticable

C

Tolerable/medium

Acceptable

D

Acceptable/low

Acceptable with no unnecessary risks

 
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