Home Engineering Small Unmanned Fixed-Wing Aircraft Design. A Practical Approach

# 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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