Safety cases & safety assessments

When a system (e.g. an aircraft) is delivered and in its pristine condition, it has an initial level of safety often justified by the designer's "System Safety Assessment", which is often archived after certification. However, safety is not self-sustaining [SAE ARP 5150] it depends on numerous factors, including the original design; manufacturing; operating crew and maintenance actions; operational and environmental effects; quality of spare parts; modifications; configuration control; etc. Once released into service, the system is continually evolving and changing and should do so under the control of the operator's live "Safety Case". Within the aviation context Duane Kritzinger explores the relationship between the Safety Assessment and the Safety Case.