This article explores reliability assessment from a systemic and hierarchical perspective. It highlights that while failures occur at the component level, they are often driven by stresses resulting from system use and environmental conditions. To address this, the U-Cycle approach is introduced, providing a structured method to link system decomposition with potential failure mechanisms. The proposed methodology, supported by five complementary views, captures both the organization of the system and its behaviour under operational stress. It integrates a top-down functional analysis, which translates mission profiles into stress variables acting on components, with a bottom-up analysis that examines how failures propagate through subsystems to affect the system-level functionality. Together, these views provide a comprehensive framework for assessing system reliability. A case study on the electrical flight-control function of an aircraft demonstrates the practical implementation of the U-Cycle approach. It shows how stress propagation and degradation mechanisms influence the function’s availability, thereby illustrating the method’s applicability for both predictive reliability modelling during design and for diagnostic reasoning during operation.
Wasnik et al. (Wed,) studied this question.