Safety-critical systems are growing ever more complex and interlinked, especially in industries such as defence. Despite increasing complexity, automation and digitisation, humans remain vital actors in these systems. People need to be considered at the system level, as part of the system architecture and design, not just as potential causes of failures. Concerns about the adequacy of existing safety assurance standards have prompted development of IEC 63187, a draft international standard that is expected to be published during 2026. IEC 63187 delivers a system safety framework tailored for complex systems in defence programmes, building on the systems engineering life cycle approach from ISO/IEC/IEEE 15288. This paper explains how IEC 63187 incorporates human factors throughout the system life cycle, outlines the supporting guidance, and compares its approach with other safety standards. It provides practical insights for engineers, safety managers, and regulators seeking to apply human factors in complex, safety-critical systems.