ABSTRACT
A safe system is one where hazards and countermeasures are effectively balanced in a stable equilibrium. Achieving such a balance in(a complex socio-technical system such as aviation is not easy, not least because, as a transportation mode, it is a system in constant evolution. Advances in, say, technology or training will have to interact with a myriad of changes elsewhere. These may include attitude, organisational size or external influences in the form of economic or regulatory pressures. Largely opaque technical systems such as aviation make the complete understanding of all influences an unrealistic aim. Risk management techniques generally acknowledge this constraint and attempt to achieve system safety through multiple defences or filters designed to break the accident chain.
