As the safety analyses of critical systems typically cease or reduce in their utility after system certification useful knowledge about the behaviour of the system in conditions of failure remains unused in the operational phase of the system lifecycle. In this paper, we show that this knowledge could be usefully exploited in the context of an online hazard-directed monitoring scheme in which a suitable specification derived from design models and safety analyses forms a reference monitoring model. As a practical application of this approach, we propose a safety monitor that can operate on such models to support the on-line detection, diagnosis and control of hazardous failures in real-time. We discuss the development of the monitoring model and report on a case study that we performed on a laboratory model of an aircraft fuel system.
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