We propose an approach to analyse the safety of avionic systems that takes into account the impact of computation and communication resource sharing. The approach is made of three main steps: use a formal notation to describe how failures propagate in the system under study, use model-checking tools to verify safety requirements and to derive allocation constraints, use a constraint solver to generate safe allocations. This approach is illustrated by the study of the Terrain Following/Terrain Avoidance (TF/TA) System of a fighter aircraft.