In aeronautics the component compliance makes assemblies widely over-constrained, and the assembly process consists in adjusting linkage devices in order to satisfy functional requirements. The paper deals with a simulation method of the assembly process integrating the flexibility of the components which leads to the optimal values of the adjustment parameters so that functional requirements are ensured. The simulation is based on the Finite Element Method but also integrates component measurements which are necessary to solve the optimization problem. The efficiency of the method is illustrated through the assembly of a sliding door of a helicopter.