Service providers for maintenance, repair and overhaul (MRO) of aircrafts and components face major challenges. The calculation of an optimal stock level for components is one of these. The optimal stock level is highly influenced by the customers demand. Furthermore the stock level influences the profitability of the processes. The challenge for MRO service provider is to respond to the changing customer base and to adapt the stock level for components dynamically. For this purpose, an approach for the determination of the spare parts demand is described in this paper. The approach consists of a hierarchical derivation of reliability parameters in the first step and finally an assessment of the derived reliability parameters. By this assessment the relevance of different impact factors can be determined. The determination of the spare parts demand and the optimal stock level can thereby be determined much more accurately.