Planning of aircraft’s maintenance activities, failure occurrences and necessary spare parts are essential for minimizing downtimes, costs and preventing accidents. The aim of this paper is to propose an approach that supports decision making process in planning of aircraft’s maintenance activities and required spare parts. Presented mathematical model is based on Weibull’s model and calculates aircraft’s reliability characteristics by using total unit time parameter provided by aircraft’s parts manufacturer. Further, by capitalizing the random nature of total unit time, the number of spare parts and the costs of negative inventory level are determined.