This paper presents a brief summary of the research results of the application of experimental and operational modal analysis (EMA and OMA) effectiveness for the assessment of the change in the condition of operating structures by modifying their dynamic characteristics. Special focus is given to the topicality of the operational modal analysis for the definition of the dynamic features of the structures (frequencies, modes and deformation) under near-natural conditions. The research was conducted using two operating laboratory models, when the first one imitated a part of a fuel/gas pipeline and the second – a helicopter blade. The results of finite-element model simulation, identification of natural mode and the influence of the two types of virtual defects on the changes in the dynamic properties are provided. The work describes the results of experimental research of the dynamic behaviour of the pipe model, using the methods of OMA and comparing them with the modelling results. The research results demonstrate how the modification of the condition of the large-scaled models impacts pipe and blade models when the defects of local and global nature are introduced.