## 4 Conclusions  
Early detection of changes in the material structures which are not visible during the visual inspection or with the conventional inspection methods is, for many applications, very important. For aircraft structures, the monitoring and diagnostics is especially important because structural damage can lead to aircraft accidents. Therefore, many diagnostic methods with a focus on non-destructive testing have been developed. Research at the Faculty of Aeronautics of the Technical University of Kosice and of the cooperating institutions dealing with academic research, together with the innovative SME EDIS vvd company in the area of NDT, is focused on the non-destructive material testing method using the developed microwire-based tensile stress sensors. The sensing element of these sensors is an amorphous magnetic microwire which is sensitive not only to the magnetic field but also to the applied tensile stress. Measurement of the tensile stress can be performed either on the material surface or inside of the material because microwires, thanks to their small dimensions, can be easily integrated into the tested construction without structural violations and changes in material characteristics. For this purpose, the original induction method with the excitation and sensing coils placed outside the sensing microwire has also been developed. Using this method, the material diagnostics can be absolutely contactless. Microwire-based tensile stress sensors can be used, for example, for construction, structural health and operational load monitoring, fatigue testing or crack detection. In the article, the original research results from the initial tests performed directly in an aircraft are presented. Thanks to the performed measurements with the microwire-based sensors and verification measurements with the accelerometer with different external temperatures, the temperature dependence of the natural frequency of the aircraft was also analyzed. It was also analytically confirmed that with the increasing temperature, the natural frequency decreases. The prepared simulation CAD/CAE model will serve as a basis for further sensor development, optimization and calibration. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Method**} & **Resonant** & **Eddy** & **Ultrasonic** & **Radiography** & **Magnetic** \\  & **Acoustic** & **Current** & **Utrasonic** & **Radiography** & **Microwires** \\ \hline \multicolumn{6}{c}{**Defect/Issue**} \\ \hline Cracks & 1 & 1 & 1 & 2 & 1 \\ Material Properties & 1 & 3 & 3 & 2 & 3 \\ Structural Integrity & 1 & 1 & 1 & 1 & 1 \\ Product Lot Variation & 2 & 2 & 1 & 1 & 1 \\ \hline \multicolumn{6}{c}{**Defect Location**} \\ \hline Surface (External) & 1 & 1 & 1 & 3 & 1 \\ Internal & 1 & 3 & 1 & 1 & 1 \\ Brazing/Bonding/Welding & 1 & 3 & 2 & 2 & 3 \\ \hline \multicolumn{6}{c}{**Speed/Cost**} \\ \hline Time Demands & 1 & 2 & 1 & 3 & 2 \\ Inspection Costs & 1 & 2 & 3 & 3 & 2 \\ \hline \multicolumn{6}{c}{**Automation Capacity**} \\ \hline Quantitative Results & 1 & 3 & 2 & 3 & 2 \\ Ease of Automation & 1 & 2 & 3 & 3 & 2 \\ \hline \multicolumn{6}{c}{**Monitoring/Diagnostics**} \\ \hline Pre-Flight/Post-Flight & 3 & 3 & 3 & 3 & 1 \\ In-Flight & 3 & 3 & 3 & 3 & 1 \\ \hline \multicolumn{6}{c}{1—excellent; 2—fair; 3—poor.} \\ \end{tabular}
\end{table} 
Table 1: Comparison of non-destructive testing methods. The main advantages of microwire-based sensors are their small dimensions, weight and manufacturing costs, which make them promising for utilization in many applications, including their utilization directly in the aircraft, as well as the possibility of being utilized on unmanned aerial vehicles. Methodology, M.S. and P.L. ; validation, P.L. and K.S. ; formal analysis, M.B. ; data curation, P.K. ; writing--original draft preparation, K.D. ; writing--review and editing, K.S. and M.B. ; visualization, K.D., M.S. and K.S. All authors have read and agreed to the published version of the manuscript.