## 2 Methods and Materials
### Non-Destructive Testing (NDT) Techniques  
Non-Destructive Testing (NDT) techniques are widely applied across multiple industries beyond aviation, including civil engineering, infrastructure monitoring, and cultural heritage preservation. A notable example of their successful application is demonstrated in the rehabilitation of the Holy Aedicule of the Holy Sepulchre [36]. This study highlighted how a combination of NDT methods--such as Infrared Thermography (IRT), Ground Penetrating Radar (GPR), Ultrasonic Testing (UT), and Digital Microscopy--was employed to assess the preservation state of a historical monument, monitor ongoing rehabilitation works, and evaluate the compatibility of conservation materials and interventions. These techniques were instrumental in detecting defects, monitoring structural integrity, and providing real-time feedback for decision-making throughout the restoration process. Selecting the most appropriate non-destructive testing technique is crucial. Each technology meets different needs; therefore, the comparison Table 1 below is essential for understanding the applicability of each method, highlighting the advantages, limitations, and types of aircraft defects that can be detected. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Reference** & **Technology** & **Test Object** & **Advantages** & **Limitations** & **Suitable Type of Defect to Be Detected** \\ \hline
[37; 38; 39] & X-ray & Internal Defects & \begin{tabular}{c} Not limited by material and geometry; most \\ sensitive or defective \\ \end{tabular} & \
\begin{tabular}{c} Defect depth \\ limitations; strict \\ installation and safety \\ requirements; high cost \\ \end{tabular} &
\begin{tabular}{c} Porosity or Voids \\ Debonding \\ Foreign Bodies \\ \end{tabular} \\ \hline
[37; 38; 39] & Ultrasonic & Surface and Internal Defects & \begin{tabular}{c} Sensitive to defects and \\ quick results \\ \end{tabular} & \
\begin{tabular}{c} Difficult to detect small, \\ thin and complex parts; \\ coupling agent and \\ direct contact with the \\ specimen is required \\ \end{tabular} &
\begin{tabular}{c} Porosity or Voids \\ Debonding \\ Foreign Bodies \\ \end{tabular} \\ \hline
[37; 38; 39] & Magnetic Particle & \begin{tabular}{c} Surface and \\ Near-Surface Defects \\ \end{tabular} & \begin{tabular}{c} Higher sensitivity than \\ ultrasonic or \\ radiographic when \\ testing surface defects \\ \end{tabular} & \
\begin{tabular}{c} Limited to \\ Ferromagnetic material; \\ difficult to measure the \\ defect depth \\ \end{tabular} &
\begin{tabular}{c} Cracks \\ \end{tabular} \\ \hline
[37; 38; 39] & Penetrant & \begin{tabular}{c} Surface opening \\ defect \\ \end{tabular} & \
\begin{tabular}{c} The operation is simple, \\ and the sensitivity is high \\ \end{tabular} &
\begin{tabular}{c} Only surface-opening \\ defects can be detected \\ \end{tabular} & Surface-opening defects \\ \hline
[37; 38; 39] & Eddy Current & \begin{tabular}{c} Surface and \\ Near-Surface Defects \\ \end{tabular} & \
\begin{tabular}{c} Highly automated \\ system; time-saving; no \\ couplant required \\ \end{tabular} &
\begin{tabular}{c} Limited to conductive \\ material; easy to give a \\ false display \\ \end{tabular} & Cracks \\ \hline
[37; 38; 39] & Acoustic Emission & \begin{tabular}{c} Surface and Internal \\ Defects \\ \end{tabular} & \begin{tabular}{c} Effective for active \\ defects \\ \end{tabular} & \
\begin{tabular}{c} Having issues with the \\ defect size and shape \\ aspects \\ \end{tabular} &
\begin{tabular}{c} Porosity or Voids \\ Delamination \\ Debonding \\ Foreign Bodies \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of the most common non-destructive testing techniques in aircraft inspection. X-ray [37; 38; 39] technology is a widely utilised non-invasive testing method due to its versatility in inspecting objects regardless of their material or geometry, combined with its high sensitivity to detecting volumetric defects.