## 2 Methods and Materials
### Non-Destructive Testing (NDT) Techniques
However, it has several limitations, including strict installation and safety requirements, difficulty measuring defect depth, and high installation costs. X-ray effectively detects internal defects such as porosity, voids, debonding, and foreign bodies. [PERSON] et al. [41] recently introduced an X-ray robotic system designed to evaluate the condition of aeronautical parts. The system featured two industrial robots mounted on 5 m tracks, each equipped with X-ray computed tomography (CT) devices. The purpose was to inspect aeronautical composite components for potential defects, such as porosity and inclusions, that might have developed during manufacturing or while in service. This method showed significant promise for using robotised X-ray tomography to detect volumetric defects. However, the accuracy of robot positioning was critical to achieving reliable results. Specifically, the source and detector should be moved simultaneously and precisely on both sides to ensure accurate reconstruction, as any positioning errors could negatively affect the imaging quality. Ultrasonic [37; 38; 39] approaches such as Phased Array Ultrasonic Testing (PAUT), Time-of-Flight Diffraction (TOFD), Pulse-Echo Testing, Guided Wave Testing, and Immersion Testing can be used in aircraft inspection to detect both surface and internal defects. Several advantages of these methods include high sensitivity to defects, quick results, and accurate defect localisation. However, some limitations exist in detecting minor defects and inspecting thin parts with complex geometries. Additionally, a coupling agent is always necessary, and direct contact with the specimen is required. These technologies can detect porosity or voids, delamination, debonding, foreign bodies, and cracks. [PERSON] et al. [42] made a significant effort to develop an autonomous UAV system integrated with a 5 MHz, dual-crystal ultrasonic transducer to inspect large aluminium plates of varying thicknesses, including those with simulated defects. The ultrasonic transducer was held in a spring-loaded mounting structure to ensure consistent contact force while transmitting ultrasonic acoustic energy through the coupling gel. The UAV ultrasonic system could autonomously inspect and measure thickness at predefined locations on the surface. A comparison between manual ultrasonic inspection and the UAV-based autonomous ultrasonic inspection revealed that the signal amplitude from the autonomous inspection was much weaker. Positioning errors for the UAV were under 87.1 mm, and alignment errors were less than 5 degrees. These issues were caused by factors such as aerodynamic effects near the surface, the ultrasound probe's sensitivity to angular misalignment, and the overloading of the UAV payload. Magnetic Particle Inspection (MPI) [37; 38; 39] is a non-destructive testing (NDT) technique that is particularly effective for detecting surface and near-surface discontinuities, such as cracks, in ferromagnetic materials. This method stands out because of its high \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 \multirow{3}{*}{[37; 38; 39]} & Infrared & & \begin{tabular}{c} Suitable with metallic and non-metallic \\ and non-metallic \\ materials no \\ contamination or contact \\ of the test piece \\ \end{tabular} & \
\begin{tabular}{c} \\ \end{tabular} &
\begin{tabular}{c} Delamination \\ Debonding \\ corrosion \\ Impact Damage \\ Cracks \\ \end{tabular} \\ \hline \multirow{3}{*}{[19; 20; 40]} & \multirow{3}{*}{3D Scanning— Laser/LiDAR} & \multirow{3}{*}{Surface Defects} & High accuracy; non-contact Inspection; quick data acquisition; detailed 3D mapping; suitable for a wide range of materials and geometries & Surface-only detection; reflectivity issues; high equipment cost; environmental sensitivity & Dents \\  & & & & \\ \hline \hline \end{tabular}
\end{table}