## 1 Introduction
### Manual Inspection  
As described earlier, aircraft maintenance, specifically aircraft inspection, is vital, as safety is a top priority in the aviation industry. There are two types of aircraft inspection: visual inspection and non-destructive inspection. Most aviation maintenance inspections--90%--are visual [15]. [PERSON] et al. [16] explained that General Visual Inspection (GVI) is a technical term used in the aerospace sector. This manual process is executed by experienced maintenance engineers using raw human senses like vision, touch, hearing, and smell. Maintenance Steering Group-3 (MSG-3) defines GVI as a _\"Visual examination of an interior or exterior area, installation or assembly to detect obvious damage, failure or irregularity, made from within touching distance and under normally available lighting condition such as daylight, hangar lighting, flashlight or drop-light\"_ [16]. A more detailed analysis of the visual inspection of aircraft was published by the Federal Aviation Administration (FAA) in 1997 [17]. There are four main reasons to perform an aircraft visual inspection:  
1. To assess the overall condition of the airframe and components;
2. Defects such as dents, delamination, cracks, and corrosion should be detected early before they reach a critical size;
3. To detect manufacturing defects, in-service defects as well as unplanned defects;  
Figure 3: Typical Maintenance Checks [13]. Finally, evidence of defects is to be collected to support information about the aircraft's condition. Depending on the type of check, the certified maintenance engineers walk around the aircraft to perform a casual visual inspection or focus on a specific area or system for an in-depth investigation. They usually use supporting inspection tools and equipment like flashlights, magnifiers, and mirrors. In cases where the inspection area is not reachable, cherry pickers, scissor litters, ladders, or scaffolding can be used [18]. As [PERSON] et al. [19] described, aircraft surface inspection is one of the most crucial maintenance tasks for detecting defects such as cracks, scratches, and dents. However, as his study mentioned, it is very challenging for a human operator to perform without supporting inspection tools. The main reason is the size of the defect, which can be hard or, in some cases, not visible to the naked eye. Low-angle light using a flashlight is a well-known technique quality engineers use to detect defects. Next, the area of interest is demarcated with a marker pen for further examination. The engineer can estimate the defect's size and depth using a dial gauge. The probe moves across the defective area until it contacts the surface. A few disadvantages of this approach include the level of experience of the person performing the measurement and the inability to accurately measure defects larger than the diameter of the dial indicator. In addition to the previous point, [PERSON] et al. [20] introduced the ruler and the high-resolution electronic indicator as supporting inspection equipment for dent depth measurement. Multiple manual readings and recordings were required for precision, making it time-consuming, especially with large areas with many dents. Furthermore, data analysis was complex, and further time was required. A circle was drawn to estimate the dent area, with the longest length used to measure the dent's diameter. Measurements varied depending on the inspector's interpretation, and multiple measurements were required for confirmation. In a recent study, [PERSON] et al. [21] presented a new thermography inspection method. Maintenance engineers conducted inspections with manually operated thermography cameras by reaching the designated areas of the aircraft using cranes and jacks. The proposed new methodology was promising; however, the manual process above was time-consuming, raised safety concerns, and depended on the inspector's skills. [PERSON] et al.