## 1 Introduction  
Competitiveness in aviation operations has depended on the constant adoption of continuous flights, requiring sustainable maintenance services. A maintenance and repair organisation (MRO) is a service establishment that provides maintenance, repair and overhaul functions to operators to keep their aircraft utilised. While airlines had their engineering departments to perform maintenance at the initial time of commercial aviation, outsourcing options have become popular due to the increment in operation volume [1]. In addition, regulatory authorities have forced companies to organise their cores as directives of regulations. MROs have introduced flexibility between aviation operation and maintenance processes, providing a variety of special maintenance centres that specialise in a particular component, such as engine or structural maintenance [2]. MROs have the vital mission in the aviation market of not only maintaining the sustainability of operations but also reducing costs because aircraft grounding impacts efficiency and \"the ratio of\" flight time to operating expenses. The daily cost of an aircraft being out of operation can be considered to be GBP 200,000 on average [3] (based on reasonable average estimates: a narrow body aircraft (200 passengers \(\times\) 8 journeys \(\times\) GBP 100 = GBP 160,000); wide body (350 passengers \(\times\) 1 journey \(\times\) GBP 800 = GBP 280,000)). Moreover, unscheduled maintenance due to unexpected problems also highly impacts expenses. According to the IATA, wide-body jetswere predicted to have a cost of more than USD 6.5 billion in unplanned maintenance owing to delays and cancellations in 2017. Appropriately scheduled maintenance, including shorter downtimes, avoiding unnecessary disassembly and minimizing unexpected delays and possible human errors, is crucial to reducing the grounding time of aircraft. Structural components are a significant element in terms of inspection and hence maintaining airworthiness. MROs spend significant time checking structural components, which accumulates to typically \(\approx\)30% of the costs [4]. Note that hidden (non-visible) defects (damages) inside the structure hinder operation in the medium to longer term. Several conventional inspection methods need a removal process that lengthens the maintenance time. Advanced non-destructive inspection/technique (NDI/NDT) methods are already popular in structural checks for detecting the occurrence of any defect in an early stage. The NDT is a powerful tool for detecting damages in the structures that formed during both the manufacturing and service stages [5; 6]. However, it is a formidable task to design a generic inspection procedure, considering the restrictions caused by regulations, operations, capabilities and costs. Obtaining more informative and determinative inspection outcomes should provide cost-effective maintenance which reduces the time and effort for individual inspection methods, especially after the pandemic period, in which there was a high workforce loss of 2.3 million personnel in global aviation [7]. Composite structures have become attractive for many industrial sectors due to their good strength and stiffness per unit weight [8; 9; 10]. In aviation, the volume of the composite structures used in an aircraft, such as the fuselage, wings, and empennage, has risen day by day. Accordingly, it is a vital requirement to identify possible damage to aircraft structures regarding airworthiness. It should be noted that understanding the structure will help in exploring the problems and related solutions. For this reason, possible defect types that can occur in a composite structure must first be understood. Then, the characterisation and measuring of a defect in the structure must be performed precisely if possible. Here, measuring refers to correctly establishing the defect's location, size and significance. Structural health monitoring (SHM) methods have ensured useful strategies for inspecting aerospace composites in various recent studies [11; 12; 13; 14].