## 2 Methods and Materials
### Material Selection  
In 2022, IATA's Airline Maintenance Cost Executive Commentary provided insights into global Maintenance, Repair, and Overhaul (MRO) spending by aircraft segment [2]. The chart below (Figure 7) shows that most spending in 2022 was directed toward engine and airframe maintenance. Forecasts for 2032 suggest a similar spending pattern, with a slight increase in engine maintenance costs. This reflects the critical focus on engine and airframe upkeep for safe and efficient aircraft operation. Figure 7: Global MRO Spend by Segment [2]. In the aerospace industry, materials have always played a crucial role in reducing weight, enhancing performance, and improving fuel efficiency. The primary aim is to lower costs while maintaining safety. In this context, [PERSON] et al. [99] published a paper outlining the material requirements for designing aircraft structures and engines, highlighting the advantages and challenges of current materials, and introducing future trends in aerospace materials. The earliest airframe materials were wooden, with the first flight occurring in 1903. In 1927, aluminium-based alloys dominated aircraft structures for the next 80 years. However, this trend shifted with the rise in composite materials. Figure 8 below illustrates material trends in Boeing aircraft over the years, showing a rapid increase in composite materials and a decrease in aluminium alloys, especially in the Boeing 787. A similar trend, documented by [PERSON] et al. [101], included data on Airbus aircraft, which have also adopted this shift. As illustrated in Figure 9, in the new generation of Airbus models, there is a notable increase in composite materials and a reduction in aluminium alloys, comprising more than 53% of the Airbus A350. However, aluminium-based alloys continue to play a crucial role in airframe structures and engines due to their cost-effectiveness, ease of manufacturing and their well-known mechanical behaviour. Additionally, these alloys can undergo heat treatment and endure relatively high-stress levels [99]. Several key factors have driven the development of new aircraft materials, such as composite materials, over the past two decades. Increasing payload capacity and extending flight range are top priorities. Improving fuel efficiency and prolonging service life are also essential goals. Reducing the overall weight of the aircraft is critical. Ultimately, these efforts aim to lower operating costs, making modern aircraft more economical and efficient [99]. Before diving into specifics, it is important to define composite materials. Composites are materials made from two or more distinct components that, when combined, offer enhanced properties that the individual materials cannot achieve on their own [102]. Many researchers, including [PERSON] et al. [103] highlighted the benefits of using composite materials in contrast with conventional metallic counterparts. A few examples are weight efficiency, low maintenance, design flexibility and integrity, corrosion resistance and  
Figure 8: Total materials used in Boeing series aircraft [100]. a lifespan of approximately 28 years. Composite materials are extensively used throughout the aircraft airframe. In the Airbus A380 (Figure 10), for example, composites are incorporated into the centre wing box, horizontal and vertical tailplanes, outer flaps, as well as the unpressurised fuselage and rear pressure bulkhead. These materials also hold great promise for engine components, contributing to further weight reduction and improved performance. The primary types of composites include ceramic matrix, metal matrix, and polymer matrix composites. Of these, polymer matrix composites (PMCs) are the most widely used. Notably, in Boeing 777 and 787 aircraft, PMCs account for up to 50% of their weight, primarily as carbon fibre-reinforced polymers (CFRP). PMCs can be further classified into two subcategories: thermoplastics and thermosets [99]. Figure 10: The use of composites in Airbus A380 [104].