## 1 Introduction  
The energy used in aviation accounts for between 2.5% and 5% of all global energy consumption [1]: increasing usage of fossil fuel will result in rising greenhouse gas emissions. The 781 Mt of CO\({}_{2}\) emitted from aeroplanes amounts to about 12% of global transport emissions. According to the International Air Transport Association (IATA), there will be 47,000 jets and about 8.2 billion passengers travelling around the world by 2036. The European Commission and IATA have developed a plan to reduce aviation emissions by 50% in 2040 [1]. Sustainable, eco-friendly fuels for aviation are necessary to reduce emissions and to ensure a safe and healthy environment [2]. Kerosene fuel is inexpensive but is the primary cause of the emission in aviation. Combusting kerosene emits the greenhouse gas CO\({}_{2}\)[3]. Fuel cells convert chemical energy from fuel (hydrogen) into electrical energy through an electrochemical reaction. A total of 20% of an aircraft's emissions are from the auxiliary power unit (APU), and both Boeing and Airbus have deployed fuel cells to replace the gas turbine APUs used when the main engines are not engaged. This has provided a reduction in both emissions and fuel use [3,4]. Fuel cell-powered aircraft such as SUNTAN and CRYOPLANE use complete hydrogen propulsion systems [3]. However, combusting hydrogen forms small amounts of NOx [5]. Although hydrogen has high gravimetric (mass-based) energy density, it has low volumetric energy density at ambient conditions compared to conventional fuels, thus requiring larger storage volumes and posing weight challenges; also, safety is another concern [1]. There are different storage technologies for hydrogen, compressed hydrogen, liquified hydrogen, and cryogenic hydrogen [6]. Hydrogen is highly flammable; thus, storage vessels must have good structural integrity [7]. Although composite overwrapped pressure vessels (COPVs) are widely adopted in industry for hydrogen storage, this study investigates metallic tanks to establish a preliminary understanding of structural performance under high-pressure conditions [8]. The objective of this study is to conduct a preliminary structural analysis of compressed hydrogen storage vessels made of stainless steel 304, 6061-T6 aluminium, and Grade 5 (Ti-6 Al-4V) titanium to evaluate their suitability for replacing conventional auxiliary power units with the PEM fuel cell system, and to identify the most promising material for safe and efficient hydrogen storage in commercial aircraft. The design of compressed hydrogen storage vessels suitable for commercial aircraft Boeing 777 producing 440 kW of power from a PEM fuel cell APU was demonstrated. The structure was analysed using ANSYS R19.2 to propose a material with high structural integrity. This will help the aviation industry choose suitable materials, optimising safety, reliability, weight, and cost, with reduced emissions. [PERSON] et al. [9] used PEM fuel cells to power aircraft at elevations of 200, 1200, and 2200 m to determine the viability of fuel cells at various altitudes. [PERSON] et al. [10] combined solid oxide fuel cells (SOFCs) with a piston engine to develop a hybrid system. The results of the analysis showed that SOFCs can be employed with a piston engine; and the system had a 52.29% energy efficiency, and fuel cell has an exergy efficiency of 92%. [PERSON] et al. [11] investigated hydrogen storage proton-exchange membrane fuel cell (PEMFC) electric aircraft and the size of the propulsion system using a real-world flight profile. Electric aeroplanes are generally 25% heavier than conventional aircraft, but weight can be reduced by enhancing energy storage methods for fuel cell technologies. [PERSON] et al. [12] addressed the technical challenges of the performance of fuel cell hybrid systems with auxiliary power gas turbines, concluding that systems' weight being greater than that of conventional APUs was the main concern. [PERSON] et al.