As all industries move towards sustainable fuels to reduce emissions, aircraft industry is still at its nascent stages. The adoption of clean energy sources has been slow particularly due to low energy density of commercially available batteries, and high volumetric density of hydrogen as a fuel. Electric aircrafts have been demonstrated but only a few are available commercially. Hydrogen powered flights have also been demonstrated, but only for experimental purposes. As the move towards clean aviation furthers, it is necessary to indicate an approach for designing a mass minimized aircraft powerplant. The aim of this thesis was to indicate exergy-gravimetric approaches towards designing an optimal minimized-mass powerplant for aircraft applications. This thesis work also aimed to use this approach to size a mass-minimized powerplant for the Pipistrel Alpha Electro aircraft. The exergy analysis of fuel cell was carried out on CycleTempo through which sources of exergy destruction were identified and quantized. A mass-minimized model was developed on MATLAB. Using these tools, a number of system configurations for varying battery combinations, endurance requirements, and fuel cell types were analysed. Overall, an exergy-gravimetric approach towards mass minimization was developed in this thesis work. Multiple fuel cell - battery hybrid powerplants were sized for the Pipistrel Alpha Electro. It was demonstrated that at least three system configurations using commercially available batteries and PEM fuel cells exist which would perform superior than the existing battery system on the aircraft, reducing the powerplant weight by as much as 20 kg from the existing battery system, while increasing endurance by about 15 minutes.