To achieve carbon neutrality in aviation by 2050, as targeted by the Green Deal, this thesis explores novel hybrid-electric powertrain solutions. Focusing on a regional aircraft, the analysis investigates the impact of combining hydrogen (used in both fuel cells and combustion), batteries, and kerosene combustion on aircraft performance and environmental impact. A Double-Hybrid Powertrain model, incorporating these energy sources, was developed and integrated within a Conceptual Design Method. Results indicate that a powertrain configuration dominated by fuel cells (80-90\% of total power), in conjunction with by either a small battery or limited kerosene combustion, offers significant potential for reducing both total energy consumption and environmental impact.