This paper addresses the feasibility of Proton Exchange Membrane Fuel Cells (PEMFC) to reduce the climate impact of civil aviation from a performance calculation perspective. PEMFC systems are considered one of the most promising and suitable approaches to mitigate climate effects as they do not cause local CO2 or NOx emissions when fueled with green hydrogen. However, the thermal management of fuel cells and electric components represents a major challenge: Despite the large amounts of waste heat generated, very limited optimal operating temperature ranges must be respected. At the same time, these components and the required heat exchanger impose a significant mass penalty that must be considered in the overall aircraft mass and thrust requirements. A PEMFC model and a heat exchanger model based on the Effectiveness-Number of Transfer Units (ϵ-NTU) method are implemented in the Modular Aircraft Engine Performance Tool for Sustainable Aviation (MAPLE) to evaluate their performance and mass. Various propulsion configurations are then developed and derived from the literature, including all-electric and gas turbine hybrid-electric fuel cell engines with low-and hightemperature PEMFC. In a preliminary analysis, an A320 with turbofan or electric fan engines and an ATR72 with turboprop or electric propeller engines are investigated along representative flight missions in terms of propulsion power requirements and waste heat generation. Optimization of the fuel cell size, thermal management system, and hybridization strategy is then applied to attainable configurations in order to minimize additional mass, fuel consumption, and thus emissions and climate impact compared to conventional gas turbine engines. Overall, the use of PEMFCs with green hydrogen is indeed promising to alleviate the climate impact of civil aviation by reducing CO2 and NOx emissions for hybrid-electric concepts, while eliminating them for all-electric architectures. However, among the all-electric configurations, only an ATR72 derivative powered by high-Temperature PEMFCs is feasible from a perfor-mance perspective, whereas an all-electric A320-sized aircraft is not viable due to prohibitive cooling requirements and mass penalties. The hybridization of both aircraft categories with both PEMFC types is possible and leads to combined CO2 and NOx reductions of 7-40% depending on the aircraft, mission, fuel cell type, and permissible gas turbine surge margins.