Hydrogen fuel cells are set to play an important role in decarbonizing the aviation sector. They operate at higher efficiencies than turbine engines, but their low operating temperature requires a dedicated thermal management system. The added drag, weight and parasitic power of such a system has a high impact on aircraft performance and its design must be integrated with overall propulsion system development. Among cooling options, flow boiling has the potential of increasing the cooling system performance, but design methods are lacking in the literature. To address this problem, this thesis presents preliminary design methods able to size both conventional single-phase and novel two-phase cooling systems and tests them on a reference regional turboprop aircraft. Sizing results show that pumped multi-phase systems result in lower system mass and drag than conventional single-phase cooling while vapour compression systems are a less attractive option due to their high power demand.