The driving force behind the presented research is the need for more ecological applications in the aviation sector. Hybrid-electric propulsion systems are considered a promising alternative technology capable to provide fuel savings, reduced emissions and lower noise levels. Despite the added benefits, there are several pitfalls such as the commonly known issues of batteries’ low energy density — compared to that of jet-fuels —, as well as the need to efficiently regulate the various heat loads generated from the electrical propulsion system. The design of the Thermal Management System (TMS), for a hybrid-electric aircraft, alone constitutes of a ubiquitous task, due to its impact on the aircraft’s total weight, cooling drag and overall performance. This study focuses on investigating the possible operating media of a selected TMS, and their impact during the conceptual design phase of the system. On one hand, the air’s capability to transfer heat with increasing altitude is investigated. On the other hand, various liquid coolants are evaluated over their performance to remove heat loads but also, to not exceed their evaporation, boiling or autoignition points, while doing so. The simulations are conducted with the use of Modelon Impact, implementing the Modelica Liquid Cooling Library, and are transient to account for the dynamic effects of temperature change and thermal inertia of the various system elements. The results show that an atmospheric air mass flow of at least 0.848 kg/s is required to sufficiently dissipate the induced heat loads. Although, that is highly dependent on the flight altitude. If a liquid medium is to be selected, the best performing would be a propylene glycol water mixture with a mass flow of 0.204 kg/s, but the final selection is also subject to other factors that vary according to the priorities given.