The article highlights the main stages of preparatory work for ground testing of a demonstrator gearbox that transmits power to the air propeller, obtained by combining the power of a gas turbine and electric engines. This hybrid turbo-electric propulsion (HTEP) is being developed with the aim of reducing the environmental impact of aircraft emissions, in line with the European Green Deal with the goal of ensuring environmental neutrality in the European Union by 2050, as well as one of the Goals of the ACARE Flight Path 2050 programme (Challenge 3. Protecting the environment and energy supply). Design work was carried out to develop a combined gearbox, research and development work was carried out to create an electric engine, select test bench equipment, and develop test plans for the HTEP to meet the objectives set out in the EFACA project. These findings are based on research conducted within the EFACA (Environmentally Friendly Aviation for All Classes of Aircraft) project funded by the European Commission under the Horizon Europe programme (Grant Agreement No. 101056866), which is part of the Climate, Energy and Mobility cluster. EFACA aims to contribute to the development of a cleaner aviation sector by developing new technologies using electric and hybrid turbo-electric propulsion and new sustainable fuels to combat climate change by 2050 (www.efaca.eu).