A hybrid-electric aircraft system is provided and includes first and second hybrid-electric engines, each of which includes an electric motor to drive operations thereof, and a supplemental power unit (SPU). The SPU is configured as a thermal engine paired with a generator and is configured to generate electrical power. The first and second hybrid-electric engines are operable normally and off, respectively, with electrical power generated by the SPU being diverted to the electric motor of the second hybrid-electric engine.