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Fuel-cell-based, hydrogen-driven aircraft can significantly reduce the climate impact of aviation but the low ambient pressures at high altitude negatively affect the performance of fuel cells. The influence of low pressures on the behavior of a direct hybrid-system that connects a fuel cell and a battery without a DC/DC converter, is examined and compared to a fuel-cell-only system. Low ambient pressure leads to a reduced fuel cell voltage and power output. The voltage change between maximum voltage and the voltage level at the operational power increases with a decreasing air pressure for both the hybrid-system and the fuel-cell-only system. However, in a direct hybrid, the battery stabilizes the system voltage level and the voltage decrease is significantly smaller for the hybrid-system than for the fuel-cell-only system. At an ambient pressure of 300 mbar, the voltage variation of a 4 kW system is 96% lower for a hybrid-system compared to a fuel-cell-only system. The lower pressure also reduces the power output of the fuel cell. In the direct hybrid the battery contributes a demand related share of power and the contribution from the fuel cell decreases with air pressure as the contribution of the battery rises. [Display omitted] • Influence of pressure dependent fuel cell behaviour on direct hybrid system is shown. • Influence of low pressure on voltage variation within a direct hybrid is analysed. • Low ambient pressure increases voltage variation especially in fuel cell only systems. • Direct hybridisation stabilizes the voltage. • Low ambient pressure reduces power contributed by fuel cell in a direct hybrid. [ABSTRACT FROM AUTHOR]