Integration of a multifunctional fuel cell into civil aircraft as on-board power generating system is a promising opportunity to improve energy efficiency while reducing emissions. In this approach, a PEMFC would provide electrical energy during on-ground operations and emergency situations, while by-products water vapour and oxygen depleted air (ODA) would be used during flight for water generation and inerting (provision of noninflammable atmosphere). This study presents a novel cathode exhaust gas dehumidification system based on an air cycle machine (compressor, heat exchanger, cyclone, turbine) which supplies liquid water and dry ODA for the aforementioned functions. First, feasibility of the multifunctional approach, quality of the generated water and necessary level of dehumidification are discussed. Next, real fluid property data and the system thermodynamic model are presented. Feasibility of the novel system is demonstrated with a parametric study on the effect of stoichiometry, compressor pressure ratio and turbine outlet pressure. Computed results show that: the target average 02 concentration of 10.5% (vol.) is maintained at stoichiometries below lambda O-2 <= 1.8; for maximum humidity ratio x(max) = 0.002 kg(H2O)/kg (ODA) there is not risk of ice formation on condenser when compressor pressure ratio is above pi >= 2.8. Finally, an estimation of added and potentially removed weight is given, also possible system configurations are derived. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.