Abstract: Redundant components, and their volume and weight, used in on board gas generating systems are avoided in a fully integrated system for generating oxygen enriched air (OEA) and nitrogen enriched air (NEA) for a variety of purposes. A first separator (16) may provide OEA to a second separator (24) directly to provide oxygen for crew consumption or alternatively may provide OEA to a compressor (42) which in turn provides high pressure OEA to storage vessels (70), (72). The separator (16) also provides NEA alternatively to aircraft fuel tanks (60) for inerting them by direct flow or, after being compressed by the compressor (42) to an NEA storage system (50). NEA from the storage system (50) may additionally be used for fuel expulsion in a fuel system (96) associated with an auxiliary power unit/emergency power unit including a hot gas generator (74), the latter also receiving oxidant for the fuel in the form of OEA from one of the OEA storage tanks (72).