A method of generating oxygen depleted air on an aircraft, and an aircraft fuel tank inerting system. The method including the steps of passing a first proportion of air through an air pressure reduction device to produce a supply of reduced pressure air, passing a second proportion along one side of a membrane of an air separation module, exposing the other side of the membrane of the air separation module to the reduced pressure air, so that oxygen is extracted from the second proportion of air across the membrane and the air separation module exhausts oxygen depleted air for inerting an aircraft fuel tank.