Aircraft fuel tank flammability reduction methods and systems and air separation methods using membranes

An aircraft fuel tank flammability reduction method includes contacting a membrane filter with air feed (17), permeating oxygen and nitrogen from the air feed through the membrane (22), and producing filtered air (21) from the filter (22). The filtered air (21) is produced from the filter as a result of the membrane removing any hydrocarbons containing six or more carbon atoms to produce a total of 0.001 ppm w/w or less. An air separation method includes feeding air into a filter containing a hollow fiber membrane that exhibits the property of resisting degradation due to exposure to hydrocarbons containing six or more carbon atoms. The filter (22) exhibits a pressure drop across the membrane (22) of less than 5 psi. The method includes feeding the filtered air (21) into an air separation module (12) containing a hollow fiber membrane that exhibits a susceptibility to degradation from exposure to hydrocarbons containing six or more carbon atoms.