An on-board inert gas generating system includes a flow control valve modulated according to changes in ambient conditions to minimize changes to oxygen content within a fuel tank. The quality of the nitrogen-enriched air stream that is provided by the air separation module varies in response to flow. Higher flow rates through the air separation module removes less oxygen relative to lower flow rates. The amount of flow through the air separation module that produces the least amount of oxygen within the fuel tank is determined for an ambient pressure and provided by modulating the flow control valve.