Aircraft on board inert gas generation system

The system receives air from a relatively low pressure source such as low pressure engine bleed air or ram air and comprises a positive displacement compressor 40 supplying an air separation module 48 for providing oxygen enriched and nitrogen enriched inert gas flows. The operating speed of the compressor 40, for adjustment across a wide range in order to provide efficient operation in cruise and descent phases of aircraft flight, and/or the flow rate from the module 48 to the space receiving the inert gas flow, e.g. fuel tanks 52, may be controlled in accordance with a sensor for monitoring he composition of least one of the gas fractions provided by the module and/or the composition in the tanks. Compressor speed is controlled by a nitrogen sensor 90 associated with the fuel tanks 52 and linked to a motor controller 88. The oxygen enriched flow is stored and/or utilized on board of the aircraft.