A method of conditioning oxygen depleted air (ODA) exhausted from a fuel cell comprising the steps of; taking fuel from an aircraft fuel tank collector cell, delivering said fuel to a rear mounted engine via a primary heat exchanger, taking exhaust ODA from a fuel cell, passing the ODA through the primary heat exchanger in the opposite direction to the fuel, such that the fuel acts as a heat sink for the ODA, to cool the ODA, passing the ODA through a dryer, to dry the ODA and using the cooled, dried ODA to inert fuel in the aircraft fuel tank.