Ullage washing is a process that requires displacing the air in the fuel tank empty space, also known as ullage, with nitrogen gas or nitrogen enriched air (NEA). NEA is a term used to describe low purity nitrogen (90-98% pure), generally generated via a gas separation process. Ullage washing would be accomplished by providing the nitrogen or NEA to a supply line that feeds a simple fuel tank gas supply manifold.

Air, and particularly oxygen, readily dissolves in fuel. When a commercial transport airplane takes off after fueling, the resulting change in altitude causes a decrease in atmospheric pressure in the fuel tank. This decrease in pressure allows for some of the air to escape solution and enter the ullage space of the fuel tank. Since oxygen dissolves more readily than nitrogen, this can increase the oxygen concentration of the fuel tank ullage above ambient, although the total amount of gas evolving from the fuel is small. This can have a profound effect on the fuel tank oxygen concentration for both inert fuel tanks as well as fuel tanks with ambient air in the ullage space. Fuel scrubbing is a process by which most of the oxygen dissolved in the fuel is displaced with nitrogen. Fuel and nitrogen are combined through a series of nozzles in a large container with the resulting combination having a very small amount of oxygen in solution. The military has used fuel scrubbing to allow for fuel tank inerting systems to operate more effectively and to increase survivability to ballistic impact in combat.