Nitrogen Generation System (NGS) is an onboard inert-gas system that removes oxygen from ambient bleed air and delivers nitrogen-enriched air into aircraft fuel tanks, preventing combustion. By exploiting the fact that ambient air is already 78% nitrogen, NGS uses proprietary air-separation-module technology to generate pure nitrogen or nitrogen-rich air for immediate use without storage, producing gas that meets global aviation standards and lowering in-tank flammability exposure.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is a nitrogen generation system in aircraft?
A nitrogen generation system removes oxygen from ambient air and blows it into the center fuel tank, generating nitrogen gas devoid of impurities and water vapor. The nitrogen-rich air reduces flammability of the void above jet fuel, and the inerting system goal is to reduce oxygen content to 12%.
A nitrogen generation system (NGS) is safety equipment that lowers the oxygen content inside the fuel tank from the normal 21% to below 12%. By ventilating fuel-vapor-laden ullage gas out of the tank and into the atmosphere, the system replaces the expelled mix with nitrogen-enriched air (NEA), so the ullage becomes inert and fuel-tank flammability is reduced. The process is not a chemical creation of new nitrogen; instead, aircraft bleed air from the compressor stage is first dried, then passed through hollow-fiber membranes or carbon molecular sieves that adsorb the larger oxygen molecules and let the smaller nitrogen molecules pass, so nitrogen is separated from all other elements. The resulting NEA is fed into the tank, while the oxygen-rich reject flow is blown into the center fuel tank or vented overboard.
On the Boeing 787, a no-bleed airplane, the same function is performed by an oxygen-depletion system that achieves the same 12% target through proprietary technology. Whether skid-mounted, wall-mounted, or portable, an NGS always comprises an air compressor, a dryer, multiple stages of filtration that remove dust and moisture, storage tanks, and the separation module - often called OBIGGS - built from air-separation membranes (ASMs). Maintenance following the manufacturer's service guidelines keeps the unit running efficiently for decades.
What is the purpose of a nitrogen generation system in aircraft?
The purpose of a nitrogen generation system is to reduce fuel vapor flammability whenever the aircraft's engines are running. Its goal is to reduce oxygen content within the fuel tank to 12%, so nitrogen-enriched air lowers the risk of combustion. NGS thereby prevents combustion and reduces the risk of fuel tank fires.
To achieve this, the system cools and separates bleed air. The system ensures air entering ASM is free from contaminants and excessive ozone, and is at correct temperature and pressure. Separation is accomplished by running air through semipermeable fibrous tubes: smaller molecules pass through membranes and are exhausted overboard, while nitrogen is not negated - it is separated from all other elements. Oxygen-enriched air is vented out; NEA makes its way to the center wing tank.
How does the nitrogen generation system work in aircraft?
The nitrogen generation system works by using hot bleed air from the aircraft's left pneumatic manifold. This air is fed into Air Separation Modules, each a semi permeable hollow fibre membrane bundle contained in a pressure containment canister. Inside the modules, separation is accomplished by running the air through semipermeable fibrous tubes. Non N2 molecules, being smaller, pass through the membranes and are exhausted overboard, while oxygen enriched air is vented out. The remaining nitrogen enriched air is delivered to the center fuel tank, where the oxygen content is reduced below 12%, a level insufficient for combustion. By lowering the flammability exposure of the tank to a level similar to or lower than that of the main wing tanks, the system lowers the risk of combustion.
