Aircrafts have fuel tanks in its wings and sometimes tail and fuselage. The fuel in tanks is the energy source required for engines, apu and indirectly all of systems.
With its full capacity, an A330 can fly 13-14 hours that refueled approximetely 140,000 liters of fuel. During the flight period, the aircraft systems especially near the fuel tank (e.g. Air condition packs) are warmed up.
It is risky that heat and fuel are in close proximity. Considering it, it would be terrible for this high amount of fuel to start burning at 36,000 feet.
Combustion reactions can take place in a very long time, such as rust, but can also occur in very short periods. In order for the combustion reaction to occur in an environment, 3 basic elements must exist. Flammable material, caustic and heat.
Fuel as our flammable material and it is indispensable for us. The second element is the heat we cannot prevent due to the working systems and finally oxygen, which is the caustic gas. If we prevent oxygen from being in the tank, one of the elements that cause combustion is eliminated and flight safety increases.
The NGS system (Nitrogen Generation System) has been developed to eliminate this oxygen and prevents the combustion reaction from occurring by filling the fuel tanks with nitrogen, preventing the presence of oxygen in the tank.
The NGS system separates nitrogen up to 99 percent purity with the air filtration method it receives from the atmosphere and sends this nitrogen to the tanks and it is kept in the fuel tank together with the fuel.