In an aircraft fuel tank, the fuel is already present, and potential ignition sources (such as electrical faults or mechanical impacts) cannot be entirely eliminated. Therefore, the key to preventing combustion lies in controlling the oxygen concentration within the tank.

The Role of Nitrogen

Nitrogen is an inert gas, meaning it does not support combustion. By introducing nitrogen into the fuel tank, the concentration of oxygen is significantly reduced. Here’s how it works:
Displacement of Oxygen: Nitrogen is injected into the fuel tank, displacing the air (which contains 21% oxygen). As the oxygen concentration decreases, the environment within the tank becomes less conducive to combustion.
Creating an Inert Atmosphere: When the oxygen concentration drops below a certain threshold (typically below 12% for most hydrocarbon fuels), the mixture becomes non-flammable. This is because there is not enough oxygen to sustain the chemical reaction required for combustion.

Practical Implementation

In modern aircraft, systems like the On-Board Inert Gas Generation System (OBIGGS) are used to continuously supply nitrogen-enriched air (NEA) to the fuel tanks. These systems work by:
Air Separation: Using air separation modules to extract nitrogen from the surrounding air.
Injection: Pumping the nitrogen-enriched air into the fuel tank, thereby reducing the oxygen concentration to a safe level.

Example Scenario

Imagine a fuel tank with a volume of 100 liters. Initially, the air inside contains 21 liters of oxygen. When nitrogen is introduced, it displaces the air, reducing the oxygen concentration. If the oxygen level is reduced to 10 liters (10%), the mixture becomes non-flammable, effectively preventing combustion even if an ignition source is present.