Under normal operating conditions, there is an explosive mixture of vaporized fuel and air in the space above the liquid fuel, referred to as the ullage. If this mixture is exposed to a flame source, such as a spark, the vapors can ignite in an explosive manner. Current aircraft fire prevention techniques have consisted primarily of the release of flame retardants such as halogenated hydrocarbons, including CCL4, CF4, FREONS, or HALONS (broadly referred to as chloro-fluorohydrocarbons), following flame ignition. In most cases, particularly in the case of a fuel tank ignition, this response is too late as the ignition rapidly evolves into an explosion.

The developed method prevents ignition of the fuel vapors rather than extinguishing them after they are ignited. This is accomplished by filling the ullage with an electronegative gas, sulfur hexafluoride (SF6), in combination with an inert gas, such as CO2 and / or N2. This not only excludes oxygen from the ullage, but also prevents transmission of fire initiating sparks.

As fuel is consumed or as the altitude of the aircraft changes, the ullage gases are fed to or extracted from the tank to maintain a continuous, controlled fuel flow.

Reference: UCLA Case No. 1997-552

US Patent Number: 5,904,190