Abstract: Abstract: Oxygen evolves from the fuel, increasing the oxygen concentration in the ullage of the fuel tank, which increases the risk of fuel vapor igniting explosion, making the aircraft fuel tank in an unsafe state. The fuel tank can be inerted by flammability reducing method. Nitrogen-riched-gas is used to reduce the oxygen concentration in the ullage of the tank below the level of 14.5%, to ensure the safety of the fuel tank. By establishing the tank ullage oxygen concentration model, considering the dissolution of dissolved oxygen in the fuel, the FRM model which the concentration changing with the altitude was established. The simulation showed that the reliability flammability of fuel tank decreases with the increase of the minimum time between failures (MTBF) of the model, and the minimum MTBF satisfying the reliable flammability is obtained. Through simulation, the minimum flow required by the model to meet the performance flammability was obtained: the minimum high flow required to satisfy the requirements in descent and the minimum low flow required to meet the requirements in other phrases.

Key words: Key words: flammability, inerting, nitrogen-riched-gas, equilibrium oxygen concentration, reliability