Abstract: A fuel ignition arrester system that has a feedback control to maintain the fuel/air mixture in the ullage of a fuel tank above the flammability range. The system uses a photoelectric detector to signal when the fuel/air mixture concentration falls or rises to a predetermined concentration value. The predetermined value is selected to be higher than the upper limit of the flammability range so that the fuel/air mixture is maintained above the range and, thus, incombustible. A fuel pump and piping system with nozzles is arranged to inject fuel into the ullage when the concentration falls to the predetermined value and to stop injecting fuel when the concentration rises to the predetermined value. A flammability detection system includes the detector and a controller that produces an annunciating signal when the fuel/air mixture concentration is equal to or between the upper and lower limits of the flammability range.