An aircraft safety system, comprising: an ullage passivation system arranged to selectively provide a gas flow into an ullage in a fuel tank ; the ullage passivation system including: a sensor configured to be disposed within the ullage of the fuel tank and arranged to provide a signal indicative of a temperature and/or a vapor composition of the ullage of the fuel tank ; a sensor configured to be disposed on or within a wing or fuselage and arranged to provide a signal indicative of ambient air humidity; and the aircraft safety system further comprising a controller in communication with the ullage passivation system , the controller being programmed to selectively operate the ullage passivation system to provide the gas flow to the ullage in the fuel tank based exclusively on the sensed ambient air humidity exceeding a humidity threshold; and at least one of the sensed temperature and the sensed vapor composition exceeding a predetermined threshold; wherein the controller is programmed with a look up table or performance map, enabling the controller to anticipate operating conditions that require operation of the ullage passivation system to avoid a thermal event or water condensation within the fuel tank, the look up table or performance map containing information that cross-references or correlates the inputs received by the controller from the sensors throughout the aircraft to operating points of the ullage passivation system , and wherein the lookup table or performance map correlates at least one of the temperature of the ullage in the fuel tank and the vapor composition of the ullage in the fuel tank to operating points of the ullage passivation system . an ullage passivation system arranged to selectively provide a gas flow into an ullage in a fuel tank ; the ullage passivation system including: a sensor configured to be disposed within the ullage of the fuel tank and arranged to provide a signal indicative of a temperature and/or a vapor composition of the ullage of the fuel tank ; a sensor configured to be disposed on or within a wing or fuselage and arranged to provide a signal indicative of ambient air humidity; and the aircraft safety system further comprising a sensor configured to be disposed within the ullage of the fuel tank and arranged to provide a signal indicative of a temperature and/or a vapor composition of the ullage of the fuel tank ; a sensor configured to be disposed on or within a wing or fuselage and arranged to provide a signal indicative of ambient air humidity; and the aircraft safety system further comprising a controller in communication with the ullage passivation system , the controller being programmed to selectively operate the ullage passivation system to provide the gas flow to the ullage in the fuel tank based exclusively on the sensed ambient air humidity exceeding a humidity threshold; and at least one of the sensed temperature and the sensed vapor composition exceeding a predetermined threshold; wherein the controller is programmed with a look up table or performance map, enabling the controller to anticipate operating conditions that require operation of the ullage passivation system to avoid a thermal event or water condensation within the fuel tank, the look up table or performance map containing information that cross-references or correlates the inputs received by the controller from the sensors throughout the aircraft to operating points of the ullage passivation system , and wherein the lookup table or performance map correlates at least one of the temperature of the ullage in the fuel tank and the vapor composition of the ullage in the fuel tank to operating points of the ullage passivation system . <EOS>