Abstract: Aircraft fuel system reduces flammability exposure of fuel tanks. In one embodiment, aircraft center fuel tank fuel is cooled in flight to reduce flammability exposure to be similar to that of an unheated metal wing tank. Fuel tanks having adjacent heat sources are insulated to minimize heat flow into the fuel. Fuel tanks having lower cooling properties, e.g., composite wing tanks, are cooled during flight so temperatures are reduced to be similar to metal wing tanks. A fuel tank, pressurized relative to outside pressure at altitude, having lower flammability exposure than unpressurized tanks and cooling fuel in the tank reduces flammability exposure of the fuel tank to be similar to that of an unpressurized metal wing tank. Recirculating flow from a tank, passing through heat exchanger, and returning to the tank cools fuel. A controller starts/stops fuel flow to tanks and uses sensors to command flow to reduce flammability.