A method of using an air vehicle, wherein the air vehicle comprises a fuel cell system; wherein the fuel cell system comprises: a fuel cell, a fuel gas system for supplying fuel gas to the fuel cell, a potential sensor, and a controller; wherein the fuel gas system comprises a fuel gas supplier; and the method comprises: monitoring, by the controller, the potential of the fuel cell; determining, by the controller, whether or not a potential of the fuel cell measured by the potential sensor, is a reversal potential; when the controller determines that the potential of the fuel cell is a reversal potential, increasing, by the controller, a fuel gas supply from the fuel gas supplier to the fuel cell; characterized in that the method further comprises: recording, by the controller, an accumulated time in which the potential of the fuel cell is in a reversal potential state; when the controller determines that the potential of the fuel cell is a reversal potential, determining, by the controller, whether or not the accumulated time is less than a predetermined amount of time; when the controller determines that the accumulated time is less than the predetermined amount of time, increasing, by the controller, the fuel gas supply from the fuel gas supplier to the fuel cell; and when the controller determines that the accumulated time is equal to or longer than the predetermined amount of time, limiting, by the controller, the output of the fuel cell, or stopping the fuel cell system and ending control of the fuel cell system, by the controller. wherein the fuel cell system comprises: a fuel cell, a fuel gas system for supplying fuel gas to the fuel cell, a potential sensor, and a controller; a fuel cell, a fuel gas system for supplying fuel gas to the fuel cell, a potential sensor, and a controller; wherein the fuel gas system comprises a fuel gas supplier; and the method comprises: monitoring, by the controller, the potential of the fuel cell; determining, by the controller, whether or not a potential of the fuel cell measured by the potential sensor, is a reversal potential; when the controller determines that the potential of the fuel cell is a reversal potential, increasing, by the controller, a fuel gas supply from the fuel gas supplier to the fuel cell; characterized in that the method further comprises: recording, by the controller, an accumulated time in which the potential of the fuel cell is in a reversal potential state; when the controller determines that the potential of the fuel cell is a reversal potential, determining, by the controller, whether or not the accumulated time is less than a predetermined amount of time; when the controller determines that the accumulated time is less than the predetermined amount of time, increasing, by the controller, the fuel gas supply from the fuel gas supplier to the fuel cell; and when the controller determines that the accumulated time is equal to or longer than the predetermined amount of time, limiting, by the controller, the output of the fuel cell, or stopping the fuel cell system and ending control of the fuel cell system, by the controller. monitoring, by the controller, the potential of the fuel cell; determining, by the controller, whether or not a potential of the fuel cell measured by the potential sensor, is a reversal potential; when the controller determines that the potential of the fuel cell is a reversal potential, increasing, by the controller, a fuel gas supply from the fuel gas supplier to the fuel cell; characterized in that the method further comprises: recording, by the controller, an accumulated time in which the potential of the fuel cell is in a reversal potential state; when the controller determines that the potential of the fuel cell is a reversal potential, determining, by the controller, whether or not the accumulated time is less than a predetermined amount of time; when the controller determines that the accumulated time is less than the predetermined amount of time, increasing, by the controller, the fuel gas supply from the fuel gas supplier to the fuel cell; and when the controller determines that the accumulated time is equal to or longer than the predetermined amount of time, limiting, by the controller, the output of the fuel cell, or stopping the fuel cell system and ending control of the fuel cell system, by the controller. recording, by the controller, an accumulated time in which the potential of the fuel cell is in a reversal potential state; when the controller determines that the potential of the fuel cell is a reversal potential, determining, by the controller, whether or not the accumulated time is less than a predetermined amount of time; when the controller determines that the accumulated time is less than the predetermined amount of time, increasing, by the controller, the fuel gas supply from the fuel gas supplier to the fuel cell; and when the controller determines that the accumulated time is equal to or longer than the predetermined amount of time, limiting, by the controller, the output of the fuel cell, or stopping the fuel cell system and ending control of the fuel cell system, by the controller.