An aircraft system (100, 200), comprising: a main fuel tank (160, 260) configured to house primary fuel; a green fuel tank (170, 270) configured to house green fuel; a mixing apparatus (180, 280) coupled to the main fuel tank (160, 260) and the green fuel tank (170, 270), the mixing apparatus (180, 280) configured to mix the primary fuel and the green fuel to result in a predetermined ratio of mixed fuel; and a controller (190, 290) configured to selectively command a first amount of primary fuel from the main fuel tank (160, 260) and a second amount of green fuel from the green fuel tank (170, 270) to result in the predetermined ratio of mixed fuel; a main aircraft engine (120, 220) coupled to the controller (190, 290), the main fuel tank (160, 260), and the mixing apparatus (180, 280), wherein the main aircraft engine (120, 220) is configured to operate, in a first main engine mode, with the primary fuel from the main fuel tank (160, 260) and to operate, in a second main engine mode, with the mixed fuel from the mixing apparatus (180, 280) based on commands from the controller (190, 290), wherein the controller (190, 290) is configured to selectively operate the main aircraft engine (120, 220) in the first main engine mode and in the second engine mode; an auxiliary power source (130, 230) coupled to the controller (190, 290) and the green fuel tank (170, 270), wherein the auxiliary power source (130, 230) is configured to operate with at least a portion of the green fuel from the green fuel tank (170, 270) based on commands from the controller (190, 290); a green fuel processor coupled to the green fuel tank and the auxiliary power source, wherein the green fuel processor is configured to receive the green fuel from the green fuel tank and to process the green fuel into hydrogen, and wherein the auxiliary power source is a fuel cell configured to operate with the hydrogen of the green fuel; and a main fuel sensor (164, 264) coupled to the controller (190, 290) and configured to measure a first level of fuel in the main fuel tank (160, 260), the controller (190, 290) configured to selectively command the first amount and the second amount based on the first level of fuel in the main fuel tank (160, 260) and such that the first amount and the second amount result in the predetermined ratio of at least 50:50 of primary fuel to green fuel, and wherein the controller is configured to consider the first level in view of an anticipated usage of the primary fuel to reach a predetermined destination, and wherein the controller is configured to select an appropriate time for a transition between the first main engine mode and the second main engine mode such that the first level is sufficient to maintain the predetermined ratio of at least 50:50 throughout the anticipated usage to the predetermined destination. a main fuel tank (160, 260) configured to house primary fuel; a green fuel tank (170, 270) configured to house green fuel; a mixing apparatus (180, 280) coupled to the main fuel tank (160, 260) and the green fuel tank (170, 270), the mixing apparatus (180, 280) configured to mix the primary fuel and the green fuel to result in a predetermined ratio of mixed fuel; and a controller (190, 290) configured to selectively command a first amount of primary fuel from the main fuel tank (160, 260) and a second amount of green fuel from the green fuel tank (170, 270) to result in the predetermined ratio of mixed fuel; a main aircraft engine (120, 220) coupled to the controller (190, 290), the main fuel tank (160, 260), and the mixing apparatus (180, 280), wherein the main aircraft engine (120, 220) is configured to operate, in a first main engine mode, with the primary fuel from the main fuel tank (160, 260) and to operate, in a second main engine mode, with the mixed fuel from the mixing apparatus (180, 280) based on commands from the controller (190, 290), wherein the controller (190, 290) is configured to selectively operate the main aircraft engine (120, 220) in the first main engine mode and in the second engine mode; an auxiliary power source (130, 230) coupled to the controller (190, 290) and the green fuel tank (170, 270), wherein the auxiliary power source (130, 230) is configured to operate with at least a portion of the green fuel from the green fuel tank (170, 270) based on commands from the controller (190, 290); a green fuel processor coupled to the green fuel tank and the auxiliary power source, wherein the green fuel processor is configured to receive the green fuel from the green fuel tank and to process the green fuel into hydrogen, and wherein the auxiliary power source is a fuel cell configured to operate with the hydrogen of the green fuel; and a main fuel sensor (164, 264) coupled to the controller (190, 290) and configured to measure a first level of fuel in the main fuel tank (160, 260), the controller (190, 290) configured to selectively command the first amount and the second amount based on the first level of fuel in the main fuel tank (160, 260) and such that the first amount and the second amount result in the predetermined ratio of at least 50:50 of primary fuel to green fuel, and wherein the controller is configured to consider the first level in view of an anticipated usage of the primary fuel to reach a predetermined destination, and wherein the controller is configured to select an appropriate time for a transition between the first main engine mode and the second main engine mode such that the first level is sufficient to maintain the predetermined ratio of at least 50:50 throughout the anticipated usage to the predetermined destination.