For example, think about your fuel system. There may be several tanks involved. While you're preflighting the aircraft, check how much fuel is in each tank. Make sure you drain each fuel drain that's displayed in the diagram. Based on the fuel levels from your preflight inspection, position the fuel selector for the most appropriate tank — generally the main tank with the most fuel in it. As you continue to visualize the system, the next step may be an auxiliary fuel pump. Do you need to turn it on? Next in line — perhaps the mixture. Think before you set it. If your engine is fuel injected, the mixture may need to be full aft as you engage the starter. If you're at a high elevation airport and your engine is carbureted, it may not want the mixture full rich during the start up. Take a second and think through each component of the system.