Why is it recommended to fill fuel tanks after the last flight of the day?

Filling fuel tanks after the last flight of the day is recommended primarily to prevent moisture condensation in the tanks. When an aircraft is fueled, it is important to fill the tanks to full capacity, as the presence of empty space can lead to the formation of water condensation. This condensation occurs when warm air in the fuel tank cools and the moisture in the air condenses on the tank walls. Over time, this can lead to water accumulating in the fuel system, which can cause a variety of issues such as engine performance problems or fuel contamination.

By filling the tanks before parking the aircraft, you reduce the amount of air space in the tank, thus minimizing the potential for condensation. This practice also ensures that the aircraft is ready for its next flight with clean, water-free fuel.

Focusing on other options illustrates why they may not address the primary concern. While maximizing fuel efficiency, reducing contamination, and easing morning refueling might be benefits or considerations in some scenarios, the prevention of moisture condensation is the most critical aspect directly influenced by the timing of fueling.