Why are fuel tanks filled up at the end of the day?

• A. To minimize condensation of water vapour when temperatures decrease overnight.
• B. To allow more time for testing.
• C. To reduce evaporation.
• D. To improve fuel quality.

Answer: (A)

Moisture control in aircraft fuel tanks is the key idea. When the tanks are not full, the air inside contains water vapor. As the aircraft temperature drops overnight, that air cools and the water vapor can condense into liquid water, which settles at the bottom of the tank and can be drawn into the fuel system or cause icing at altitude. By filling the tanks at the end of the day, you reduce the amount of air in the tank, leaving less opportunity for condensation to form, and any moisture can be found and drained from the sump.

Evaporation is a less significant factor here, and the other options aren’t related to how moisture behaves in cool-down conditions, so the primary purpose is to minimize condensation of water vapor as temperatures fall.