Why does gravity refueling allow for less fuel compared to pressure refueling?

Gravity refueling is a process that relies on the natural force of gravity to fill the fuel tanks of an aircraft. The reason that less fuel may be allowed during gravity refueling compared to pressure refueling specifically relates to the design and positioning of the fuel tanks.

When it is stated that the Aft Cabin Tank sits higher, it highlights a critical aspect: if the tank's positioning is elevated, it can create a situation where it is more challenging to fully utilize the gravitational flow to fill that tank efficiently. This can lead to limitations on how much fuel can be added, resulting in a smaller total volume of fuel taken on during gravity refueling operations.

In contrast, pressure refueling employs a system that uses pressurized fuel to push fuel into the tanks, which can overcome the influence of gravity regarding tank positioning. This allows for a more complete fill of the tanks without being hindered by the physical constraints associated with gravity. Thus, the correct answer ties directly into the operational dynamics of gravity refueling versus pressure refueling.