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© Copyright 1998, Jim Loy
An airplane's fuel tanks are normally inside its wings. And, planes are normally refueled (or fueled) from the bottom. The hose, from the fuel truck, is shoved upward into a valve at the lowest point of the fuel tank. And fuel is pumped into the fuel tank, from below. Why? Of course, it is easier to work under the plane than on its wings. But, isn't it more efficient to pour the fuel in from the top? Doesn't the pump have to push harder against all the weight of the fuel already in the tank?
The answer to that is, "No." It is more efficient (it requires less energy) to pump the fuel in at the bottom. What the pump is working against is "pressure." This pressure is measured in pounds per square inch. It is no greater at the bottom of a wide tank than it is at the bottom of a narrow tank. In fact, it is the same pressure as a tank the same diameter as the hose. The pump is lifting this cylindrical column of liquid, which is the same diameter as the hose. This column is as high as the top of the liquid in the tank. Pouring the fuel in at the top would require lifting a column that is higher than the top of the wing, which would take more energy.