Determining Landing Weight and Balance
As fuel is burned during the flight, the airplane's weight and balance will change. Normally, most of the fuel load is carried in the wings, very near the center of gravity. This reduces the movement of the center of gravity during the flight. It is possible for the airplane to takeoff within it's center of gravity limitations, but land outside of them, however. This is especially true if the CG is close to the CG limit.
To calculate the landing weight and balance for the airplane, subtract the expected fuel burn during the flight. Calculate the new center of gravity with this fuel missing, and make sure this landing CG is still within limits.
In addition to the CG position upon landing, some airplanes are designed with a maximum structural landing weight. When this limitation exists, enough fuel must be burned during the flight so that the airplane's landing weight does not exceed this limitation.