See it

A loading that changes in flight

Every gallon burned takes 6 lb off the aircraft at the fuel tanks’ arm (48 in here). The weight falls — and the CG moves, because weight is leaving from one place.

Here the tanks are aft of the loaded CG, so burning fuel moves the CG forward. On the envelope the point slides down and to the left along a straight line to the zero-fuel point.

That is why a preflight check looks at the landing condition, not only the takeoff: both ends of that line must be inside the envelope (PHAK Ch. 10).