How fuel burn moves aircraft CG
Center of gravity does not always stay in the same place throughout a flight. When fuel is consumed, both total aircraft weight and total moment change. The direction and amount of CG movement depend on where the fuel is located relative to the current CG.
Fuel forward of CG
If fuel is carried forward of the aircraft's current CG, burning that fuel removes forward moment. Other things equal, the CG tends to move aft.
Fuel aft of CG
If fuel is carried aft of the current CG, burning it removes aft moment and tends to move the CG forward.
Fuel near the CG
If the fuel tank arm is close to the aircraft CG, fuel burn may change total weight significantly while producing only a small CG shift.
Multiple tanks complicate the picture
Aircraft with wing, tip, auxiliary or fuselage tanks may have different tank arms. Transfer sequences or tank-selection procedures can therefore influence CG movement as well as fuel availability.
How to calculate a later condition
Start with the loaded total weight and moment, subtract the weight and moment of fuel burned, then divide the new total moment by the new total weight. Compare the resulting CG against the approved envelope.
Check the whole flight when required
An acceptable takeoff CG does not prove every later condition is acceptable. Some aircraft or operations require checking landing or zero-fuel conditions because the point can move toward an envelope limit.
Related PilotDesk guides
W&B envelope · Fuel weight · Moment and CG
Burning 20 pounds of fuel
Use made-up values to see the arithmetic, not to set an aircraft limit. Suppose the starting weight is 2,400 lb and the loaded CG is at 40.00 in. The starting moment is 2,400 × 40.00 = 96,000 lb-in. If 20 lb of fuel at a tank arm of 30 in is burned, subtract 600 lb-in and 20 lb. The new weight is 2,380 lb and the new moment is 95,400 lb-in, so the CG is 95,400 ÷ 2,380 = 40.08 in (rounded). Because the tank arm is forward of the starting CG, the CG moves slightly aft as that fuel is removed.
For an actual loading check, use the aircraft's measured starting weight and moment, tank arms, fuel weight and approved envelope. Some aircraft require checking multiple fuel states or following a specific tank-use sequence.