Fuel, Payload and Cargo Distribution Effects
Fuel effects
Fuel is carried in tanks at specific arms. As fuel burns, the aircraft mass decreases and the CG moves - the direction depends on tank location relative to the CG. Wing-tank fuel near the CG has little effect; fuselage or tail-tank fuel can shift CG significantly. Questions often ask for the CG after a given fuel burn; treat burned fuel as a negative mass at the tank arm.
Payload effects
Passengers and cargo are distributed by zone. Moving a mass aft moves the CG aft proportionally: shift = (mass × distance) ÷ total mass. Loading a heavy item in a forward hold moves CG forward; in an aft hold, aft. Cargo density matters: a light bulky load may fill a hold before reaching its mass limit (volume-limited), while dense cargo may reach the structural floor limit first.
Traps
- Assuming fuel burn always moves CG forward - it depends on tank position.
- Ignoring the effect of passengers moving during flight (e.g., to the rear lavatory).
- Forgetting that cargo compartment mass limits are separate from total payload limits.
- Volume-limited vs mass-limited cargo: check both.
- Using block fuel instead of trip fuel for the landing mass calculation.
Worked example
Aircraft mass 12,000 kg, CG at 5.0 m. A 200 kg crate is moved from arm 3.0 m to arm 9.0 m (distance 6.0 m). CG shift = (200 × 6.0) ÷ 12,000 = 0.10 m aft, so new CG = 5.10 m.
Rule of thumb
To move CG by a desired amount, the mass to move = (desired shift × total mass) ÷ distance available.