Originally Posted by LarryJ

The C.G. is calculated before takeoff. It is calculated both at takeoff weight and at Zero Fuel Weight (ZFW). This ensures that you are within limits both at takeoff and at landing, no matter how much of your fuel you burn. The actual C.G. moves during flight as fuel is burned and the forward and aft limits change based on aircraft weight.

I can't think of any reason why a change would be needed after takeoff other than a mistake in loading the aircraft that wasn't discovered until after takeoff.


There are no sharp lines between "safe" and "unsafe". A hard line is drawn but it is based on certification requirements with built in buffers.

With regard to balance, as the C.G. moves aft, the airplane because more efficient*, less stable, and the elevator control has greater authority. As C.G. moves forward, the airplane becomes less efficient, more stable, and the elevator control has less authority. The forward and aft limit lines are drawn where the negative affects of movement in that direction have reached the certification limits.

(* The MD11 had a system called 'relaxed stability'. It pumped fuel into trim tanks in the tail at cruise to move the C.G. farther aft at cruise to improve efficiency. That fuel was later pumped back into the main tanks for descent and landed when the greater stability was needed. Concord also had tail trim tanks though I'm not sure how it was used operationally. There are likely more examples.)