It is possible to load most modern airplanes so the CG shifts outside of the allowable limit. Placards and loading instructions in the weight and balance data inform the pilot of the restrictions that prevent such a shift from occurring. A typical placard in the baggage compartment of an airplane is shown in Figure 1. When the CG of an aircraft falls outside of the limits, it can usually be brought back in by using ballast.

Center of Gravity Change After a Repair or Alteration
Figure 1. Typical baggage compartment placard

Temporary Ballast

Temporary ballast, in the form of lead bars or heavy canvas bags of sand or lead shot, is often carried in the baggage compartments to adjust the balance for certain flight conditions. The bags are marked “Ballast XX Pounds—Removal Requires Weight and Balance Check.” Temporary ballast must be secured so it cannot shift its location in flight and the structural limits of the baggage compartment must not be exceeded. All temporary ballast must be removed before the aircraft is weighed.

Temporary Ballast Formula

The CG of a loaded airplane can be moved into its allowable range by shifting passengers or cargo or by adding temporary ballast.

Permanent Ballast

If a repair or alteration causes the aircraft CG to fall outside of its limit, permanent ballast can be installed. Usually permanent ballast is made of blocks of lead painted red and marked “Permanent Ballast—Do Not Remove.” It should be attached to the structure so that it does not interfere with any control action and attached rigidly enough that it cannot be dislodged by any flight maneuvers or rough landing.

Two things must first be known to determine the amount of ballast needed to bring the CG within limits: the amount the CG is out of limits, and the distance between the location of the ballast and the limit that is affected.

If an airplane with an empty weight of 1,876 pounds has been altered so its EWCG is +32.2, and CG range for weights up to 2,250 pounds is +33.0 to +46.0, permanent ballast must be installed to move the EWCG from +32.2 to +33.0. There is a bulkhead at fuselage station 228 strong enough to support the ballast.

Center of Gravity Change After a Repair or Alteration
Figure 2. Formula for determining ballast

To determine the amount of ballast needed, use the formula in Figure 2.

A block of lead weighing 7.7 pounds, attached to the bulkhead at fuselage station 228, moves the EWCG back to its proper forward limit of +33. This block should be painted red and marked “Permanent Ballast—Do Not Remove.”