Ballast is used in an aircraft to attain the desired CG balance, when the center of gravity is not within limits or is not at the location desired by the operator. It is usually located as far aft or as far forward as possible to bring the CG within limits, while using a minimum amount of weight. Ballast that is installed to compensate for the removal or installation of equipment items and that is to remain in the aircraft for long periods is called permanent ballast. It is generally lead bars or plates bolted to the aircraft structure. It may be painted red and placarded: PERMANENT BALLAST—DO NOT REMOVE. The installation of permanent ballast results in an increase in the aircraft empty weight, and it reduces the useful load.
Temporary ballast, or removable ballast, is used to meet certain loading conditions that may vary from time to time. It generally takes the form of lead shot bags, sand bags, or other weight items that are not permanently installed. Temporary ballast should be placarded: BALLAST, XX LB. REMOVAL REQUIRES WEIGHT AND BALANCE CHECK. The baggage compartment is usually the most convenient location for temporary ballast.
Whenever permanent or temporary ballast is installed, it must be placed in an approved location and secured in an appropriate manner. If permanent ballast is being bolted to the structure of the aircraft, the location must be one that was previously approved and designed for the installation, or it must be approved by the FAA as a major alteration before the aircraft is returned to service. When temporary ballast is placed in a baggage compartment, it must be secured in a way that prevents it from becoming a projectile if the aircraft encounters turbulence or an unusual flight attitude.
To calculate how much ballast is needed to bring the center of gravity within limits, the following formula is used.
Figure 4-24 and Figure 4-27 show an aft extreme condition check being performed on an airplane. In this previously shown example, the airplane’s center of gravity was out of limits by 0.6″. If there were a need or a desire to fly the airplane loaded this way, one way to make it possible would be the installation of temporary ballast in the front of the airplane. The logical choice for placement of this ballast is the forward baggage compartment.
The center of gravity for this airplane is 0.6″ too far aft. If the forward baggage compartment is used as a temporary ballast location, the ballast calculation will be as follows:
When ballast is calculated, the answer should always be rounded up to the next higher whole pound, or in this case, 47 lb of ballast would be used. To ensure the ballast calculation is correct, the weight of the ballast should be plugged back into the four column calculation and a new center of gravity calculated.
The aft limit for the airplane was 99″, and the new CG is at 98.96″, which puts it within acceptable limits. The new CG did not fall exactly at 99″ because the amount of needed ballast was rounded up to the next higher whole pound. If the ballast could have been placed farther forward, such as being bolted to the engine firewall, less ballast would have been needed. That is why ballast is always placed as far away from the affected limit as possible.
In evaluating the ballast calculation shown above, the following key points should be recognized.
- The loaded weight of the aircraft, as identified in the formula, is what the airplane weighed when the CG was out of limits.
- The distance the CG is out of limits is the difference between the CG location and the CG limit, in this case 99.6″ minus 99″.
- The affected limit identified in the formula is the CG limit which has been exceeded. If the CG is too far aft, it is the aft limit that has been exceeded.
- The aft limit for this example airplane is 99″, and the ballast is being placed in the baggage compartment at an arm of 60″. The difference between the two is 39″, the quantity divided by in the formula.
Viewed as a first class lever problem, Figure 4-29 shows what this ballast calculation would look like. A ballast weight of 46.68 lb on the left side of the lever multiplied by the arm of 39″ (99 minus 60) would equal the aircraft weight of 3,034 lb multiplied by the distance the CG is out of limits, which is 0.6″ (99.6 minus 99).