Aircraft ballast and center of gravity
Ballast is weight intentionally added to an aircraft to help place the center of gravity within an acceptable range. The amount of weight matters, but its location is equally important because the distance from the aircraft datum determines how much moment the ballast creates.
Weight, arm, and moment
The basic relationship is moment = weight × arm. A 20-pound ballast item at a 100-inch arm creates 2,000 pound-inches of moment. The same 20 pounds at a 50-inch arm creates only 1,000 pound-inches, so the farther location has twice the effect on the aircraft's total moment.
How ballast moves the CG
Adding weight aft of the current CG tends to move the loaded CG aft. Adding weight forward of the current CG tends to move it forward. The exact change depends on current total weight, current total moment, ballast weight, and ballast station.
Solving for ballast weight
If you know current weight and CG, a desired target CG, and the allowable ballast arm, the weight-and-moment equation can be rearranged to solve for the ballast required. PilotDesk's ballast calculator performs that arithmetic, but the target still must fall within the approved envelope.
Why “more ballast” is not always the answer
Adding ballast increases total aircraft weight. That can create a different problem even if it moves CG in the desired direction. Maximum takeoff weight, landing weight, baggage-area limits, floor-loading restrictions, structural limits, and station limits all still apply.
Permanent versus temporary ballast
Aircraft may use permanent ballast installed as part of the approved configuration or temporary/removable ballast under approved procedures. The installation method, attachment, station, and documentation matter; loose weight is not an acceptable substitute for an approved ballast arrangement.
Check the final loading condition
After adding ballast, recalculate total weight and total moment from the beginning and confirm the final point is within the aircraft's approved CG envelope. Also consider how fuel burn may move CG during the flight.
Related PilotDesk tools
Moment & CG · Weight & Balance Builder · Percent MAC guide