3.4 Weight and Balance Equations and Records

Weight and Balance Equations and Records

Weight and balance is one of the most calculation-heavy General subjects, but the core idea is direct. Every item has a weight, a distance from the datum called an arm, and a moment. Moment equals weight times arm. The aircraft center of gravity, or CG, is found by dividing total moment by total weight. The maintenance reason is equally direct: an aircraft outside weight or CG limits may have unsafe performance, stability, or control characteristics.

The datum is an imaginary reference plane chosen by the manufacturer. Arms measured aft of the datum are usually positive. Arms forward of the datum may be negative, depending on the aircraft data. A negative arm creates a negative moment. Keep the signs with the numbers through the entire calculation. Do not drop a negative sign because the answer choices all look positive.

Weighing procedures start before the scales are used. The aircraft must be in the proper empty weight configuration described by the manufacturer or applicable data. Fuel, oil, unusable fuel, equipment, drainable fluids, ballast, and tare items must be treated correctly. Scales must be appropriate and calibrated. The weighing area must be level, sheltered enough for accurate readings, and suitable for jacks or platforms if required.

Adverse loading questions ask whether a worst reasonable combination creates an out-of-limit condition. A forward CG case may involve minimum fuel, front seats occupied, and aft seats empty. An aft CG case may involve baggage and rear seats loaded. The exact scenario depends on aircraft data, so read the problem before assuming which condition is worst.

Equipment changes are common maintenance triggers for weight and balance revision. Removing a radio, adding avionics, changing an alternator, or installing an interior component can change empty weight and empty weight CG. The mechanic must account for removed and installed items. A removed item is subtracted from weight and moment. An installed item is added. The new totals produce a new CG.

A simple calculation workflow is:

1. List the beginning weight, arm, and moment.
2. Subtract removed equipment weight and moment.
3. Add installed equipment weight and moment.
4. Divide final moment by final weight for CG.
5. Compare the result to current limits.
6. Update required records and equipment information.

Ballast problems use the same lever principle. If a CG must be moved, the correcting moment is created by placing known or unknown weight at a known arm. The farther the ballast is from the current CG, the less weight may be needed, but the location must be structurally and procedurally acceptable. Never treat ballast as a casual weight dropped into the aircraft.

For exam preparation, practice with a table. Put weights in one column, arms in the next, and moments in the third. Label pounds, inches, and pound-inches. If the problem uses a moment index, apply the stated divisor consistently. If the final CG is outside the limit, the correct answer may be the amount to move, remove, or add weight rather than the CG number itself.

Weight and balance is not just arithmetic. It is a return-to-service topic because the calculated condition must match the aircraft records, placards, equipment list, and limitations.