Weight and Balance
Weight and balance rewards careful arithmetic more than memorization. The vocabulary: the datum is an arbitrary reference plane; a station is a location measured from it; an arm is the distance from the datum to an item's center of gravity; a moment is weight × arm; and the CG is total moment ÷ total weight. Arms ahead of the datum are negative, which is the single most common sign error.
Empty weight and empty-weight CG (EWCG) are established by weighing the aircraft after equipment changes. Empty weight includes unusable fuel and full oil but not the pilot, passengers, or payload. Useful load is maximum certificated weight minus empty weight. Tare is the weight of blocks and chocks and is subtracted from the scale readings.
The weight-shift formula does most of the work: weight moved ÷ total weight = change in CG ÷ distance moved. Solve for whichever quantity is missing.
Loading rules. When adding weight, new CG = (old moment + added moment) ÷ (old weight + added weight). Removing weight uses the same relationship with subtraction. Ballast must be fixed, recorded, and entered in the weight-and-balance record.
Traps. You must check both maximum weight and CG limits; being inside the CG range does not excuse overweight. Forward CG raises stall speed and elevator forces; aft CG degrades stability and is the more critical controllability problem.
Worked example. A 3,400-lb aircraft shifts 100 lb of baggage 40 inches aft. Change in CG = (100 × 40) ÷ 3,400 = 1.18 inches aft.