Mass and Balance
Mass and balance questions are arithmetic with defined rules. Master the definitions first: basic empty mass, dry operating mass, take-off mass, landing mass, zero-fuel mass, and the maximum limits for each. Then learn the load sheet logic and the CG envelope.
Key concepts: moment = mass × arm; the CG is total moment divided by total mass. Moving an item shifts the CG by (mass moved × distance moved) ÷ total mass. Adding or removing mass shifts the CG toward or away from the added/removed item's station. Understand forward and aft CG effects on performance and handling - forward CG increases stall speed and reduces range; aft CG reduces stability and can reduce stick forces. Know that MZFW limits payload, MTOW limits fuel plus payload, and MLW limits arrival mass.
Traps: mixing units (kg vs lb, inches vs metres); forgetting to include crew and crew baggage in dry operating mass; treating unusable fuel as usable; and using the wrong limit when a question gives both structural and performance-limited masses - the lower of the two governs.
Worked example: total mass 50 000 kg, CG at 20.0 m. Move 500 kg aft from station 15 m to station 25 m. Shift = (500 × 10) ÷ 50 000 = 0.1 m aft, so the new CG is 20.1 m. Rule of thumb: small mass, big lever - a modest item at a long arm can move the CG more than a large item near the datum.