Mass and Balance
Mass and balance questions are arithmetic with traps. Know the definitions of basic empty mass, dry operating mass, take-off mass, landing mass, and the difference between maximum structural masses and performance-limited masses. Understand that the centre of gravity must lie within the certified forward and aft limits, and why the forward limit is a control authority limit while the aft limit is a stability limit.
Learn to compute the CG using moments: mass times arm, summed and divided by total mass. Be comfortable converting between arm, station and index units, and using the load sheet format. Understand the effect of fuel burn on CG as fuel is consumed from tanks at different arms.
Traps: forgetting to include crew and catering in dry operating mass; treating maximum zero-fuel mass as optional; and mixing units (kg versus lb, metres versus inches).
Worked example: basic empty mass 1200 kg at arm 2.0 m; pilot 80 kg at 1.5 m; fuel 200 kg at 2.5 m. Moments: 2400 + 120 + 500 = 3020 kg·m; total mass 1480 kg; CG = 3020/1480 = 2.04 m. Check against limits before accepting.
Rule of thumb: always compute total mass and total moment in the same units, and sanity-check that the CG lies between the forward and aft limits before answering.