Mass and balance
This domain tests whether you can load an aircraft so it is both under its mass limits and inside its centre-of-gravity envelope. Almost every question reduces to mass × arm = moment and a comparison with the AFM limits.
The rules
- Mass limits: MTOW, maximum landing mass, maximum zero-fuel mass, and any seat or baggage limit.
- Arm is horizontal distance from the datum; aft arms are positive.
- Total moment = sum of (mass × arm). CG = total moment ÷ total mass.
- The CG envelope is usually given in metres aft of datum or as %MAC: %MAC = (CG − LEMAC) ÷ MAC × 100.
- Empty mass and empty CG come from the current weighing; modifications change both.
Worked example
Empty 700 kg at 1.55 m. Pilot 80 kg at 1.00 m; rear passenger 70 kg at 2.00 m; baggage 10 kg at 2.80 m; fuel 80 kg at 1.50 m. Moments: 1085 + 80 + 140 + 28 + 120 = 1453 kg·m. Total mass 940 kg; CG = 1453 ÷ 940 = 1.546 m. With MTOW 950 kg and limits 1.40–1.80 m, the load is legal. Add 20 kg baggage: mass 960 kg (over MTOW) although CG becomes 1.572 m, still inside.
Traps
Exceeding MTOW while chasing the CG; forgetting fuel mass; using the wrong datum; applying baggage limits after summing rather than per compartment; assuming a forward CG is always safe (it increases rotation and landing distances and can reduce elevator authority).