Mass and Balance (Loading, CG, and Structural Limits)
This domain tests whether you can build a lawful load sheet: the structural limits that cap mass, and the centre-of-gravity calculation that keeps the aircraft controllable.
Definitions and the CG calculation
- MTOW, MLW, MZFW, OEW and useful load cap different phases; the lowest applicable limit governs.
- Moment = mass × arm. CG = total moment ÷ total mass.
- %MAC = (CG − LEMAC) ÷ MAC × 100.
- Forward CG: higher stall speed, longer takeoff, heavier stick forces. Aft CG: reduced stability and a greater risk of deep stall or spin.
Worked example
OEW 1,050 kg at 2.10 m; pilot 85 kg at 1.60 m; fuel 120 kg at 2.60 m; baggage 20 kg at 3.20 m. Moments: 2,205 + 136 + 312 + 64 = 2,717 kg·m. Total mass 1,275 kg. CG = 2,717 ÷ 1,275 = 2.131 m. With limits 1.95–2.25 m, the aircraft is within limits and 0.119 m forward of the aft limit.
Traps
- Checking takeoff mass against MTOW but never against MZFW.
- Forgetting that fuel burn moves the CG and that you must check the landing case too.
- Treating litres as kilograms; density varies with temperature and fuel grade.
- Getting the sign wrong when an arm lies ahead of the datum.
- Ignoring floor loading, seat limits or compartment maxima because the total mass was fine.