Mass and Balance
The Mass and Balance domain tests whether you can build a legal loading from basic empty mass to take-off mass, place the centre of gravity (CG) inside its forward and aft limits, and explain how loading affects performance and handling. The exam expects correct use of datum, arm, moment and the structural mass limits, not merely arithmetic.
Core definitions and limits
- Basic empty mass (BEM) includes airframe, engines, fixed equipment, unusable fuel and full oil.
- Dry operating mass (DOM) = BEM + crew + operational items; it excludes usable fuel and traffic load.
- Zero-fuel mass (ZFM) = take-off mass minus usable fuel; it must not exceed MZFM.
- Take-off mass must not exceed MTOM; landing mass must not exceed MLM.
- Moment = mass × arm; CG arm = total moment ÷ total mass. %MAC = (CG arm − leading-edge arm) ÷ MAC length × 100.
Forward CG increases rotation and landing-flare loads and can reduce range; aft CG reduces static stability and can make stall/spin recovery more difficult. The forward and aft limits are both legal limits.
Worked example
BEM 900 kg at 2.00 m; pilot 80 kg at 1.50 m; passenger 70 kg at 1.50 m; baggage 20 kg at 3.20 m; fuel 100 kg at 2.40 m. Moments: 1,800 + 120 + 105 + 64 + 240 = 2,329 kg·m. Mass = 1,170 kg. CG = 2,329 ÷ 1,170 = 1.991 m. With limits 1.95–2.25 m, the loading is inside the envelope. ZFM = 1,170 − 100 = 1,070 kg, so compare it with MZFM as well as MTOM.
Traps
Using fuel volume as mass without applying density; forgetting that unusable fuel is in BEM but usable fuel is not; measuring arms from the wrong datum; checking only take-off CG and not zero-fuel or landing CG; and assuming an aft CG is 'better' because it reduces stick force.