Mass and Balance
Mass and balance test whether the planned loading stays within structural and CG limits and whether performance data is based on the correct mass. The exam covers MTOW, MLW, MZFW, index methods, load sheets and the effect of CG on performance.
Limits and terms
- MTOW, MLW, MZFW and MTW are structural; regulated masses may be lower because of performance.
- CG must remain within forward and aft limits for take-off, in flight and landing.
- Forward CG increases stick force and take-off distance; aft CG reduces stability and can reduce stall margin.
Index method
Operators use index = (mass × (arm − reference arm))/constant + base index. Load indices are summed on the load sheet; the final index must lie inside the operator's envelope.
Worked example
Empty mass 35,000 kg at 18.0 m; pax 12,000 kg at 22.0 m; cargo 3,000 kg at 30.0 m; fuel 8,000 kg at 16.0 m. Total mass = 58,000 kg. Total moment = (35,000 × 18) + (12,000 × 22) + (3,000 × 30) + (8,000 × 16) = 630,000 + 264,000 + 90,000 + 128,000 = 1,112,000 kg·m. CG = 1,112,000/58,000 = 19.17 m. If the certified range is 17.5–24.0 m, the loading is within limits. In index terms, the same total is compared with the load sheet envelope.
Traps
- Using dry operating mass instead of zero-fuel mass, or omitting crew.
- Forgetting that fuel density changes with temperature; use kg, not litres, for mass.
- Ignoring last-minute changes after the load sheet is signed.
- Assuming an aft CG is always better because it reduces take-off distance.