Mass and Balance
Mass and Balance tests whether you can compute aircraft mass, centre of gravity and load distribution within certified limits. The examiner looks for correct use of datum, arm, moment and the difference between empty, zero-fuel, takeoff and landing mass.
Definitions and rules
- Mass is in kilograms; moment is mass multiplied by arm from the datum. CG = total moment ÷ total mass.
- Empty mass includes unusable fuel and operating equipment. Zero-fuel mass excludes usable fuel. Takeoff mass adds taxi fuel and trip fuel as applicable.
- Limits are forward and aft CG limits, maximum zero-fuel mass, maximum takeoff mass and maximum landing mass. Floor loading and compartment limits also apply.
- Last-minute changes must be recorded and checked; a load sheet is invalid if the actual distribution differs.
- Forward CG generally increases stall speed and reduces performance; aft CG reduces static stability and can affect rotation and stall recovery.
Worked example
Empty mass 40,000 kg at 12 m; passengers 12,000 kg at 18 m; cargo 3,000 kg at 20 m; fuel 8,000 kg at 15 m. Moments: 480,000 + 216,000 + 60,000 + 120,000 = 876,000 kg·m. Total mass = 63,000 kg. CG = 876,000 ÷ 63,000 = 13.90 m. Compare with the certified forward and aft limits; the arithmetic alone does not prove the load is legal.
Traps
Using pounds and kilograms interchangeably; omitting taxi fuel; confusing zero-fuel mass with landing mass; and checking only total mass while ignoring CG. A load can be under maximum mass but outside the CG envelope.