Mass and Balance
Mass and Balance tests mass limitations, centre of gravity, moment arms, index systems and load sheets. The goal is to ensure MTOM, MLM, MZFW and CG limits are respected for takeoff, in flight and landing, because both mass and CG position affect performance and handling.
Core rules
- Mass limits include MTOM, MLM and MZFW, plus regulated takeoff and landing masses from performance.
- Moment equals mass times arm; CG equals total moment divided by total mass.
- Percent MAC equals (CG arm minus leading-edge arm) divided by MAC length times 100.
- Fuel density varies with temperature; use the operator density or volume-to-mass conversion.
- The load sheet must reflect actual distribution, not only total mass.
- A forward CG increases nose-down authority demand; an aft CG reduces static margin.
Worked example
Basic mass is 2,000 kg at arm 2.0 m; payload is 500 kg at arm 3.0 m. Moments: 2,000 times 2.0 = 4,000 kg m; 500 times 3.0 = 1,500 kg m. Total mass = 2,500 kg; total moment = 5,500 kg m. CG = 5,500 divided by 2,500 = 2.2 m from datum. Check that 2.2 m lies inside the certified forward and aft limits.
Traps
- Using volume instead of mass for fuel.
- Forgetting crew, baggage or taxi fuel at the correct arm.
- Mixing metres and millimetres, or kilograms and pounds.
- Checking only MTOW and ignoring MZFW or CG.