Mass and Balance
Mass and Balance
Mass and Balance tests the calculation and control of aircraft mass, centre of gravity (CG) and loading limits. The exam expects precise use of datum, arm, moment and the difference between mass and weight, plus an understanding of how loading affects performance and stability.
Core rules
- Mass is in kilograms or pounds; weight is force. Balance is calculated by moments: moment = mass × arm, and CG = total moment ÷ total mass.
- The empty mass and empty CG are fixed for a given aircraft; crew, passengers, baggage, fuel and cargo are added at their respective arms. Fuel may have a different arm from baggage, and fuel burn changes both mass and CG.
- Maximum take-off mass, maximum landing mass and maximum zero-fuel mass are separate limits. The CG must remain within the forward and aft limits for take-off, in flight and landing.
- Forward CG increases stall speed, pitch-down tendency and take-off distance; aft CG reduces static stability and can make stall recovery difficult.
Worked example
Empty mass 700 kg at arm 1.00 m; pilot 80 kg at 0.50 m; fuel 100 kg at 2.00 m. Total mass = 880 kg. Total moment = 700 + 40 + 200 = 940 kg·m. CG = 940 ÷ 880 = 1.068 m. If the aft limit is 1.05 m, the aircraft is out of balance and must be reloaded, even though the mass is below maximum.
Traps
- Forgetting fuel or including fuel in the zero-fuel mass.
- Mixing inches and metres, or pounds and kilograms.
- Using the empty CG as the final CG.
- Assuming a heavy aircraft is automatically out of balance; mass and CG are separate checks.