Mass and balance - principles and limitations
Mass and balance - principles and limitations tests the definitions and structural limits that make a load legal: maximum take-off mass, maximum landing mass, maximum zero-fuel mass, CG limits, datum, arm, moment and the effect of CG on performance.
Definitions and limits
- MTOM, MLM and MZFM are structural or performance limits; the lowest applicable limit governs.
- The datum is a reference point; arm is distance from datum; moment = mass × arm.
- Total CG = total moment ÷ total mass. The CG must lie within the forward and aft limits for take-off, landing and zero fuel as applicable.
- A forward CG increases rotation effort and may reduce performance; an aft CG reduces static stability and can affect stall recovery.
Worked example
50,000 kg is loaded at arm 18 m and 10,000 kg at arm 30 m. Total mass is 60,000 kg. Total moment = 900,000 + 300,000 = 1,200,000 kg·m. CG = 1,200,000 ÷ 60,000 = 20 m. If the forward limit is 17 m and the aft limit is 23 m, the CG is legal. If zero-fuel mass is 55,000 kg against an MZFM of 56,000 kg, the zero-fuel case is also legal.
Traps
Do not mix kilograms and pounds or metres and inches. Do not use an index as a mass. Check zero-fuel mass before adding fuel. An aft CG may reduce trim drag but is not automatically beneficial; stability and recovery margins matter.