Mass and Balance
What the exam tests
Mass and balance questions test datum, arm, moment, centre of gravity, forward and aft limits, maximum weights, fuel load and loading calculations. The exam expects correct units and a clear statement that the CG remains within limits for takeoff and landing.
Core rules
- Mass x arm = moment. Total mass is the sum of masses; total moment is the sum of moments; CG = total moment / total mass.
- Maximum takeoff mass, maximum landing mass, maximum zero-fuel mass and maximum ramp mass are separate limits.
- Zero-fuel mass excludes usable fuel. Fuel burn moves CG because fuel has an arm.
- The CG must remain within the certified forward and aft limits throughout the flight, not only at dispatch.
- Load sheets must be checked for last-minute changes, baggage and passenger seating.
Worked example
Empty mass 800 kg at arm 2.00 m; pilot 80 kg at 1.50 m; fuel 100 kg at 2.50 m; baggage 20 kg at 3.00 m. Moments: 1,600 + 120 + 250 + 60 = 2,030 kg m. Total mass = 1,000 kg. CG = 2,030 / 1,000 = 2.03 m. If the aft limit is 2.20 m and the forward limit is 1.80 m, the loading is within limits. If 20 kg of baggage is moved from 3.00 m to 1.00 m, total moment falls by 40 kg m and CG moves to 1.99 m.
Traps
Do not mix kilograms and pounds or metres and inches. Do not omit fuel from takeoff mass but include it in zero-fuel mass. Do not assume a CG inside limits at takeoff remains inside after fuel burn. Check the most forward and most aft cases.