Mass and Balance Fundamentals: Definitions and Units
Mass, weight and units
Mass is the quantity of matter, measured in kilograms; weight is the force that mass produces in a gravitational field, measured in newtons. W = m × g, with g = 9.81 m/s². A 1 000 kg mass therefore weighs 9 810 N, or 9.81 kN. EASA mass and balance work is done in kg and kg·m; some manufacturers' charts use pounds (1 kg = 2.2046 lb) and inch-pounds, so convert before mixing.
The mass definitions the examiner expects
- Basic empty mass (BEM): airframe, engines, systems, fixed equipment, unusable fuel, and full oil and other operating fluids.
- Dry operating mass (DOM): BEM plus crew, crew baggage, catering, removable service equipment and potable water. It contains no usable fuel and no traffic load.
- Zero-fuel mass (ZFM): DOM plus traffic load.
- Take-off mass (TOM): DOM plus traffic load plus take-off fuel.
- Landing mass: the mass at touchdown, i.e. TOM minus trip fuel, still including any alternate, final reserve and contingency fuel on board.
- Traffic load (payload): passengers, baggage, cargo and mail. Useful load: traffic load plus usable fuel.
Worked example
DOM 41 500 kg, traffic load 13 200 kg, take-off fuel 9 800 kg, trip fuel 7 100 kg. ZFM = 41 500 + 13 200 = 54 700 kg. TOM = 54 700 + 9 800 = 64 500 kg. Landing mass = 64 500 − 7 100 = 57 400 kg. Useful load = 13 200 + 9 800 = 23 000 kg. If crew and operational items total 900 kg, the BEM is 40 600 kg.
Traps that cost marks
Quoting "empty mass" when the question means DOM; leaving oil out of the BEM; subtracting all fuel to get landing mass and forgetting reserve fuel that is still on board; treating payload as including fuel. Remember also that a change of 1 kg in mass changes weight by 9.81 N, not by 1 N.