General Mass Calculations: Fuel Density, Standard Masses, and LMC

031-02-03Pablo Asensio Martínez2026-04-011 min

Calculating Total Mass

The total mass of the aircraft is the sum of all its components:

  1. Start with Basic Empty Mass (BEM).
  2. Add Crew and Pantry to get Dry Operating Mass (DOM).
  3. Add Traffic Load (Passengers + Bags + Cargo) to get Zero Fuel Mass (ZFM).
  4. Add Take-off Fuel to get Take-Off Mass (TOM).

Fuel Calculations

Fuel is often measured in volume (Litres or US Gallons) but must be converted to mass (kg or lbs) for weight and balance calculations.

  • Formula: $\text{Mass} = \text{Volume} \times \text{Density (SG)}$
  • Specific Gravity (SG): The ratio of the density of the fuel to the density of water.
    • Jet A-1 SG $\approx 0.8$ (ranges 0.78 - 0.84).
    • Avgas SG $\approx 0.72$.

Example: 1000 Litres of Jet A-1 at SG 0.8 $ \text{Mass} = 1000 \times 0.8 = 800 \text{ kg} $

Passenger Mass

For large aircraft, standard masses are often used for passengers and baggage to simplify calculations, rather than weighing each individual.

  • Standard Mass: A statistical average value set by the authority (e.g., EASA) which includes hand baggage.
  • Actual Mass: Used when standard masses are not applicable (e.g., small aircraft with fewer than 10 seats, or non-standard flights).

Adjustments

Always account for:

  • Last Minute Changes (LMC): Additions or removals of passengers/cargo after the load sheet is finalized.
  • Total mass must be recalculated to ensure limits are not exceeded.