General Mass Calculations: Fuel Density, Standard Masses, and LMC
Calculating Total Mass
The total mass of the aircraft is the sum of all its components:
- Start with Basic Empty Mass (BEM).
- Add Crew and Pantry to get Dry Operating Mass (DOM).
- Add Traffic Load (Passengers + Bags + Cargo) to get Zero Fuel Mass (ZFM).
- 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.