Fuel Calculation Procedures
Specific Gravity (SG) / Density
Fuel is sold by volume (liters/gallons) but calculated by mass (kg/lbs) for performance.
- Formula: $Mass = Volume \times Density$
- Jet A1 Density: Typically around 0.8 kg/L (varies with temperature).
- Hot fuel is less dense (more volume for same weight).
- Cold fuel is more dense (less volume for same weight).
Fuel Flow Calculation
Integrated Range
For long flights, fuel consumption reduces aircraft weight, which in turn reduces fuel consumption.
- Calculation: Done step-by-step (typically hourly) or using accumulated tables (Flight Planning Tables).
- Nautical Air Miles (NAM): Distance flown through the air mass.
- $NAM = TAS \times Time$
- $Fuel = NAM \times (Fuel Flow / TAS)$
- Nautical Ground Miles (NGM): Distance over ground (corrected for wind).
- $NGM = GS \times Time$
Holding Fuel
Calculated at "Holding Speed" usually at 1,500 ft above destination elevation (for Final Reserve) or at cruise altitude (for Contingency holding).
- Holding speed is usually lower than cruise speed ($V_{MD}$ usually for max endurance) to minimize flow.
Computer Flight Plans (CFP)
Modern systems automate these calculations, providing a log with:
- Zone Fuel: Fuel for each leg.
- Min Fuel: Regulatory minimum.
- Take-off Fuel: Actual fuel on board at T/O.
- Burn-off: Expected consumption.