Flight planning and fuel
The fuel plan
A flight plan builds fuel from several blocks: taxi fuel, trip fuel, contingency fuel (often 5% of trip fuel), alternate fuel, final reserve (typically 30 minutes holding at 1,500 ft), and extra fuel. The sum of taxi, trip, contingency and alternate fuel plus final reserve gives the minimum required fuel; extra fuel is added at the commander's discretion. A trap is the order of the blocks: contingency is based on trip fuel, not total fuel, and the alternate fuel is calculated from the destination to the alternate at the planned mass.
Reading the plan
Flight planning combines the route distance, wind component, temperature and altitude to give true airspeed, ground speed, time and fuel. The wind triangle is central: track, heading, true airspeed and wind give ground speed and drift. Fuel flow times time gives trip fuel. A trap is the difference between still-air distance and ground distance, and the effect of a headwind on one leg and a tailwind on the return. Another trap is units: litres versus kilograms, and the fuel density used for conversion.
Worked example and rule of thumb
A route of 900 nm is flown at a ground speed of 450 kt, giving 2.0 hours. At a fuel flow of 2,200 kg/h, trip fuel is 4,400 kg. Contingency at 5% is 220 kg. Alternate fuel for a 150 nm leg at 400 kt and the same flow is 0.375 h × 2,200 = 825 kg. Final reserve for 30 minutes holding at 1,500 kg/h is 750 kg. Minimum fuel is 4,400 + 220 + 825 + 750 = 6,195 kg, before taxi fuel. Rule of thumb: contingency is 5% of trip fuel, and final reserve is 30 minutes of holding.