Fuel Planning with a Changing Forecast
You are planning a VFR flight of 2 hours 30 minutes in a single-engine piston aircraft. The forecast wind at your cruising altitude is 250° at 25 knots, and your intended track is 090° true. Your true airspeed is 120 knots. The aircraft burns 32 litres per hour. You plan to carry 160 litres, of which 10 litres are unusable. Your departure aerodrome has no suitable alternate within 30 minutes, and the destination forecast is marginal VFR with a 2000 ft ceiling and 8 km visibility. During your pre-flight briefing, you note that the wind forecast has changed to 280° at 35 knots, and the destination forecast now includes a temporary 1500 ft ceiling and 5 km visibility.
1. Calculate the new fuel requirement and determine whether the planned fuel is sufficient.
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First, calculate the headwind or tailwind component. The track is 090° and the wind is 280° at 35 knots. The angle between track and wind is 170°, which is nearly a direct headwind. The headwind component is 35 times the cosine of 10°, approximately 34.5 knots. The ground speed is 120 minus 34.5, approximately 85.5 knots. The still-air distance for 2 hours 30 minutes at 120 knots is 300 nautical miles. At 85.5 knots, the flight time becomes 300 divided by 85.5, approximately 3.5 hours. Fuel burn is 32 litres per hour, so the trip fuel is 3.5 times 32, approximately 112 litres. Add 10 litres unusable, giving 122 litres. Add a 30-minute reserve at 32 litres per hour, which is 16 litres, and a 10% contingency of 11.2 litres, giving about 149 litres. The planned 160 litres is sufficient, but only just, with about 11 litres margin. However, the deteriorating destination forecast means you should consider carrying more fuel or planning a diversion.
2. What is the effect of the changed wind on your fuel planning, and what action should you take?
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The changed wind from 250° at 25 knots to 280° at 35 knots increases the headwind component from about 22 knots to about 34.5 knots. This reduces ground speed from about 98 knots to about 85.5 knots, increasing flight time from about 3.1 hours to about 3.5 hours. The fuel required increases by about 13 litres. The planned 160 litres is still sufficient, but the margin is reduced. Given the marginal VFR forecast and the temporary deterioration, you should either delay the flight, carry additional fuel, or plan a diversion to an aerodrome with better weather. The decision should be based on the updated fuel calculation and the weather trend. If you cannot carry more fuel, you should not depart.