Marginal Weather and Fuel at Nightfall
You are planning a twin-engine turboprop flight with one destination and one alternate. Trip fuel is 420 kg, taxi fuel 20 kg, and contingency is 5% of trip fuel. The alternate is 35 minutes away at a fuel flow of 240 kg/h. Final reserve is 30 minutes at 240 kg/h. Fuel on board at start-up is 760 kg, and MTOM is not limiting. The destination TAF includes TEMPO 1500 m in snow and BKN 600 ft for your ETA, while the alternate forecast is BKN 900 ft with 8 km visibility and a 20 kt crosswind. The destination has a non-precision approach with minima of 600 ft and 2,000 m. Your ETA is 30 minutes before last light. ATC advises that holding may be required for 20 minutes due to inbound traffic.
1. Calculate the minimum fuel required and compare it with the fuel on board.
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Minimum fuel = taxi + trip + contingency + alternate + final reserve.
Taxi = 20 kg Trip = 420 kg Contingency = 5% × 420 = 21 kg Alternate = 35/60 × 240 = 140 kg Final reserve = 30/60 × 240 = 120 kg
Total = 20 + 420 + 21 + 140 + 120 = 721 kg. With 760 kg on board, surplus = 39 kg. At 240 kg/h that is 39/240 × 60 = 9.75 minutes. The forecast holding of 20 minutes would need 80 kg, so the planned fuel is 41 kg below the minimum if that hold is used.
2. Would you dispatch as planned? Explain the decision and the alternatives.
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I would not dispatch as planned. The destination TEMPO visibility of 1,500 m is below the 2,000 m minima, and cloud at 600 ft is at the minima, so a diversion is plausible, not remote. The alternate is only marginally better and has a 20 kt crosswind. The fuel surplus is under 10 minutes while ATC has already warned of a 20-minute hold; last light is 30 minutes after ETA, adding night approach risk. Legal fuel figures are not enough when the weather is marginal. Options: add fuel if mass and runway allow, choose a destination with a forecast above minima, or carry an alternate with more favourable weather. In flight, make the diversion decision before entering holding, not after fuel becomes critical.