Fuel planning on a marginal VFR leg
You are flying a single-engine piston aircraft (cruise TAS 120 kt, fuel burn 34 L/h) on a VFR cross-country of 210 NM. Forecast wind gives a 15 kt headwind component for the whole leg. Usable fuel at start is 110 L. Company/operator planning uses 45 min final reserve plus 20 min contingency. The aircraft must land with at least 30 min of fuel remaining at the destination. En-route there is a 12-minute diversion to an alternate if the destination becomes unusable. Expected departure is at the current time; no refuelling is available at the destination.
1. Compute the planned trip fuel, contingency, final reserve and total required fuel. Is the departure legal and safe with 110 L?
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Groundspeed = 120 − 15 = 105 kt. Trip time = 210 / 105 = 2.00 h. Trip fuel = 34 × 2.0 = 68 L.
Contingency (20 min) = 34 × 20/60 = 11.3 L. Final reserve (45 min) = 34 × 0.75 = 25.5 L.
Required = 68 + 11.3 + 25.5 = 104.8 L. With 110 L you have ~5 L margin - technically above the minimum, but only ~9 minutes of fuel. That is not a safe margin for a 2-hour leg with no refuelling at destination. The 30-min landing minimum (17 L) is also close to being breached if any delay occurs. Recommendation: refuel or reduce payload/route before departure.
2. In flight, ATC puts you in a 15-minute hold 40 NM from destination. Recompute the fuel situation and decide the correct course of action.
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Hold fuel = 34 × 0.25 = 8.5 L. Remaining after hold = 110 − 68 − 8.5 = 33.5 L (assuming trip fuel already burned to the hold point is included in the 68 L figure; adjust if the hold occurs earlier).
33.5 L = ~59 min endurance. After landing you would have 33.5 − 17 (30-min minimum) = 16.5 L usable margin - below the 45-min final reserve of 25.5 L. You are now below final reserve. Declare minimum fuel / MAYDAY FUEL as appropriate, request priority, and plan to divert to the 12-min alternate if the hold extends. Do not continue to a destination that may add further delay.
3. What is the single biggest planning error in the original flight plan?
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Planning to arrive with only ~5 L above the legal minimum on a leg with no refuelling at destination and a headwind that could increase. The headwind is the dominant risk: a 5 kt increase in headwind adds ~0.1 h (≈3.4 L) to trip fuel, wiping out the margin. A robust plan would carry at least the contingency plus a personal buffer of 15–20 min, and would identify a fuel stop or alternate with fuel before departure.