Fuel plan with a strengthening headwind
You plan a 185 NM leg at a TAS of 115 kt with a forecast 15 kt headwind. Fuel burn is 8.5 US gal/h. Allow 0.2 h for taxi, 1.5 gal for climb, 5% of trip fuel for contingency, and a 22 NM alternate with the same wind and fuel flow. Final reserve is 45 min. Usable fuel on board is 30 US gal. The aircraft has no fuel totaliser, so you use clock and dipstick. The alternate has a 1,900 ft runway and fuel available; weather is VMC. About 40 min after departure the headwind strengthens to 30 kt. You must decide whether to continue, divert, or land for fuel, and whether the original plan had a safe margin.
1. Calculate the fuel required by the original plan and the margin over the 30 US gal on board.
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Ground speed with a 15 kt headwind = 115 − 15 = 100 kt.
Trip time = 185 ÷ 100 = 1.85 h. Trip fuel = 1.85 × 8.5 = 15.7 gal.
Contingency = 5% × 15.7 = 0.8 gal.
Alternate time at 100 kt = 22 ÷ 100 = 0.22 h; alternate fuel = 0.22 × 8.5 = 1.9 gal.
Taxi fuel = 0.2 × 8.5 = 1.7 gal. Climb allowance = 1.5 gal. Final reserve = 45 min = 0.75 × 8.5 = 6.4 gal.
Total required = 1.7 + 1.5 + 15.7 + 0.8 + 1.9 + 6.4 = 28.0 gal.
Margin = 30.0 − 28.0 = 2.0 gal, about 14 min at 8.5 gal/h. That is a thin but positive planning margin, and it assumes the forecast wind is correct.
2. Recompute the fuel needed if the 30 kt headwind persists, and estimate the fuel remaining about 40 min after departure.
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With a 30 kt headwind, ground speed = 115 − 30 = 85 kt.
Revised trip time = 185 ÷ 85 = 2.18 h. Revised trip fuel = 2.18 × 8.5 = 18.5 gal.
Contingency = 5% × 18.5 = 0.9 gal. Alternate fuel at 85 kt = (22 ÷ 85) × 8.5 = 0.26 × 8.5 = 2.2 gal.
Total required = taxi 1.7 + climb 1.5 + trip 18.5 + contingency 0.9 + alternate 2.2 + reserve 6.4 = 31.2 gal.
That exceeds the 30 gal on board even before considering fuel already burned. At 40 min, approximately 0.67 h of cruise fuel has been used: 0.67 × 8.5 = 5.7 gal. Including taxi and climb, used fuel ≈ 1.7 + 1.5 + 5.7 = 8.9 gal, leaving about 21.1 gal.
Distance covered ≈ 100 kt × 0.67 h = 67 NM, leaving about 118 NM. At 85 kt that is 1.39 h and 11.8 gal. Continuing would leave 21.1 − 11.8 − 2.2 − 6.4 = 0.7 gal above the alternate and reserve - effectively no margin.
3. What is your decision, and what does it imply about the original fuel plan?
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I would divert to the alternate now and refuel. The fuel remaining after the diversion is about 21.1 − 2.2 = 18.9 gal, which still leaves the 6.4 gal final reserve intact. Continuing is not defensible because the projected landing fuel is within about 0.7 gal of the final reserve, and the headwind could strengthen further or the alternate forecast could change.
The original plan was not unsafe on the forecast wind, but its 2 gal margin was too small for a leg where a 15 kt wind error consumes about 3 gal. A better plan would have carried at least 31–32 gal or identified a nearer en-route fuel stop. The case also shows why the alternate and reserve must be recalculated when the wind changes, not treated as fixed add-ons.