Fuel, Endurance and Alternate Planning in Marginal Wind
You are planning a 120 NM leg in a helicopter cruising at 105 kt TAS. Forecast wind is 25 kt from 240°; your track is 060° (true). The aircraft burns 45 L/h. Usable fuel is 145 L; you plan to land with a 30-minute reserve. Taxi and run-up use 4 L. The destination has one runway and no fuel; the alternate is 25 NM away on a track of 150° (true) from the destination, and the same wind applies. A front is forecast to move through, and the wind is expected to veer to 300° and increase to 35 kt by your arrival time.
1. Compute the fuel required for the planned flight including reserve and determine the fuel available for the leg.
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Wind: 240° at 25 kt, track 060°. The wind is from the left rear quarter - the angle between track and wind direction is 180° (240 − 60 = 180? No: track 060, wind from 240 means wind is blowing toward 060, so it is a direct tailwind). Careful: wind direction is where it comes FROM. 240° from means it blows toward 060°, which is your track - a pure tailwind of 25 kt. Groundspeed = 105 + 25 = 130 kt. Time = 120 / 130 = 0.923 h = 55.4 min. Trip fuel = 0.923 × 45 = 41.5 L. Reserve = 30 min × 45 L/h = 22.5 L. Taxi/run-up = 4 L. Total required = 41.5 + 22.5 + 4 = 68 L. Available usable = 145 L. Fuel available for the leg beyond reserve and taxi = 145 − 22.5 − 4 = 118.5 L, which is ample for the 41.5 L trip. There is no fuel problem on the numbers - the risk lies in the wind shift.
2. Recompute the leg with the forecast veered wind and assess the effect on the reserve.
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New wind 300° at 35 kt, track 060°. Angle between track and wind direction = 300 − 60 = 240°, i.e. the wind is from 60° left of track (a headwind component). Headwind component = 35 × cos(60°) = 35 × 0.5 = 17.5 kt. Groundspeed = 105 − 17.5 = 87.5 kt. Time = 120 / 87.5 = 1.371 h = 82.3 min. Trip fuel = 1.371 × 45 = 61.7 L. Required total = 61.7 + 22.5 + 4 = 88.2 L - still within 145 L, so the flight remains feasible. But the arrival reserve is intact only if the wind shift occurs as forecast; a stronger veer or a slower cruise (e.g. 95 kt TAS) would erode it. The alternate leg must also be checked: track 150°, wind 300° at 35 kt gives an angle of 150° - a headwind component of 35 × cos(30°) = 30.3 kt, groundspeed 74.7 kt, time 25/74.7 = 0.335 h = 20 min, fuel 15 L. That must come from the 22.5 L reserve, leaving only ~7.5 L - about 10 minutes - at the alternate. That is a genuine problem.
3. Given the alternate fuel calculation, what decision do you make and how do you brief it?
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The planned 30-minute reserve is not sufficient to reach the alternate and hold. Options: (1) carry more fuel - but usable capacity is fixed at 145 L, so the only lever is reducing payload or landing short to refuel en route; (2) nominate a closer alternate with a more favourable wind; (3) depart earlier to arrive before the veer, and re-clear the fuel plan in flight with an actual wind update. Decision: depart early, file the 25 NM alternate but brief that if the wind has veered by the halfway point, you will divert to a closer aerodrome with fuel. In-flight, take a wind estimate at each waypoint and recompute groundspeed and fuel remaining against the alternate requirement; set a hard decision point at which, if fuel remaining minus alternate fuel minus reserve is negative, you divert immediately. Brief the passenger on the possibility of a fuel stop. Land with no less than the reserve - the reserve is a planning figure, not a target to spend.