Fuel and Alternate Planning with a Changing Forecast
You are planning a 2-hour 45-minute flight in a retractable single. Planned true airspeed 145 kt, forecast wind 270/25 at cruise, route mostly eastbound. Fuel capacity 64 gal usable, cruise burn 11.5 gal/hr, taxi and climb allowance 3 gal. The destination forecast at your ETA is 3,000 ft broken, 5 SM visibility, with a 40% chance of 1,500 ft and 2 SM by one hour after your ETA. The nearest suitable alternate is 35 NM away with an ILS and forecast 2,000 ft and 6 SM. Your personal minimum is 1,000 ft ceiling and 3 SM above the lowest usable minimums.
1. Compute a realistic fuel plan and identify the margin.
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Estimate groundspeed: eastbound with a 270/25 wind gives a tailwind component; for a mostly eastbound route assume ~15–20 kt tailwind, so groundspeed ~160–165 kt. At 160 kt, 2:45 of still-air time becomes roughly 2:30 in the air - but plan conservatively and use the planned time. Fuel: 2.75 hr × 11.5 = ~31.6 gal plus 3 gal allowance = ~34.6 gal. On 64 gal usable that leaves ~29 gal, which looks generous - but the margin that matters is reserve to the alternate plus 45 minutes at cruise. Alternate 35 NM at ~150 kt ≈ 14 min ≈ 2.7 gal; 45 min ≈ 8.6 gal; total reserve ≈ 11.3 gal. Your contingency is comfortable on paper, but the forecast trend (40% chance of 1,500/2 by one hour after ETA) is the real risk: if you arrive and must divert, you need the alternate fuel plus the ability to hold. Verify the alternate weather is above your personal minimums (2,000/6 vs. 1,000/3 minimum - it is, but only just) and carry fuel to hold 20–30 minutes beyond the reserve.
2. Does the destination forecast meet your personal minimums, and what does that imply for the alternate?
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Your minimum is 1,000 ft ceiling and 3 SM above the lowest usable minimums. At the forecast 3,000/5, if the approach minimums are, say, 600/1, then 1,000/3 above that means you need ~1,600/4 - the 3,000/5 forecast clears it. But the 40% chance of 1,500/2 by one hour after ETA would put you below your personal minimums (1,500 is above 1,600? No - 1,500 is below 1,600, and 2 SM is below 4). So the destination is forecast to degrade below your minimums within the window. That makes the alternate mandatory in practice, not just legally, and it must be forecast to stay above your minimums for your ETA plus reserve. The 2,000/6 alternate is acceptable but has little cushion; identify a second alternate or plan to arrive early enough to beat the degradation.
3. What decision points do you set before departure and en route?
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Pre-departure: confirm the alternate forecast holds for ETA + 1 hour; if the destination trend is toward your minimums, plan to arrive at or before the earlier ETA or delay. En route: get an updated destination and alternate forecast about 45–60 minutes out. Set a hard diversion trigger: if the destination is forecast at or below your personal minimums at your arrival time, divert to the alternate without attempting an approach. Also set a fuel trigger: if you would land with less than reserve + 20 minutes, divert early rather than hold. The judgement being tested is committing to the alternate before you are low on fuel and options, not after a missed approach.