Fuel Planning and Alternate Selection Under Changing Weather
You are planning a 3.2-hour IFR flight in a twin turboprop from a southern Canadian airport to a northern destination. Forecast enroute winds are 300° at 40 knots at 18,000 ft, giving a 25-knot headwind component. Destination forecast: 600 ft overcast, 2 sm visibility, temporary 300 ft and 1 sm in snow. Alternate airport is 45 nm away with an ILS to 200 ft and 1/2 sm, forecast 400 ft and 1 sm. Your aircraft burns 600 lb/hr at cruise, has 3,600 lb usable fuel, and company reserves are 45 minutes. The destination has no precision approach; the alternate does.
1. Compute your fuel requirement and determine whether you can legally and safely plan this flight.
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First, compute trip fuel: 3.2 hours at 600 lb/hr = 1,920 lb. Add the 45-minute company reserve: 0.75 hours at 600 lb/hr = 450 lb. Add an approach and missed approach allowance, typically 15 minutes or 150 lb. Add taxi fuel, typically 50 lb. Total planned fuel: 1,920 + 450 + 150 + 50 = 2,570 lb. Usable fuel is 3,600 lb, so you have 1,030 lb of margin - about 1.7 hours of extra fuel. Fuel is not the limiting factor.
The real question is alternate legality. The destination forecast is 600 ft and 2 sm, with temporary 300 ft and 1 sm. The destination has no precision approach, so the alternate minimum applies. Under Canadian rules, when the destination forecast is below the approach minimum, you need an alternate. The alternate forecast is 400 ft and 1 sm, and its ILS minimum is 200 ft and 1/2 sm, so the alternate is above its minimum and acceptable.
However, the temporary 300 ft and 1 sm at the destination is below the destination's non-precision minimum, which means you must carry enough fuel to fly to the alternate and hold. Your fuel margin covers that. The flight is legal and safe on fuel, provided you monitor the destination weather and are prepared to divert.
2. What human factors and decision-making traps should you watch for?
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Several traps are present. First, plan continuation bias: you have invested in the flight plan and the fuel works out, so you may be tempted to press on even if the destination weather deteriorates. Set a clear decision point - for example, if the destination is below 600 ft and 2 sm at the top of descent, divert.
Second, the temporary 300 ft and 1 sm in snow is a warning sign. Snow showers can reduce visibility rapidly and produce icing. If the destination is reporting snow, the actual conditions may be worse than the forecast.
Third, the alternate is only 45 nm away, which is comfortable, but you must verify its weather is still above minimums before you need it. Weather can change in the time it takes to fly the approach and miss.
Fourth, fatigue and night considerations: if this is a night flight, the approach into a 600 ft ceiling with snow is demanding. Brief the approach thoroughly, use the autopilot to reduce workload, and set personal minimums above the published ones.
Finally, fuel: although you have margin, a diversion plus a missed approach plus holding can eat into it. Monitor fuel continuously and declare minimum fuel early rather than late.