Descent Planning into a Marginal Alternate
You are planning a 2-hour IFR flight at FL280 to an airport with one ILS runway. The destination forecast at arrival is 600 ft ceiling and 1,600 m visibility, with a temporary condition of 300 ft and 800 m. The only suitable alternate is 45 minutes away, with a forecast of 900 ft ceiling and 3 km visibility, but its ILS is unserviceable and only a non-precision approach is available. Your aircraft has fuel for 3 hours total, including 45 minutes reserve. The destination's ILS minima are 200 ft and 550 m; the non-precision minima at the alternate are 500 ft and 1,500 m. There is no GNSS approach at the alternate.
1. Can the destination be used without an alternate, and what does the forecast imply?
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No. The destination forecast includes a temporary condition of 300 ft and 800 m, which is below the ILS minima of 200 ft and 550 m only in visibility terms if the temporary condition is treated as prevailing for planning. More importantly, the forecast ceiling of 600 ft is above the ILS decision altitude, but the temporary deterioration to 300 ft and 800 m means the approach may not be usable at arrival. The safe planning assumption is that an alternate is required. The forecast must be interpreted conservatively: a temporary condition is still part of the forecast and can affect the approach. The crew should not rely on the destination alone when the temporary condition falls below minima.
2. Is the alternate legally and practically suitable, and does fuel allow the diversion?
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The alternate forecast of 900 ft and 3 km is above the non-precision minima of 500 ft and 1,500 m, so it is legally suitable if the forecast is valid at the expected arrival time. Practically, however, the alternate has no ILS and no GNSS approach, so the crew must fly a non-precision approach to 500 ft in 900 ft ceiling and 3 km visibility. That is feasible but leaves little margin if the weather deteriorates. Fuel is the limiting factor: 3 hours total with 45 minutes reserve leaves 2 hours 15 minutes of usable flight time. The flight is 2 hours, leaving only 15 minutes of contingency before reserve, while the alternate is 45 minutes away. The crew cannot divert to the alternate and still land with reserve fuel if the destination approach fails. The plan is not acceptable without additional fuel or a closer, better-equipped alternate.