Alternate Planning When the Destination Forecast Is Marginal
You are planning a 2-hour IFR flight to an airport with one ILS approach to Runway 09 and a circling approach to Runway 27. The TAF for your arrival window reads: wind 090/08, visibility 4 SM, ceiling 1,500 ft broken, temporarily 2 SM and 800 ft. The ILS minimums are 200 ft and 1/2 SM; the circling minimums are 600 ft and 1 SM. Your aircraft is category B. The destination has no ATIS and the approach is not authorized when the tower is closed. The tower closes at 2200 and your ETA is 2145. The nearest alternate is 30 minutes away with an ILS to 200 ft and a TAF of 1,000 ft and 3 SM.
1. Does the destination require an alternate, and if so, is the planned alternate adequate?
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The destination forecast includes a temporary condition of 2 SM and 800 ft, which is above the ILS minimums of 1/2 SM and 200 ft but below the circling minimums of 1 SM and 600 ft for category B. Because the forecast is marginal and includes a temporary deterioration, a conservative reading is that an alternate is required. The standard rule is to require an alternate when the destination forecast is below the approach minimums or when the forecast is marginal; here the temporary condition and the single usable approach justify carrying an alternate.
The planned alternate has an ILS to 200 ft and a TAF of 1,000 ft and 3 SM, which is comfortably above its minimums. Thirty minutes away is reasonable for a 2-hour flight. So the alternate is adequate, provided you have the fuel to reach it and still hold reserve.
2. What is the significance of the tower closing at 2200 relative to your 2145 ETA?
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The approach is not authorized when the tower is closed, so you must be on the ground or committed to the approach before 2200. With a 2145 ETA you have only 15 minutes of buffer. If you are delayed by ATC, weather, or a missed approach, you could arrive after the tower closes and find the approach unavailable, forcing a diversion to the alternate.
Plan to arrive with margin: request a slightly earlier ETA, or plan to hold and arrive before 2200. If you cannot make 2200, plan the diversion to the alternate rather than arriving at a closed airport with no approach. Confirm the tower's closing time and the approach authorization with ATC or the airport facility before departure.
3. How would you plan fuel for this flight?
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Compute trip fuel for 2 hours, then add reserve fuel (typically 45 minutes at cruise), then add alternate fuel for 30 minutes, and finally add any holding or contingency fuel. If the flight is 2 hours at 200 kt and 400 lb/hr, trip fuel is 800 lb. Reserve at 45 minutes is 300 lb. Alternate fuel at 30 minutes is 200 lb. Contingency of 10% of trip is 80 lb. Total is about 1,380 lb.
Because the destination forecast is marginal and the tower closes at 2200, add holding fuel for at least 15 minutes to cover a delay or a missed approach. That adds about 100 lb. Confirm the total against the aircraft's usable fuel and the landing weight limit, and verify the alternate fuel is sufficient for the actual distance and winds.