Interpreting a Terminal Forecast for an Alternate Decision
A flight departs at 21:00 UTC and arrives at destination A at 23:40 UTC. The TAF for A reads: wind 220/12 kt, visibility 8 km, few cloud 800 ft, scattered 2,500 ft, temporarily 22:00–02:00 visibility 2,500 m in rain and mist, broken 600 ft. The alternate B, 25 minutes away, has a TAF with visibility 10 km, no significant cloud, wind 200/8 kt, but a temporary 20:00–00:00 period of visibility 4 km in mist and broken 1,200 ft. Company minima: destination 5 km and 1,000 ft; alternate 5 km and 800 ft. Instrument approaches at both airfields have a decision height of 200 ft.
1. Decide whether A can be used as destination and whether B is a legal alternate, showing the reasoning.
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At A, the prevailing forecast is 8 km and few 800 ft, which meets the 5 km/1,000 ft destination minima only if you apply the prevailing group. However, the temporary period 22:00–02:00 covers the whole arrival window (23:40), and during it visibility falls to 2,500 m with broken 600 ft. Since the TEMPO overlaps the expected time of arrival, a conservative and standard planning assumption is that the TEMPO conditions may apply at arrival. Under those conditions A is below the 5 km/1,000 ft minima, so A cannot be relied upon as destination without an alternate.
For B: prevailing 10 km and no significant cloud meets the alternate minima, but the TEMPO 20:00–00:00 overlaps the arrival window and gives 4 km in mist and broken 1,200 ft. This is below the 5 km alternate minimum and the cloud base is marginal relative to the 800 ft alternate requirement. B is therefore not a valid alternate for this arrival time unless the forecast is amended or a different alternate is selected.
2. The crew considers departing anyway because the destination TEMPO is 'only temporary'. Evaluate this reasoning and list what you would require before departure.
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'Temporary' in a TAF means a fluctuation expected to last less than an hour in each occurrence, but it can recur throughout the stated period. The period 22:00–02:00 completely brackets the arrival, so the crew cannot assume the temporary conditions will not be present. Planning on the prevailing group alone is optimistic and would leave the flight without a usable alternate.
Before departure I would require: a valid alternate whose forecast, including any TEMPO or BECOMING groups overlapping the arrival window, meets the 5 km/800 ft minimum; sufficient fuel for destination, alternate and holding; confirmation of the approach minima at both airfields and the aircraft's ability to fly them; and a review of the actual weather trend before descent, including the latest METAR and any SIGMET. If no acceptable alternate exists, the flight should be delayed.