TAF TEMPO, Crosswind Limit and a VFR Cross-Country
You plan a 120 nm VFR cross-country departing at 10:00 local. Destination TAF: 0900/1800 25012G22 9999 SCT025 BKN060 TEMPO 1000/1400 25025G40 4000 TSRA BKN012CB. Destination runway 22 is 220° magnetic. Aircraft crosswind limit is 15 kt. You have 1 hour reserve fuel. No instrument rating; aircraft is not approved for flight in icing. The route has terrain up to 3,500 ft; you plan 6,500 ft. The TAF is issued at 08:00. Another pilot says TEMPO means the bad weather will not happen. The destination has one runway; the alternate is 35 nm away with no TS forecast but a crosswind of 20 kt.
1. Interpret the TAF, especially the TEMPO group, and state the main hazards for a VFR flight.
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Base forecast: wind 250° at 12 kt gusting 22, visibility 10 km or more, scattered cloud at 2,500 ft, broken at 6,000 ft. TEMPO 1000/1400 means temporary conditions between 10:00 and 14:00: wind 250° 25 kt gusting 40, visibility 4,000 m, thunderstorms with rain, broken CB at 1,200 ft. TEMPO is not “won’t happen”; it is a temporary fluctuation with significant probability. A CB at 1,200 ft and TSRA would make VFR flight unsafe and potentially illegal under VFR cloud and visibility minima.
2. Calculate the crosswind component at the base gust and at the TEMPO gust for runway 22.
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Runway 22 is 220° magnetic. The angle between wind 250° and runway 220° is 30°. Crosswind = wind speed × sin 30° = 0.5 × wind speed. At the base gust 22 kt: 11 kt, within the 15 kt limit. In the TEMPO gust 40 kt: 20 kt, which exceeds the aircraft limit by 5 kt. At the mean TEMPO wind 25 kt: 12.5 kt, within limit, but gusts are what matter. The crosswind limit is a maximum, not a target.
3. What go/no-go decision follows, and why is the alternate not automatically a solution?
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Plan not to arrive during the TEMPO window. Because TEMPO runs 10:00-14:00 and your departure is 10:00 for a 120 nm flight, you would likely arrive inside it. Either delay until after 14:00 and re-check the TAF, or cancel. If you go, you need a safe alternate outside the TS area, but the alternate has a 20 kt crosswind, so it may not be usable. Do not rely on “TEMPO means it won’t happen”. The safe decision is delay or cancel unless the forecast improves and you can arrive with clear weather and crosswind within limits.