Reading the TAF and the Crosswind Limit
You are planning a VFR arrival at an aerodrome with a single runway, 09/27, 1,800 m long. The current TAF reads: TAF YXXX 121100Z 1212/1312 24012KT 9999 SCT030 TEMPO 1212/1216 24020G35KT 4000 SHRA BKN012 PROB30 1214/1218 25025G45KT 2000 TSRA BKN008CB. The current METAR reads: YXXX 121250Z 24018G28KT 6000 -SHRA SCT015 BKN025 18/15 Q1008. Your aircraft has a demonstrated crosswind limit of 15 kt, a landing distance of 600 m at ISA sea level, and you are at maximum landing weight. The aerodrome elevation is 1,200 ft, temperature 18°C.
1. Decode the wind and weather in the TAF and METAR, and determine the crosswind component on runway 27 at the time of the METAR.
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TAF decode:
- Base: from 1212Z, wind 240° at 12 kt, visibility 10 km or more, scattered cloud at 3,000 ft.
- TEMPO 1212/1216: temporarily wind 240° at 20 kt gusting 35 kt, visibility 4,000 m, showers of rain, broken cloud at 1,200 ft.
- PROB30 1214/1218: 30% probability of wind 250° at 25 kt gusting 45 kt, visibility 2,000 m, thunderstorm with rain, broken cumulonimbus at 800 ft.
METAR decode: wind 240° at 18 kt gusting 28 kt, visibility 6,000 m, light rain showers, scattered at 1,500 ft, broken at 2,500 ft, temperature 18°C, dew point 15°C, QNH 1008 hPa.
Crosswind on runway 27: runway heading 270°, wind 240° - the angle is 30°. Crosswind = 18 × sin 30° = 9 kt steady, and in the gust 28 × sin 30° = 14 kt. Both within the 15 kt limit, but the gust is at the limit.
2. Assess whether the PROB30 thunderstorm group should affect your go/no-go decision, and evaluate the landing distance at 1,200 ft and 18°C.
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PROB30: A 30% probability is not a forecast of no hazard - it means there is a real chance of a thunderstorm with 45 kt gusts, 2,000 m visibility and CB at 800 ft. At 800 ft CB, the cloud base is below the 1,000 ft AGL needed for a VFR circuit, and 2,000 m visibility is marginal. The 45 kt gust on runway 27 gives a crosswind of 45 × sin 30° = 22.5 kt, well above the 15 kt limit. The PROB30 must therefore be treated as a possible no-go, and the TEMPO gusts of 35 kt give 17.5 kt crosswind, also above the limit. Decision: the crosswind limit will likely be exceeded during the arrival window - plan to hold, divert, or delay.
Landing distance: Density altitude at 1,200 ft and 18°C: ISA temperature at 1,200 ft is 15 − 2.4 = 12.6°C, so the deviation is +5.4°C. Density altitude ≈ 1,200 + 120 × 5.4 ≈ 1,850 ft. Landing distance increases roughly 10% per 1,000 ft of density altitude and the runway is wet (showers), adding about 15%. 600 m × 1.185 × 1.15 ≈ 820 m. The 1,800 m runway is adequate, but the margin is reduced. The limiting factor is the crosswind, not the runway length.