Reading the TAF Against the Observation
A pilot is planning a flight from an aerodrome with a TAF valid 0600–1800. The TAF reads: wind 24015KT, visibility 9999, few 3,000 ft, becoming 25020G30KT 4000 SHRA BKN015 from 1200 to 1400, temporarily 2000 +SHRA BKN008 from 1300 to 1600. The 1200 observation reports wind 25018G28KT, visibility 8 km, scattered 2,500 ft, broken 4,000 ft, temperature 12 °C, dewpoint 9 °C, QNH 1008. The pilot plans to arrive at 1330 and has an alternate 40 nm away with a TAF of CAVOK.
1. Interpret the TAF and the observation, and state whether the arrival at 1330 is likely to be affected.
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The TAF indicates a deterioration from 1200–1400: wind increasing to 25020G30KT, visibility falling to 4,000 m in showers of rain, and broken cloud at 1,500 ft. The temporary condition from 1300–1600 is worse: visibility 2,000 m in heavy showers and broken cloud at 800 ft. The 1200 observation shows the deterioration has begun but is not yet at the forecast level: wind 25018G28KT, visibility 8 km, and cloud bases at 2,500 and 4,000 ft. The trend is consistent with the TAF, so by 1330 the pilot should expect visibility near 4,000 m, broken cloud at 1,500 ft and possibly the temporary heavy showers with 800 ft cloud. The arrival is likely to be affected, and the pilot should plan for an instrument approach or a diversion.
2. What are the operational implications for the approach and landing?
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The wind of 25020G30KT gives a crosswind component that depends on runway orientation; if the runway is 250°, the crosswind is small, but if it is 160°, the crosswind could exceed the aircraft's limit. The pilot should compute the crosswind for the expected runway and check the aircraft's limit. The cloud base of 1,500 ft and visibility of 4,000 m are within many instrument approach minima, but the temporary 800 ft and 2,000 m may be below them. The pilot should brief the approach, including the missed approach, and should be prepared to divert to the alternate. The alternate's CAVOK forecast provides a reliable escape, but the pilot must confirm the alternate is within fuel range and that the forecast is current.
3. How should the pilot use the observation to update the plan before departure?
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The observation is the best evidence of the actual state of the atmosphere and should be compared with the TAF's initial conditions. The wind and visibility are close to the TAF's becoming group, which suggests the forecast is on track. The pilot should obtain the latest observation and any amended TAF before departure, and should set a decision point at the top of descent: if the destination report shows the temporary condition (visibility 2,000 m, cloud 800 ft) or worse, divert to the alternate without attempting an approach. The pilot should also carry enough fuel to hold for 20–30 minutes and to divert 40 nm, and should brief the passenger on the possibility of a diversion. The key judgement is to treat the TAF as a forecast and the observation as fact, and to make the diversion decision early rather than at the missed approach point.