Radiation fog before a 0645 arrival
You are the duty pilot for a night freight run into a valley aerodrome (elevation 300 ft) with no ILS; the only instrument approach is RNAV with an MDA of 520 ft and a visibility minimum of 1,600 m. At 2000 local the aerodrome reports temperature 7°C, dewpoint 6°C, wind 040/03 kt, sky clear, QNH 1026 hPa. The forecast is for clear skies, wind becoming calm, and a minimum temperature of 1°C at about 0500; sunrise is 0630. The alternate is 40 nm away, has an ILS, and requires 30 minutes' flying plus a 20-minute hold allowance. Your ETA at the destination is 0645 after a 7-hour flight, and you must plan fuel for the approach, missed approach, and alternate. The destination has no runway lighting beyond low-intensity edge lights and no ATIS after 2200.
1. Assess the likelihood and timing of radiation fog at the destination.
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The ingredients are present: a clear sky, a long autumn night, a moist boundary layer with only a 1°C dewpoint spread at 2000, light wind, and a valley setting that favours cold-air drainage. With clear skies the surface will cool roughly 1°C per hour, so saturation is possible within an hour or two; in practice fog often appears after further cooling and slight mixing, so onset is likely between 2200 and midnight, becoming dense by 0300–0500. The forecast minimum of 1°C is well below the initial dewpoint of 6°C, so the air will be saturated for several hours. Expect visibility of a few hundred metres and a shallow fog layer, perhaps 50–150 m deep, with the valley floor most affected. The light wind is enough to mix a shallow layer but not enough to disperse it.
2. Will the 0645 arrival be able to complete the RNAV approach? Use the minima and the forecast timing.
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The RNAV MDA is 520 ft at an aerodrome elevation of 300 ft, so the aircraft must be visual by about 220 ft above the aerodrome. Radiation fog of the expected depth will almost certainly extend through that level, and visibility will be well below the 1,600 m minimum. The ETA is 0645, before sunrise at 0630 has had time to heat and mix the fog; any improvement after sunrise usually begins with fog lifting into low stratus, not with immediate 1,600 m visibility. A missed approach is therefore the most likely outcome, and the pilot should not plan on the destination being available at 0645. The low-intensity edge lights and lack of ATIS after 2200 make a late change even less likely.
3. What fuel and alternate planning would you require, and would you change the departure time?
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Because the destination is forecast to be below minima at ETA, I would treat the alternate as mandatory, not merely a legal box. Fuel must cover the destination approach, missed approach, climb, cruise to the 40 nm alternate, an approach there, plus the 20-minute hold allowance and final reserve. I would also add contingency fuel for a possible hold at the destination if early fog is slower to form. A better operational choice is to delay departure so arrival is after about 0900, when solar heating should have lifted the fog; if that is not possible, plan to divert without attempting the RNAV approach. The decision should be made before top of descent, using the latest METAR/TAF and not just the 2000 observation.