Microburst and Gust Front on Final
You are on a VFR cross-country, 25 nm west of destination, at 4,500 ft, with fuel for 1 h 10 min plus 45 min reserve. Destination runway 27 is 1,800 m, dry. ATIS: wind 250/25G45 kt, visibility 4 km in heavy rain, few cumulonimbus, temperature 24°C, dew point 21°C, QNH 1002 hPa. A preceding light aircraft reports moderate to severe turbulence and 600 ft/min sink at 500 ft AGL on short final to runway 27. A cumulonimbus cell with a dark, rain-filled base lies 5 nm west of the field and is moving east at 20 kt. Your aircraft has a demonstrated maximum crosswind of 17 kt and a best glide of 65 kt. The alternate is 35 nm north, weather clear, with fuel to reach it plus 30 min. You are cleared for a visual approach but are not yet established.
1. Calculate the steady and gust crosswind components on runway 27 and compare them with the aircraft limit. What does this tell you about the landing?
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Wind 250° versus runway 270° gives a 40° angle. Crosswind component = wind speed × sin 40°; sin 40° ≈ 0.64. Steady: 25 × 0.64 ≈ 16 kt, just inside the 17 kt demonstrated maximum. Gust: 45 × 0.64 ≈ 29 kt, well outside it. The headwind components are 25 × cos 40° ≈ 19 kt and 45 × cos 40° ≈ 34 kt. The steady wind is at the edge of the aircraft limit, but the gust is not; the reported gust crosswind alone exceeds the demonstrated maximum by about 12 kt. With heavy rain, turbulence, and a gusty shear layer, the approach would not be within the aircraft's demonstrated crosswind capability. A landing might be possible in a lull, but it is not a reliable plan; the safe assumption is that the crosswind limit can be exceeded at any point on final.
2. Interpret the preceding aircraft's report of 600 ft/min sink at 500 ft AGL in terms of microburst and gust-front hazard. Why is a go-around not a guaranteed escape?
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A 600 ft/min sink at 500 ft AGL is a warning sign. At 65 kt, 500 ft at 600 ft/min takes about 50 seconds, covering about 0.9 nm. A microburst downdraft can exceed that rate and is accompanied by a strong horizontal shear: headwind to tailwind, or tailwind increase, can cause a sudden loss of airspeed and lift at low altitude. The reported sink is already at the limit of a normal approach; if it increases, the aircraft may not be able to climb out because the downdraft and shear can exceed climb performance. A go-around is not guaranteed to escape because the aircraft may still be inside the downdraft and may encounter the worst shear during the transition. The CB is 5 nm west, moving east at 20 kt, so the field will be affected within about 15 minutes. The hazard is not just the cell overhead; the gust front can arrive first.
3. Decide whether to continue, hold, or divert. State the fuel and alternate reasoning.
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Divert to the alternate 35 nm north, which is clear. Fuel is 1 h 10 min plus 45 min reserve; the alternate is within range and leaves a reserve. Holding near the field is poor because the storm is moving toward the field and the wind is already gusting to 45 kt; conditions will get worse before they get better. Continuing the approach would place the aircraft at low altitude in a known microburst and shear environment with a crosswind gust beyond the demonstrated limit. If the destination were the only option, the correct technique would be to stay well above 1,000 ft AGL until the field is assured, fly a stabilized approach, and go around early at the first sign of sink or airspeed loss; but the better decision is not to enter that environment. Divert now, and do not attempt a low-level diversion through the gust front.