Runway 04, Wind Shift
You are a cadet in the control tower during a training approach. A trainer is on final for runway 04: 1,650 m long, dry asphalt, 1% upslope. Its dry landing distance at sea level ISA is 1,100 m. The performance manual gives corrections of +8% for 1,000 ft pressure altitude, +5% for each 10°C above ISA, +15% on a wet runway, +20% for each 10 kt of tailwind, and −10% for 1% upslope. The airfield is at 1,000 ft and ISA+20°C; the runway is wet. The tower had reported wind 040/08, but the aircraft now reports 240/12 gusting 20. Runway 04 heading is 040°. The aircraft has 25 minutes of fuel; the alternate is 20 minutes away. The pilot asks, 'Can we continue?' You must advise.
1. Calculate the effective landing distance using the worst credible wind. State your assumptions.
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Tailwind component: runway 04 is 040°; wind 240° is 200° different, so it is almost directly behind. Use the gust, 20 kt, for a conservative calculation; the steady 12 kt already gives a tailwind of about 12 kt. Corrections: altitude +8% (1.08); temperature ISA+20°C = +10% (1.10); wet +15% (1.15); tailwind 20 kt = +40% (1.40); upslope 1% = −10% (0.90). Effective distance = 1,100 × 1.08 × 1.10 × 1.15 × 1.40 × 0.90. 1,100 × 1.08 = 1,188; × 1.10 = 1,307; × 1.15 = 1,503; × 1.40 = 2,104; × 0.90 = 1,894 m. With the steady 12 kt tailwind the result is about 1,677 m. Both figures exceed the 1,650 m runway. The upslope helps, but not enough to restore a safe margin. The calculation assumes the manual corrections are independent and that no reverse thrust or extra braking is available; even so, 1,677 m against 1,650 m leaves no margin for a long flare or poor braking.
2. Recommend a course of action and the radio calls.
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Recommend a go-around and diversion. The aircraft is already on final; a go-around is a normal manoeuvre, not a failure. Climb to a safe altitude, clean up, and tell the tower: 'Going around, wind check, request diversion to alternate.' Because fuel is 25 minutes and the alternate is 20 minutes away, there is no time for a second approach or holding. I would ask for priority and declare an urgency if the controller cannot give immediate routing. Do not accept a 'try it and see' approach: a wet runway with a tailwind and no stopping margin risks an overrun. The tower should also update the wind and warn following traffic. The decision protects the aircraft, crew, and runway; a missed sortie is cheaper than an excursion.
3. What would change if the wind were still 040/08 as first reported?
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With wind 040/08, the tailwind is zero and the crosswind is 8 kt. The distance becomes 1,100 × 1.08 × 1.10 × 1.15 × 0.90 = 1,353 m. That leaves about 297 m, or 18%, against the 1,650 m runway. I would continue, but brief the crosswind, keep the wet-runway technique, and be ready to go around if the touchdown is long or braking is poor. The reasoning shows why the wind report mattered: the same aircraft, temperature, and runway change from a comfortable margin to an unacceptable one when the wind swings from 040° to 240°. A good decision is sensitive to the latest information, not anchored to the first report.