Standing Water, a Downslope and 40 Nautical Miles to Decide
You are 40 nm from destination in a Cessna 172 with two persons on board and a planned landing mass of 1,050 kg. Destination: elevation 900 ft, QNH 1,011, OAT 33°C, runway 27, 1,050 m long, 1% downslope toward 27, light rain with standing water in patches, braking action reported medium to poor on the middle third, wind 250°/08 kt. The POH chart (1,100 kg, 1,000 ft pressure altitude, 30°C) gives 420 m ground roll and 700 m over 50 ft. Your SOP corrections are +10% per 2 kt of tailwind, +10% per 1% downslope, and a multiplier of 1.4 for a wet runway. There is an alternate 40 nm away with an 1,800 m runway, a 5 kt headwind and no rain. Fuel now: 70 L; cruise burn 34 L/h; ground speed to the alternate 110 kt. It is 15:40 local and rain is forecast to continue for three hours. You must decide before entering the destination's traffic pattern.
1. Calculate the landing distance required at the destination and compare it with the available landing distance.
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Tailwind component on runway 27 from 250°/08 kt: 8 × cos 20° ≈ 7.5 kt. Round to 8 kt for conservatism → 8 ÷ 2 × 10% = +40%. Upslope: 1% downslope → +10%. Surface: wet → × 1.4.
Distance over 50 ft = 700 × 1.40 = 980 m; × 1.10 = 1,078 m; × 1.40 = 1,509 m. Ground roll = 420 × 1.40 × 1.10 × 1.40 = 905 m.
LDA is 1,050 m. The required distance exceeds it by 459 m (44%). Even the ground roll alone consumes 905 m, leaving only 145 m of stopping margin to the runway end - and that assumes the aircraft touches down exactly at the chart point. The landing is not acceptable. This is not a case for a firm landing and good brakes; the aircraft would run off the end.
2. Make the go/divert/hold decision, with fuel numbers.
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Divert now.
Fuel: 40 nm at 110 kt = 0.36 h; 34 × 0.36 ≈ 12.5 L to the alternate, leaving about 57 L on landing - over 1.5 hours of endurance, which covers any holding at the alternate or a further diversion.
Holding at the destination is the wrong answer for two reasons. Burn is 34 L/h, so 30 minutes of holding costs 17 L and takes you to about 53 L with no improvement guaranteed; and the rain is forecast for three hours, far longer than the fuel allows. Waiting also puts you in the pattern at a runway where a go-around from a poor approach would leave you with even less margin.
Use of the alternate is exactly what it is for. Note that no alternate was required at the planning stage - that does not mean none exists when the weather changes. Amend the flight plan, advise ATC of the diversion and your fuel state, and brief the approach at the alternate as a fresh approach, not a repeat of the abandoned one.
3. Which correction dominates the result, and what assumptions in the chart are you now relying on?
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The two large factors are the wet-runway multiplier (×1.4) and the tailwind (+40%); together they multiply the book figure by 1.96 before the slope is even applied. Removing the tailwind by landing on runway 09 would cut the requirement to about 700 × 1.10 × 1.40 = 1,078 m - still marginal or over the 1,050 m LDA, which shows how much the wet multiplier alone is doing.
The chart also assumes: maximum braking from the moment of touchdown, a dry hard surface, sea-level pressure altitude, ISA temperature, no reverse thrust, and a mass of 1,100 kg. Your actual mass is 50 kg lighter, which helps by only a few percent and cannot rescue a 44% shortfall. On top of that, the reported braking action is 'medium to poor' - worse than the generic wet factor - and standing water introduces aquaplaning and directional-control risk, with rubber deposits near the touchdown zone reducing friction further. The honest conclusion is that the chart is optimistic for this runway, and the diversion is the conservative, correct decision.