Wet landing distance and a tight diversion fuel margin
A twin-jet is planned to land at a destination with LDA 2,350 m, elevation 400 ft, QNH 1010 hPa, OAT 24°C, runway 24. The forecast is wet with braking action medium and wind 240/18 gusting 30 kt. The operator's policy is to multiply the demonstrated dry landing distance by 1.67 and then by 1.15 for a wet runway; reverse thrust is not credited. At the expected landing weight of 61,500 kg, the manufacturer's dry chart gives 1,450 m. LDA is 2,350 m. The alternate is 120 nm away with a dry 3,200 m runway and CAVOK. Fuel remaining is 3,200 kg; fuel to alternate is 1,500 kg, final reserve 900 kg, and holding burns 800 kg/h. A 20-minute hold is available. The crew must decide whether to land at destination, hold, or divert.
1. Compute the required landing distance and determine whether the destination is legal at 61,500 kg.
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Required landing distance = demonstrated dry distance × 1.15 (wet) × 1.67 = 1,450 × 1.15 × 1.67 = 2,785 m. LDA is 2,350 m, so the destination is not legal at 61,500 kg. To fit 2,350 m, the maximum chart distance would be 2,350 / (1.15 × 1.67) = 1,224 m. At roughly 0.024 m per 1,000 kg on this part of the chart, that corresponds to a landing weight of about 52,000 kg, some 9,500 kg below the expected weight. The crew cannot burn that much fuel with the available hold, and reverse thrust is not credited.
2. Would a 20-minute hold make the landing legal? Show the arithmetic and decide.
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A 20-minute hold burns 800 × 20/60 = 267 kg. Landing weight becomes 61,233 kg; the chart distance falls by only about 6 m to about 1,444 m, and the required distance is still about 2,774 m, well above 2,350 m. Holding does not solve the problem. Diverting now to the 3,200 m dry runway uses 1,500 kg, leaves 3,200 − 1,500 = 1,700 kg, of which 900 kg is final reserve, leaving about 800 kg margin. If the crew holds 20 minutes first, fuel remaining is 2,933 kg and the margin after diversion is still about 533 kg, so the diversion is still fuel-feasible, but the hold serves no purpose. The correct decision is to divert now while the fuel margin is greatest.
3. Explain the landing and go-around climb requirements and how a contaminated runway would change the decision.
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For a twin-engine aeroplane the landing climb requirement is 3.3% and the approach climb requirement is 2.1%, with go-around thrust and the appropriate configuration. A wet or contaminated runway increases the chance that a go-around is needed, and the missed-approach obstacle data must be checked. On a contaminated runway, use the manufacturer's contaminated landing-distance data, not the wet factor; reverse thrust is not credited; braking action may be worse than reported; and crosswind limits may be lower. If braking action is medium and deteriorating, or if the go-around climb is obstacle-limited, divert to the dry runway. The decision is not just about stopping distance; it is also about the ability to go around safely.