Landing Distance on a Contaminated Runway with a Tailwind
A twin-turboprop is landing at an airport with a runway 09/27, 1,600 m long, dry asphalt, elevation 2,000 ft, QNH 1010 hPa, OAT 25 degrees C. The crew plans to land on runway 27 with a reported wind of 090/10 kt, giving a tailwind component. The aircraft mass is 16,500 kg, and the AFM gives a landing distance of 1,050 m at sea level, ISA, dry runway, no wind. The airport pressure altitude is about 2,100 ft, and the temperature is 25 degrees C, which is about 10 degrees C above ISA. The AFM provides a 1.15 factor for a wet runway and a 1.4 factor for a contaminated runway. The runway is dry but has a 200 m displaced threshold on runway 27. The crew has 1,100 kg of fuel remaining and a required reserve of 400 kg.
1. Calculate the landing distance required on runway 27 with the tailwind and the pressure altitude/temperature correction.
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Start with the sea-level ISA dry landing distance of 1,050 m. The pressure altitude of 2,100 ft increases the true airspeed at touchdown and the ground roll, typically by about 3 percent per 1,000 ft, so 1,050 x 1.063 = 1,116 m. The temperature 10 degrees C above ISA adds about 2 percent per 10 degrees C, so 1,116 x 1.02 = 1,138 m. The tailwind component is 10 kt, which increases the ground speed and the landing distance by about 10 percent per 5 kt, so 1,138 x 1.20 = 1,366 m. The runway available is 1,600 m minus the 200 m displaced threshold, giving 1,400 m. The landing distance required of 1,366 m is within the 1,400 m available, but the margin is only 34 m, which is less than the 15 percent safety margin often required. The crew should consider runway 09 with a headwind instead.
2. Assess the effect of using runway 09 with the reported wind and compare the landing distance.
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Runway 09 has a magnetic heading of 090 degrees, and the wind is from 090 degrees at 10 kt, so the headwind component is 10 kt and the crosswind is zero. A 10 kt headwind reduces the landing distance by about 20 percent, so the corrected distance of 1,138 m falls to 1,138 x 0.80 = 910 m. The runway available on runway 09 is the full 1,600 m, because the displaced threshold is on runway 27 and does not affect runway 09. The landing distance required of 910 m is well within the 1,600 m available, with a margin of 690 m. Runway 09 is therefore the better choice, despite the tailwind on runway 27 being within limits. The crew should also check the crosswind limit, which is not a factor here, and the missed approach procedure for runway 09.
3. Explain how the fuel remaining affects the landing decision and the go-around option.
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The aircraft has 1,100 kg of fuel remaining and a required reserve of 400 kg, so the usable fuel above reserve is 700 kg. If the crew must go around, the aircraft will burn fuel during the climb and the circuit, typically 150 to 250 kg for a twin-turboprop. A single go-around would leave about 850 to 950 kg, still above the 400 kg reserve. However, if the go-around is followed by a diversion to an alternate 20 minutes away, the fuel burn could be 400 to 500 kg, leaving 350 to 450 kg, which may infringe the reserve. The crew should therefore plan the approach to minimise the chance of a go-around, use runway 09 with the headwind, and be prepared to declare a fuel emergency if the reserve is likely to be infringed. The landing decision should not be based on fuel alone, but fuel is a factor in the choice of runway and the go-around plan.