Wet-runway take-off with a tailwind and obstacle
A twin-engine turbofan with MTOW 78,000 kg is departing runway 27 from an airport at 1,100 ft elevation. OAT is 31°C, QNH 1002 hPa, and the ATIS gives wind 260/12 kt; the tower reports 255/08 kt. TORA is 2,650 m, TODA 2,750 m, ASDA 2,650 m, with a 100 m clearway and no stopway. The runway is wet and braking action is reported medium. Planned TOW is 71,500 kg with flaps 5 and assumed temperature 48°C. The operator's wet-runway policy requires wet V1 and gives no credit for reverse thrust in the accelerate-stop distance. The first officer used the dry-runway chart with a 5 kt tailwind. An obstacle at 1,950 ft is 9.5 km beyond the end of TODA on the extended centreline. The crew must decide whether to depart as loaded or change configuration, thrust or weight.
1. Identify the calculation errors and explain their effect on V1 and the accelerate-stop and take-off distances.
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Three errors stand out.
- Dry chart on a wet runway. The wet take-off distance is measured to 15 ft, not 35 ft, and braking is degraded. More importantly, V1 must be reduced so that the accelerate-stop distance still fits the ASDA without reverse-thrust credit. A dry V1 can be higher than the wet V1 and may make a stop impossible.
- Tailwind understated. The tower gives 8 kt and ATIS 12 kt; using 5 kt understates the tailwind by 3–7 kt. A rule of thumb is that 10 kt of tailwind increases take-off distance by about 20%, so the error is roughly 6–14% before other corrections.
- Density altitude and assumed temperature. Pressure altitude is about 1,100 + 11 × 30 = 1,430 ft, and ISA temperature there is about 12.8°C, so OAT 31°C is about ISA+18. Reduced thrust at assumed 48°C must be checked against the minimum-control and climb limits, not just field length.
The take-off must be recalculated with wet data, the actual tailwind, and full or properly limited thrust.
2. Estimate whether the 1,950 ft obstacle is likely to be limiting and state what you would change before departure.
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Required average net gradient to the obstacle = (1,950 − 35) ft / 9,500 m = 1,915 / 31,168 ft ≈ 6.1%. That is far steeper than a normal second-segment gradient for a twin at high density altitude and reduced thrust, which is likely to be in the 2–3% range and further reduced by the tailwind. The obstacle is therefore very likely limiting.
Actions: reduce TOW until the obstacle-limited weight is met; use the AFM obstacle data for a different flap setting (often a higher take-off flap improves the second-segment gradient but costs field length); use full thrust; wait for a headwind or use a more favourable runway; or offload. Do not depart on the present figures. A 100 m clearway does not help obstacle clearance; it only affects TODA.
3. Explain why the dry V1 must not be used on the wet runway, even if that V1 is below VR.
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V1 is the decision speed: at V1 the aircraft must be able to continue to 35 ft (or 15 ft wet) or stop within the ASDA. On a wet runway the stopping performance is worse and reverse thrust is not credited, so the accelerate-stop distance for a given V1 is longer. To keep the stop within ASDA, V1 must be reduced. Using the dry V1 would mean that, after an engine failure at that speed, the remaining runway might be insufficient to stop; the crew would also be committing to a higher speed before the aircraft is safely airborne. V1 may not exceed VR, and it must satisfy VMCG; reducing it too far can create a control problem. The correct wet V1 is the lowest of the ASDA, VMCG and VR limits.