Wellington take-off: mean wind versus gust spread
Wellington (NZWN) runway 16, TODA 2,081 m, dry runway with no clearway. A twin turboprop is planned at MTOW 16,400 kg. Performance data for the planned flap setting give a base TODR of 1,380 m at 15°C, sea level, zero wind, dry runway. Corrections: temperature +1.8% per °C above 15°C; pressure altitude +0.8% per 100 ft; wet runway +15%; headwind 3% reduction per 5 kt; tailwind 20% increase per 10 kt; upslope +2% for the 0.5% upslope. QNH 1000 hPa, OAT 18°C, wind 160/15G25, runway slope 0.5% up. The crew plans to use the 15 kt mean wind. Aerodrome pressure altitude may be taken as 390 ft.
1. Calculate the TODR using the mean wind and determine whether the take-off is legal from runway 16.
Read the worked answer
Apply the corrections to the 1,380 m base TODR.
- Temperature: 18°C is 3°C above 15°C → +5.4% = +74.5 m
- Pressure altitude 390 ft → +0.8% per 100 ft = +3.12% = +43.1 m
- Wet runway → +15% = +207.0 m
- Headwind 15 kt → −3% per 5 kt = −9% = −124.2 m
- Upslope 0.5% → +2% = +27.6 m
Total factor = 1 + 0.054 + 0.0312 + 0.15 − 0.09 + 0.02 = 1.1652.
TODR = 1,380 × 1.1652 = 1,608 m.
TODA is 2,081 m, so the TODR is within TODA by about 473 m. On the mean wind, the take-off is legal and has margin.
2. The wind is variable 140° to 190° and gusting 25 kt. Recalculate using the most adverse wind and assess the departure.
Read the worked answer
Runway 16 is 160° magnetic. A wind of 190° at 25 kt has a tailwind component of 25 × cos 30° ≈ 21.7 kt. Use at least 20 kt tailwind:
- Tailwind correction: +20% per 10 kt → +40%
- Remove the headwind credit.
- Factor = 1 + 0.054 + 0.0312 + 0.15 + 0.02 + 0.40 = 1.6552
- TODR = 1,380 × 1.6552 = 2,284 m
This exceeds TODA 2,081 m by about 203 m. Even using only the 15 kt mean wind at 190° gives a 13 kt tailwind component, a +26% correction and a TODR of about 2,187 m, still beyond TODA. The departure cannot be based on the mean wind when the gust spread can produce a tailwind. Options are to wait for the wind to settle, use another runway if the wind favours it, reduce weight, change flap setting if approved performance data exists, or reject the take-off. The most adverse wind in the reported variability must be used.