Wet Grass Runway at High Density Altitude
A light single-engine aircraft is scheduled to depart from a 1,400 m grass runway at elevation 1,200 ft, QNH 1003 hPa, temperature 28°C. The runway is 1.5% uphill in the takeoff direction and is wet after rain. Wind is reported as 150°/12 kt; the aircraft will use runway 15. The AFM gives a sea-level ISA dry-runway takeoff distance of 850 m at the planned mass of 1,150 kg. The operator uses the following simplified corrections: pressure altitude +10% per 1,000 ft; temperature +10% per 10°C above ISA; headwind −10% per 5 kt (tailwind +20% per 5 kt); uphill slope +5% per 1%; wet grass +30% (combining grass and wet surface); no clearway or stopway. The pilot plans to use the same mass and configuration. Company procedure requires a 15% runway margin beyond the corrected takeoff distance. Determine whether the departure can be made and, if not, what single change would most effectively restore legality.
1. Determine pressure altitude and ISA temperature deviation at the runway.
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Pressure altitude = elevation + 30 ft per hPa below 1013. With QNH 1003 hPa, the difference is 10 hPa, so pressure altitude = 1,200 + 30 × 10 = 1,500 ft.
ISA temperature at 1,500 ft ≈ 15 − 2 × 1.5 = 12°C. The actual temperature is 28°C, so the deviation is 28 − 12 = +16°C above ISA.
2. Apply the stated corrections and compare the corrected takeoff distance plus 15% margin with the runway length.
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Correction factors:
- Pressure altitude: +15% → 1.15
- Temperature: +16% → 1.16
- Headwind 12 kt: −24% → 0.76
- Uphill 1.5%: +7.5% → 1.075
- Wet grass: +30% → 1.30
Corrected distance = 850 × 1.15 × 1.16 × 0.76 × 1.075 × 1.30 = 1,204 m.
Required distance with 15% margin = 1,204 × 1.15 = 1,385 m. The runway is 1,400 m, so the departure is legal under the stated procedure, but the margin is only about 15 m (about 1% of runway length).
3. Assess the operational risk and identify the single change that would most effectively restore a meaningful margin.
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The calculation is legal but operationally fragile. A 2 kt reduction in headwind changes the headwind factor from 0.76 to 0.80, increasing the corrected distance to about 1,268 m and the required distance to about 1,458 m, which exceeds the runway. A small temperature rise or a wetter surface could have the same effect.
The most effective crew-controlled change is to delay until conditions improve, preferably until the temperature falls by at least 4°C or the headwind increases by at least 3 kt, then recompute. Reducing mass would also help, but the available AFM mass-correction data are not given, so it cannot be assumed. The departure should not be accepted on the current 15 m margin without a re-check.