Runway Surface, Slope, Wind and Contamination Effects
Surface and slope
Runway surface condition changes the friction coefficient and therefore accelerate-stop and landing distances. Dry concrete gives the best braking; wet, slush, snow and ice degrade it progressively. Slope is normally quoted as a percentage; 1% uphill reduces take-off distance by roughly 5% and 1% downhill increases it by roughly 5% (a rule of thumb, not exact). Landing is the reverse: uphill shortens, downhill lengthens.
Wind
Headwind reduces ground roll; the effect is largest at low speed. Take-off distance varies approximately as the square of the ground speed, so a 10% headwind reduces ground roll by about 19%. Crosswind does not shorten the ground roll but consumes rudder authority and may limit the operation. Wind gradient and wind shear near the surface can destroy performance on short final or after rotation - a classic accident cause.
Contamination
Standing water can cause aquaplaning (hydroplaning) at a speed depending on tyre pressure and water depth; slush and snow add drag and reduce braking. Contaminated runways require the use of contaminated runway performance data, and where none exists the operation may be prohibited. Braking action reports (good, medium, poor, nil) are used with the appropriate chart.
Worked example
Dry TODR 1,800 m; 1% uphill runway; 10 kt headwind on a 120 kt V2. Slope: 1,800 × 0.95 ≈ 1,710 m. Headwind: ground speed ratio (120 − 10)/120 = 0.917; distance scales with the square, 1,710 × 0.84 ≈ 1,436 m. Crosswind does not enter this calculation.
Traps
Applying wind correction to the air distance rather than the ground roll; ignoring slope because 'it is only 1%'; treating a wet runway as dry; assuming aquaplaning speed is fixed.