Take-off Distance: Conditions and Corrections

Every take-off distance in this chapter is quoted for one particular set of conditions: full power, dry paved level runway. Anything else is a correction you apply yourself, and the corrections for surface are large enough that leaving them out is not a rounding error.

At a glance

• Maximum demonstrated crosswind25 kts
• Headwinddecrease 10 % per 14 kt (7.2 m/s)
• Tailwindincrease 10 % per 3 kt (1.5 m/s)
• Dry grass 5 cm (2 in)ground roll +10 %
• Dry grass 5-10 cm (2-3.9 in)ground roll +15 %
• Dry grass 25 cm (9.8 in)ground roll +25 %
• Wet grassa further +10 % on the dry grass figure
• Soft groundground roll +45 %
• Uphill slopeground roll +12 % per 1 % of slope

The tabulated conditions

Both power levers at MAX; flaps UP or APP; runway dry, paved and level; nose wheel lift-off at V_R; airspeed for initial climb at V₅₀.

Wind

Decrease the distance by 10 % for each 14 kt (7.2 m/s) of headwind, and increase it by 10 % for each 3 kt (1.5 m/s) of tailwind. A tailwind costs you far more per knot than a headwind gives back — roughly five times as much.

NOTE — These factors already carry a built-in discount and a built-in penalty: only half of any headwind is credited to you, while any tailwind is counted at one and a half times its value.

So the asymmetry is deliberate: the factors already discount the benefit of a headwind and inflate the penalty of a tailwind.

Reading the wind off the diagram

The wind components diagram converts flight direction and reported wind into headwind and crosswind components. Its worked example takes a flight direction of 360° with the wind at 32° at 30 kts, and gives a crosswind component of 16 kts. The diagram also carries the maximum demonstrated crosswind component of 25 kts.

Surface corrections, applied to the ground roll

Grass

• Dry grass 5 cm (2 in) long — increase by 10 %.

• Dry grass 5 cm (2 in) to 10 cm (3.9 in) — increase by 15 %.

• Dry grass 25 cm (9.8 in) — increase by 25 %.

• Grass above 25 cm (9.8 in) — do not attempt to take off at all.

• Wet grass — increase the dry grass calculation by a further 10 %.

Ground and slope

• Soft ground — increase the ground roll by 45 %, in addition to the grass runway calculation where applicable.

• Uphill slope — increase the ground roll by 12 % for each 1 % of slope, that is 1 m per 100 m or 1 ft per 100 ft.

If the brakes are not held while power is applied, the distances apply from the point at which the full power setting is complete — not from where the aeroplane started moving.

Three cautions on using all this

WARNING — Do not treat the ground roll as the number that matters. A take-off is only safe where the runway you actually have is as long as, or longer than, the tabulated distance to clear an obstacle 50 ft (15 m) high.

WARNING — Poor maintenance condition, deviation from the given procedures, an uneven runway, and unfavourable external factors such as rain and unfavourable wind including crosswind will all increase the take-off distance.

CAUTION — The correction factors are typical values. On wet ground or wet soft grass the take-off roll may become significantly longer than stated, and the pilot must in any case allow for the condition of the runway.

CAUTION — Handle the slope corrections carefully. A published slope figure is normally an average taken end to end, and a real runway can be steeper in places and flatter in others. Where it is steeper than the average, your roll will be longer than these tables suggest; where it is flatter, shorter.

Check yourself

• What four conditions do the tabulated take-off distances assume?

• How much does 3 kt of tailwind cost, and how much does 14 kt of headwind give back?

• What grass length makes a take-off inadvisable altogether?

• How do the soft ground and grass corrections combine?

• What runway length does the manual require for a safe take-off?

Study aid — always fly your aircraft's current AFM/POH.