Take-off performance
The take-off picture
Take-off performance is a chain of limiting speeds and distances. V1 is the decision speed: a failure before V1 means stopping within the accelerate-stop distance available; after V1 you must be able to continue and climb. VR is rotation speed, V2 the take-off safety speed reached by 35 ft. The declared distances matter: TORA (take-off run available), TODA (take-off distance available, includes clearway) and ASDA (accelerate-stop distance available, includes stopway). Gross figures come from the flight manual; the net take-off flight path is gross gradient reduced by 0.9% for twins, 0.8% for three engines and 0.5% for four.
What actually changes the numbers
Factors that lengthen the take-off run: higher mass, higher airfield pressure altitude, higher temperature (density altitude), tailwind, upslope, wet or contaminated runway, lower flap setting, and engine bleed for air conditioning or anti-ice. Headwind and lower mass shorten it. A trap is the wind component: questions often give a wind at an angle, so resolve the headwind component, and remember a 10 kt tailwind can add roughly 20% to the ground roll.
Worked example and rule of thumb
A runway has TORA 2,400 m and a clearway of 150 m, so TODA is 2,550 m. The manual gives a dry take-off distance to 35 ft of 2,300 m at the actual mass and conditions. Net distance is not reduced for a dry runway, so 2,300 m is compared with TODA 2,550 m: the take-off is legal with 250 m margin. Rule of thumb: each 1,000 ft of pressure altitude or each 10 °C above ISA adds roughly 10% to the take-off distance.