Take-off
Take-off is the most chart-intensive Performance domain. The exam asks you to convert declared distances and AFM data into a legal take-off, and to explain how mass, pressure altitude, temperature, wind, slope and runway condition move the limiting speed and distance. Questions often hinge on one definition or one factor, so precision matters more than a rough answer.
Declared distances and the required distances
- TORA is the take-off run available; TODA includes any clearway; ASDA includes any stopway.
- For a Class A aeroplane under CS-25/FAR 25 on a dry runway, the take-off distance required (TODR) is the greater of 1.15 × the all-engine take-off distance to 35 ft and the one-engine-inoperative (OEI) take-off distance to 35 ft.
- The take-off run required must fit within TORA; the accelerate-stop distance required must fit within ASDA; TODR must fit within TODA.
- The balanced field length is the field length at which the accelerate-stop distance equals the OEI take-off distance; V1 at that point is the balanced V1.
V speeds
V1 is the decision speed: at or above V1, continue after an engine failure; below V1, stop. V1 must be at least Vmcg, not greater than VR, and not greater than Vmbe. VR is the rotation speed and is never below V1. V2 is the take-off safety speed at 35 ft; it must be at least 1.13 Vs1 and at least 1.08 Vmca. A common exam trap is to treat V2 as a climb speed to be held after all engines fail - it is the one-engine-inoperative target.
Worked example
A runway has TORA = TODA = ASDA = 2,000 m. At V1 = 140 kt, the accelerate-stop distance is 1,850 m and the OEI take-off distance is 1,950 m. At V1 = 145 kt, they are 1,950 m and 1,850 m respectively. Interpolating, the balanced V1 is about 142.5 kt and the balanced field length is about 1,900 m. Since 1,900 m is below the available 2,000 m, the field is not limiting. If the balanced field length were 2,050 m, the take-off mass would have to be reduced until it fell to 2,000 m or less.
Traps that cost marks
Using TODA as though it were TORA; forgetting the 1.15 all-engine factor; applying a wind correction to a chart that already includes wind; assuming clearway can be used for the ground run; selecting V1 above VR; and ignoring slope direction. On a wet or contaminated runway, use the approved wet/contaminated data, not the dry figures.