Clearing the field is only the entry fee. Once the wheels are up, the certified take-off must still climb, on one engine, through a sequence of gates the rulebook has fixed in advance: the segments, each with its own configuration, thrust and minimum gradient, and beneath the whole path the obstacles that the net version of it must clear by a margin. This lesson works the climb-out, and then assembles every limit met so far into the single number dispatch actually uses.
Thrust with the temperature in it
Before the segments, one chart-reading key. Modern engines are flat-rated: up to a kink-point temperature the fuel control holds the certified thrust constant, so performance charts show a flat part where mass limits do not change with OAT. Above the kink point the thrust must fall to protect the engine, and the charts bend into the non-flat-rated part, where every extra degree costs payload. Recognising which side of the kink the day sits on is half of every WAT-chart question.
The segments and their gradients
The actual (gross) take-off flight path runs from 35 ft to at least 1,500 ft above the surface, with the critical engine failed, and CS-25 cuts it into segments:
First segment — from 35 ft until the gear is up: V2, take-off flap, take-off thrust, the gradient merely required to be positive for a twin.
Second segment — gear up to the acceleration altitude, at least 400 ft: still V2, take-off flap and take-off thrust, and the gross gradient must be at least 2.4 per cent for two engines, 2.7 for three, 3.0 for four. This is normally the limiting segment.
Third segment — level at the acceleration altitude: accelerate, retract the flap on schedule, and move the thrust from the time-limited take-off rating to maximum continuous.
Final segment — clean, at maximum continuous thrust, climbing at the final-segment speed to 1,500 ft: at least 1.2 per cent gross for a twin, 1.5 for three engines, 1.7 for four.