Take-Off Climb Segments and Gradients

The four segments in order

Each segment is defined by what the aeroplane is doing, not by a fixed altitude band, which is why the boundaries matter more than any single number.

• First segment: from lift-off to gear fully retracted, one engine inoperative, take-off thrust, speed building towards V2. A positive gradient is required, but no specific minimum percentage applies to a twin at this stage.
• Second segment: gear up, take-off thrust, climbing at V2 from 35 ft, the certification screen height, to the acceleration altitude. This is where the numerical minimum gross gradient is set, 2.4 per cent for a twin.
• Third segment: level acceleration at the acceleration altitude, flaps retracting from the take-off setting to clean, speed increasing from V2 towards the final segment speed. No height is gained here; it is traded for configuration and speed.
• Final segment: clean configuration, maximum continuous thrust, climbing from the acceleration altitude to 1500 ft at the final take-off speed. Its minimum gross gradient for a twin is 1.2 per cent.

Gross versus net gradient

Every gradient quoted above is a gross figure, the gradient the aeroplane actually demonstrates. Obstacle clearance calculations do not use the gross figure directly; they use a net gradient, the gross figure reduced by a defined margin, commonly 0.8 per cent for a twin, to build in a safety buffer.

An aeroplane that meets the gross certification minimum exactly still clears obstacles on the reduced net path, not on the gross one. A question about the certification minimum wants the gross gradient, and a question about obstacle clearance wants the net gradient, and the two are not interchangeable.

Converting a gradient into a rate of climb

Charts and stems sometimes state a requirement as a gradient and sometimes as a rate of climb, so converting between the two is a recurring skill. The link runs through groundspeed: a 1 per cent gradient at a groundspeed of 100 kt gives a rate of climb of approximately 100 ft per minute, so in general, gradient in per cent multiplied by groundspeed in knots gives an approximate rate of climb in feet per minute. Three per cent at 200 kt, for instance, is approximately 600 ft per minute.

The word approximately matters. The relationship uses groundspeed, so any wind component has to be applied to TAS first, and the true conversion factor is closer to 101 than 100, which is why this is always treated as a close estimate rather than an exact value.