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I am very interested in the principles of take-off performance optimization. During my research I found an interesting picture, showing the software used by a European carrier.

However, the output presented shows a value I do not understand:

performance tool screenshot

In the lower right corner, the following values are shown:

PTOW: 68.5

Field Length: 73.2

Sec. Segment: 71.0

Obstacle: 68.5

I do not understand what the Second Segment limit stands for.

I know about the different phases of the take-off run, however I am not aware how the Second Segment could be limited by any other weight than the Obstacle Clearing weight.

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The second segment of the takeoff is defined as (source: skybrary.aero):

Second Segment - begins when the landing gear is fully retracted. Engines are at takeoff thrust and the flaps/slats are in the takeoff configuration. This segment ends at the higher of 400' or specified acceleration altitude. In most cases, the second segment is the performance limiting segment of the climb.

The minimum climb gradient is defined for each segment of the takeoff (same source):

Each segment of the one engine inoperative takeoff flight path has a mandated climb gradient requirement. For example, a gross second segment climb gradient capability of 2.4%, 2.7% or 3.0% is required for two, three and four engine aircraft respectively. Similarly, the required gross gradients for the fourth segment are 1.2%, 1.5% and 1.7% respectively.

I'm not familiar with the performance tool you show, but I would assume that the given value is the maximum TOW, which still ensures this required climb gradient can be met with one engine inoperative, so in your example a maximum of 71.0t allows 2.4% gross climb gradient for the second segment.

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