What is a Second Segment Weight Limit?

Question

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:

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.

Answer 1

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.

Answer 2

OBSTACLE LIMITED CLIMB: It is natural to think of a required gradient to clear an 'Obstacle'. But that gradient arises from the geometry of the runway, obstacle height/distance and flightpath. The closer, and/or taller the obstacle, the higher the gradient required to clear it with the required margin. Interestingly, it should be pointed out that the required gradient will depend on the actual liftoff point ass the distance to the obstacle would change. 2nd Segment is a different matter . . .

SECOND SEGMENT LIMITED CLIMB: is about the capability of the airplane to maintain a minimum climbing flight path almost immediately after liftoff, with engine out, and gear up, takeoff flaps and takeoff thrust AND flying V2. This is governed by density altitude. It is equivalent to the WAT (Weight, Altitude, Temperature) charts on smaller airplanes where rwy length is rarely a factor. That gradient of 2.4% for twins has to be met no matter where in the world the rwy is located.

2nd segment can become the limiting factor at high temperatures and higher altitudes. This can become limiting even for a 10 to 12,000 ft RWY at merely 1000 ft elevation on hotter days.