Originally Posted by Feno
2.4% is the minimum net gradient requirement on the 2nd segment for twin engines.
Wrong. It is a gross gradient. It is obtained at the start of the 2nd segment. The gradient of the flight path reduces with height due to the thrust reducing with altitude. Therefore WAT-limit gradient is not equal to the average gross flight path gradient in the second segment.
Originally Posted by hvogt
A lot of confusion arises from the fact that the take-off performance requirements of CS-25 do not define any 'segments' as such.
I think the confusion arises from failure to appreciate the differences between the take-off climb requirements of 25.121 and the gradient of the flight path defined in 25.115.
25.121(a), (b) and (c) contains three minimum
steady climb gradient requirements out of ground effect in still air which together define the maximum take-off weight for the altitude and temperature, or WAT-limit. That is an airworthiness limitation that exists independent of the presence of obstacles. It replaces a provision in the predecessor regulation (CAR 4b) that required a minimum rate of climb as a function of the stall speed prior to the age of turbine engines.
In the take-off flight path the gradient is never steady. The ground effect changes with height above the surface. The drag changes when the configuration or speed changes. The thrust changes with ambient pressure and temperature as the airplane climbs, and with changes of airspeed. The take-off flight path is also subject to wind.
25.121(a) takes speed, configuration, and thrust at different points of the 'first' segment:
- speed is at liftoff,
- configuration is at the point of maximum drag in the gear retraction cycle - typically wheel well doors open and the gear half-way retracted,
- thrust is at the point where L/G retraction is begun - typically three seconds after liftoff.