Kristian17, the purpose of derated take off is to generate higher take off weight if you are ASD limited in first place and additional benefit is that you still can perform a derated take off on contaminated RWY, which you cant do with FLEX. See the extraction from Airbus manual. Of course you still use derated option on the normal RWY to preserve engine life and so on.
Definition:
“AMJ 25-13 / AC 25-13
(4)(b) Derated takeoff thrust, for an aeroplane, is a takeoff thrust less than the maximum takeoff thrust, for which exists in the AFM a set of separate and independent takeoff limitations and performance data that complies with all requirements of Part 25.” In this case, “the thrust for takeoff is considered as a normal takeoff operating limit.”
Explanation:
A reduction in the minimum control speeds sometimes generates a takeoff performance benefit (higher MTOW) when taking-off on a short runway. Indeed, the decision speed V1 is the maximum speed at which it is still possible to reject the takeoff and stop the aircraft within the runway limits. Nevertheless, V1 must be greater than VMCG, and the Accelerate Stop Distance is often the most constraining limitation on a short runway. A reduction of the VMCG can then permit a reduction of the ASD for a given takeoff weight, and lead to better takeoff performance when the MTOW without derate is ASD/VMCG limited.
Procedure:
To carry out a derated takeoff, the actual takeoff weight and speeds have to be checked against the Maximum permissible takeoff weight computed for the given derate level (specific RTOW chart or equivalent computerized system).
One more thing is you don't select TOGA unless you are above F speed (Airbus) because the corresponding speeds are different from TOGA ones.
Cheers.
