From the horses mouth so to speak...340...cut from airbus manuals
FLEX TAKE OFF:
Takeoff at reduced thrust is permissible only if the airplane meets all applicable performance requirements at the planned takeoff weight with the operating engines at the thrust available for the assumed temperature.
Thrust reduction must not exceed 25 % of the full rated takeoff thrust. To meet this requirement, the flexible temperature must not be higher than ISA + 40°C (T MAX FLEX).
The assumed temperature must not be lower than the flat rating temperature or the actual OAT.
Takeoff reduced thrust is not permitted on contaminated runways.
Takeoff at reduced thrust is allowed with any inoperative item affecting the performance only if the associated performance shortfall has been applied to meet all performance requirements at the takeoff weight, with the operating engines at the thrust available for the flex temperature.
DERATED TAKE OFF:
Derated takeoff may be used when the takeoff weight is limited by VMCG, enabling benefit to be taken from the reduction in VMCG associated with the new rating.
The use of flexible thrust is not permitted when derated thrust is used. Moreover the level of derate is entered on the MCDU PERF TO page in the DRT TO/FLX TO field.
When a derated takeoff is performed, selection of full takeoff thrust by setting thrust levers at TOGA is not permitted below the speeds specified in engine failure procedure (FCOM 3.02.10 page 4)
The use of derated takeoff is allowed on dry, wet and contaminated runway.
The minimum control speeds VMCG and VMCA are reduced for two reasons :
The derated thrust is lower than the maximum takeoff thrust
The effect of temperature on VMCG and VMCA is taken into account (which is not the case for takeoff without derate, due to the flexible takeoff concept)
The effect of the derate on the maximum takeoff weight is different depending on whether VMCG or VMCA is limiting. Indeed the effect on maximum takeoff weight is the result of a thrust decrease (downgrading the takeoff performance) and of a VMC decrease (improving the takeoff performance). As VMCG only concerns the accelerate stop distance, the VMCG decrease by far compensates the thrust loss. The VMCG limited weight is then improved by derating.
But as VMCA mainly concerns the airborne phase of the takeoff, the effect of the thrust decrease is more important and not compensated by the effect of a lower VMCA. Therefore derated takeoff would not improve TOW if VMCA limited.
When VMCG limited, an optimum derate can be determined as shown below.
Hope this helps
