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A specific derate level corresponds to the basic maximum thrust that is reduced by a corresponding percentage value. New minimum control speeds (VMCG, VMCA) can then be established.
Reducing the minimum control speeds sometimes improves takeoff performance (higher MTOW), when taking off on a short runway. Indeed, the V1 decision speed 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 in VMCG then permits a reduction in the ASD for a specific takeoff weight, and can improve takeoff performance when the MTOW (without derate) is ASD/VMCG-limited.

Flight Operations Briefing Notes
Understanding Takeoff Speeds
For a derated takeoff, the limitations, the procedures, and the performance data must be included in the Aircraft Flight Manual (AFM). For each derate level, a specific RTOW chart can be defined for a each runway, in order to take into account such new limitations as minimum control speeds.
Note:
The objective of flexible thrust differs from that of derated thrust. Both types of thrust cannot be used interchangeably.
Flexible Thrust is a thrust reduction, designed to save engine life. This thrust is reduced to take advantage of the available runway length, when full thrust is not necessary (from a performance perspective), but takeoff speeds with full thrust still apply.

III  Operational and Human Factors Affecting Takeoff Speed Computation and Utilization
The following factors are often observed when analyzing takeoffs in which takeoff speeds were not respected. Two cases can be observed:

III.1 Error in Takeoff Speed Computation:
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Data, issued from a computerized system, is rarely challenged. However, incorrect inputs may occur, and could result in inadequate takeoff speeds values.

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In takeoff speed calculations, Zero Fuel Weight (ZFW) is sometimes mistaken for Gross Weight (GW). This is particularly true when a last minute change occurs in cargo loading, or when time pressure and workload are high. Therefore, calculated speeds will be much lower than expected, and will lead to: Tailstrikes, “heavy aircraft” sensation, and high-speed rejected takeoffs. 

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Takeoff speeds calculations are based on specific configurations. Any change in the parameters of these configurations will invalidate takeoff speeds. Examples of such parameters include a runway change, a wet runway that becomes contaminated, or a takeoff from an intersection.

 

III.2 Error in Takeoff Speed Utilization:
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When a last minute-change occurs, takeoff speeds are sometimes modified and crosschecked during pushback or taxi. During such phases of flight, the PF workload is high. As a result the PF may not have sufficient time or resources to perform efficient crosschecks.
　
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