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A reduced VMCG allows for a reduced V1, which, in turn, allows for a reduced accelerate-
stop distance (ASD).
In a situation where the takeoff weight is ASD-limited, a reduced thrust helps in reducing
the ASD and therefore in increasing the takeoff weight.

Of course, while thrust reduction improves aircraft controllability, it degrades both climb
performance and accelerate-go performance. Indeed, both of them require thrust. It is
understood that excessive thrust reduction can be damaging for takeoff performance.

It must also be understood that a gain in takeoff weight is only noticeable when the
performance is VMCG-limited. That is to say, typically on short and contaminated
runways.

The following page shows three takeoff charts. The first one has been established for
.NO DERATE., the second one for a derated level of 8% (D08) and the last one for a
derated level of 24% (D24).

Step 1:
Check the red circles (#1).
Step 2:
Compare minimum speeds... Check the blue circles (#2). The higher the derate level, the lower the minimum speeds.
Step 3:
Compare the MTOW... Check the green circles (#3).
MTOW D08 is greater than MTOW No Derate. The lower VMCs generate lower takeoff
distances, allowing a higher takeoff weight.

MTOW D24 is less than MTOW No Derate. The thrust reduction is too significant. The
lack of thrust helps controllability but reduces climb performance.

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C6.2 Performance determination
Different methods for performance determination on a contaminated runway are available, depending on the operator system. The main difference is based on the medium: Paper or computer.
In order to determine the MTOW on a contaminated runway, an operator can choose to use:

A takeoff chart established for a dry runway with optimized V2/VS ratio, then apply some conservative corrections from the FCOM, or 
A takeoff chart established for a dry runway, then apply the corrections for the runway conditions available on the chart, (available only for A319, A321, A330 and A340 aircraft) or, 
3  A takeoff chart established for a contaminated runway, or, 
4  The Less Paper in the Cockpit (LPC) program. 

Of course, each operator has the flexibility of developing its own original method, nevertheless, these methods can generally be separated into the four main types mentioned above.
The following example illustrates the various methods and is based on these assumptions:
Aircraft : A330-223 Airport : Airbus City Runway QFU : 31 Runway condition : Covered with 5 mm slush Outside Air Temperature : 5°C No wind, Standard QNH CONF 2
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Example of method 1: Takeoff chart dry + FCOM corrections

- Takeoff chart DRY-

- FCOM Correction -
- SPECIAL OPERATIONS chapter -
　
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