getting to grips with aircraft performance 如何掌握飞机性能 1(40)

7.2. Derated Takeoff
7.2.1. 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.”
For a derated takeoff, the limitations, procedures and performance data must be included in the Aircraft Flight Manual (AFM). For each derate level, a specific RTOW chart can be established for a given runway, taking into account such new limitations as the minimum control speeds.
7.2.2. Minimum Control Speeds with Derated Thrust
A given derate level corresponds to the basic maximum thrust reduced by a given percentage. Therefore, the new maximum available thrust at any point of the takeoff flight path is cut back, compared to the non-derated thrust. New minimum control speeds (VMCG, VMCA) can then be established, as per JAR/FAR 25.149.
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.
Figure C33 illustrates A340 performance with and without derated thrust (from 4% to 24%). In this example, the optimum derate level (highest MTOW) corresponds to 20% of derate.

7.2.3. Derated Takeoff and Runway State
A derated takeoff is considered to be a normal takeoff with the engines at their normal operating limits. New limitations, procedures, and performance data are provided in the AFM for each derate level and each runway surface. Therefore, it is possible to determine MTOW on a dry, wet, or contaminated runway, simply by using a specific takeoff chart established for a specific derate level and a specific runway state (Figure C34).
So, derated takeoff is allowed on both a wet and contaminated runway.

7.2.4. Derated Takeoff Procedure
Derated takeoff is not available for all Airbus aircraft models. It is basic on all A330 and A340 models1, but doesn’t yet exist on the other Airbus aircraft types2.
When derated takeoff is available, 6 certified levels exist, ranging from (TOGA-4%) to (TOGA–24%) with a constant four percent increment (4%, 8%, 12%, 16%, 20% and 24%)3. This means that the AFM must contain a set of performance data for TOGA, and a set for each derate level (TOGA - X%).
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). The derate level must then be entered in the MCDU (Multipurpose Control and Display Unit) during the takeoff preparation phase (Figure C35). At the brake release point, the thrust throttles must be pushed to the FLX position (Figure C36).
　
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