getting to grips with aircraft performance 如何掌握飞机性能 1(39)

Weight Thrust
MTOW

25%
actual TOW
reduction max
available thrust  flat rated thrust 
EGT limit 
needed 
thrust 
 
Figure C30: Flexible Temperature Principle

OAT
OAT Tref Flex temp T Flex max
Consequently, the flexible temperature is the input parameter through which the engine monitoring computer adapts the thrust to the actual takeoff weight. This method is derived from the approved maximum takeoff thrust rating, and thus uses the same certified minimum control speeds.
In addition, thrust reduction cannot exceed 25% of the maximum takeoff thrust, thus leading to a maximum flexible temperature, as shown in Figure C30.
To comply with the above requirements, flexible takeoff is only possible when the flexible temperature fulfils the following three conditions:
TFlex > TREF TFlex > OAT TFlex ≤ TFlex Max
Regulations require operators to conduct periodic takeoff demonstrations, using the maximum takeoff thrust setting, in order to check takeoff parameters (N1, N2, EPR, EGT). The time interval between takeoff demonstrations may be extended, provided an approved engine condition-monitoring program is used.
7.1.2. Flexible Takeoff and Runway State
“AMJ 25-13 / AC 25-13
(f) The AFM states that [reduced thrust takeoffs] are not authorised on contaminated runways and are not  authorised on wet runways unless suitable performance accountability is made for the increased stopping distance on the wet surface".
Airbus operational documentation (RTOW, FCOM) provides performance information for flexible takeoffs on wet runways. As a result, a flexible takeoff is allowed on a wet runway, while it is forbidden on a contaminated one.

7.1.3. Flexible Temperature Determination
The following example illustrates how to determine a flexible temperature, with the use of a RTOW chart (Figure C29).
Example: Flexible Temperature and Speeds Determination
DATA
.
Takeoff from Paris-Orly, Runway 08

.
 Slat/Flap configuration: 1+F

.
Actual TOW = 66 tons

.
OAT = 24oC

.
Wind = +20 Kt headwind

.
QNH = 1013 hPa

.
Air conditioning: Off

.
Runway state: Dry

RESULT

.
Flex Temp = 68oC

.
 V1 = 145 Kt, VR = 145 Kt, V2 =150 Kt

Note: In case of deviation from the chart reference conditions (QNH, air conditioning…), corrections have to be applied to the flexible temperature.
7.1.4. Flexible Takeoff Procedure
To carry out a flexible takeoff, which is always at the discretion of the pilot, a flexible temperature has to be determined from an RTOW chart computed with no derate or an equivalent computerized system. This temperature value must then be entered in the MCDU (Multipurpose Control and Display Unit) during the takeoff preparation phase (Figure C31). At the brake release point, the thrust throttles must be pushed to the FLX position (Figure C32) as per the Standard Operating Procedure (SOP). TOGA thrust remains available at any moment during the takeoff phase. But, in the event of an engine failure after V1, its selection is not required.
　
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