getting to grips with fuel economy(32)

*  A300/A310/A320 330kias              A330/A340 340kias
Comparing these tables to the equivalent climb comparison tables in chapter 5.2.4, it can be noticed that descent techniques often have a greater effect on fuel and time than climb techniques.
5.4.3 MANAGED MODE DESCENT

The FMS computes the Top Of Descent (TOD) as a function of the cost index. We notice that the higher the cost index:
The steeper the descent path (the higher the speed)
The shorter the descent distance
The later the top of descent.
Descent performance is a function of the cost index; the higher the cost index, the higher the descent speed. But contrary to climb, the aircraft gross weight and the top of descent flight level appear to have a negligible effect on the descent speed computation.
It can be noticed that time to descent is more dependant on cost indices than the time to climb.
On the Effect of Descent Technique on Fuel and Time chart, the cost index has been annotated for each speed. It can be seen that the minimum fuel is at a CI of 0, and the minimum time occurs with a high CI, as would be expected. 
For the A300, A310 and A320 family the speed at zero cost index is about 250kias. For the A330/340 it is about 270kias. Max speed normally corresponds with a high cost index of 60 to 120. Once more it can be seen how, in the managed mode, the cost index is used to choose the balance between fuel burn and flight time.
 
 
5.4.4 EARLY DESCENT

 
If the aircraft begins its descent too early, the aircraft would leave its optimal flight level, where fuel consumption is at its best, and would have to cruise at a lower altitude to arrive at the same point.
                                  Cruise Flight Level
         .        

 
 
 
 
  
FL 100

Early Descent

TOD

       Two descent situations were simulated:
. Descent commenced 15nm (or about 2 minutes) early followed by a level-off at FL100.
. Cruise continued from the early descent point until the optimum start of descent, followed by the descent 
At 10000ft, the cruise speed could be selected between LRC and max speed. If in managed mode, one could continue at the same cost index, or select the 250kias below 10000ft limiting speed. The following table compares the two options.
　
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