getting to grips with the cost index(19)

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 (TOD).

In order to be more accurate, we have to examine the influence of the cost index on descent for each aircraft type and this is done in the following section.

In order to be more accurate, we have to examine the influence of the cost index on descent for each aircraft type and this is done in the following section.

8.2 Variation of descent parameters with the cost index
As for the climb, descent performance is a function of the cost index; indeed, the higher the cost index, the higher the descent speed. But contrary to the climb, the aircraft gross weight (as shown in Figure 37 below by means of an A340-300 example) and the TOD flight level appear to have a negligible effect on the descent speed computation.

The following Table 15 shows the different relevant accurate descent parameters (time, speed, distance, fuel,...) computed by in-flight performance software (not FMS computation) for the entire Airbus family. The above figure should convince us that reducing the cost index before descent (as practiced by some airlines) to avoid unlikely overspeed warnings is unjustified. The limit of VMO - 10kt (320kt for the A330/A340, 330kt for the A300/A310, 340kt for the A320 family) is reached with a ceiling cost index depending on the aircraft and is not bound to vary with even higher values.

Table 15. Descent parameters from FL 370
(ISA conditions, no wind)
(250kt below FL 100)

AIRCRAFT TYPE (T/OFF weight)  COSTINDEX (Kg/min)  ONLY DESCENT SEGMENT  DESCENT WITH CRUISE SEGMENT  MACH/CAS 
FUEL (Kg)  TIME (min)  DISTANCE (NM)  FUEL (Kg)  TIME (min) 
A 300-600  0  317  19,3  108  317  19,3  .790 / 258 
(PW 4158)  30  298  17,3  102  357  18,1  .793 / 285 
(120 000 Kg)  60  282  15,8  96  403  17,4  .800 / 310 
100  276  15  93  427  17  .800 / 325 
A 310  0  284  21,4  116  284  21,4  .756 / 246 
(CF6-80)  30  263  19,3  111  301  19,9  .801 / 269 
(110 000 Kg)  60  239  17  103  353  18,7  .806 / 300 
100  218  15,3  95  406  18  .810 / 332 
A 320  0  138  19  105  138  19  .764 / 252 
(CFM 56)  20  125  17  99  157  17,8  .779 / 278 
(60 000 Kg)  40  112  14,9  90  187  16,8  .786 / 311 
60  137  14,6  92  207  16,4  .796 / 339 
　
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