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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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