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Values for time, distance, Mach, fuel consumption do vary much with flight conditions such as TOD flight level temperature and wind but are less variable with respect to gross weight.
Similar to the climb, delta values with regard to time and distance are largely the same whatever the initial flight conditions.
First of all, we note that time to descent between low and high cost indices is more sensitive than for the climb varying from 4 minutes (A300, A330 and A340) up to 7 minutes for the A310.
However, in order to have a representative comparison of these different types of descent, we have to take into consideration the short cruise segments between the "low cost index TOD" and the "high cost index TOD" (see descent profiles in Section 8.1)
The following table provides parameters and differences in terms of time and fuel from a similar geographical point (TOD corresponding to cost index=0) to summarize descent laws between CI=0 (0 to 50 for the A340) and high cost indices (i.e. cost indices from which descent laws are the same : >60 for A320, >100 for A300, A310 and A330, >150 for A340).
Table 16. Descent from FL 370
Time (min) / Fuel (kg)from 1st TOD (CI = 0) Difference between low and high cost index
Cl = 0 High cost index Time gain Fuel increment
A300-600 19.3 317 17.0 427 2 min 20 s 110
A310 21.4 284 18.0 406 3 min 20 s 120
A320 19.0 138 16.3 210 2 min 40 s 70
A330 23.5 449 21.4 580 2 min 10 s 130
A340 23.2 550 21.2 663 2 min 110
However, in order to obtain the best TOD computation by the FMS and in order to avoid a nominal flight path overshoot (embarassing especially with high cost indices), a good insertion of descent winds is of vital importance even if the FMS adjusts the descent speed in a 20kt range according to the winds encountered.
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