. a benefit in time and money, however small.

.  
Saving 11 minutes but spending 1400kg to stay 5 h 30 at FL 310 after taking off at 240t results in :

–  
saving 11 x 20$/min = 220$

–  
spending 1400kg x 0.24$/kg = 336$

. a cost.

To sum up, spending too much time below optimum altitude results in a fuel used/time saved ratio not profitable in terms of costs, but spending the right time (see Table 11) below optimum altitude results in a fuel used/time saved ratio profitable in terms of both time and costs.
If applied for "raw operational judgement" the cost index can be instrumental in facilitating cost-beneficial fuel-time evaluations. This should come as no surprise for a concept that balances time and fuel-related costs.
.  Returning to the above example for cases below 230t (240t for models fitted with CFM56-5C4 engine) there is, however, no gain in staying at lower flight levels (FL 310 or 330) because time savings are not worthwhile compared to the fuel increment.
The best strategy is therefore to climb initially to FL 350 or FL 370 (whether it is a westbound or eastbound flight) to avoid congested flight levels (FL 310 and especially FL 330 when referring to paragraph 6.3.a and figure 16).

7. COST INDEX AND CRUISE MANAGEMENT
The FMS manages cruise speed according to the aircraft gross weight, flight level, wind and of course the cost index. In this chapter we will review the influence of these four parameters on the ECON speed including differences between "selected" and "managed" cruise mode with a view towards adapting the flight towards external conditions.
7.1 Cost index - cruise speed relationship
In general, we can say that, at a given cost index :
-
the higher the flight level, the higher the ECON Mach,

-
the higher the aircraft gross weight, the higher the ECON Mach.

The following graphs (ECON Mach=f(CI)) as adapted to each Airbus model will illustrate this point best.
(a) At a given gross weight
On the following figures, we can duly appreciate ECON Mach variation at different cost indices for a range of flight levels.

A320/V2500

Figure 23. ECON cruise Mach = f(CI)
A330-322/PW4158

Figure 25. ECON cruise Mach = f(CI)
A340-313E/CFM56-5C4

These figures clearly depict the importance of the optimum altitude follow-up. We can indeed notice that ECON speed is very sensitive to the cost index when flying below optimum altitude especially for low cost indices, a sensitivity effect which is rather reduced around and above optimum flight level .
Moreover, optimum speed slowly increases with flight level for higher cost indices resulting in linear Mach variations when performing step climbs.
　
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