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(b)  Contrary to the common belief of many pilots, the variation of ECON Machwith gross weight variation (due to fuel burn) at a given flight level is verysmall (see graphs in Chapter 7.1.b). It is hence always possible to fly at a fixed cost index which results in a negligible speed variation.
However, since use of the cost index as a speed control is not recommended, pilots should select the necessary Mach Number via the FCU, in case of ATC speed request prior to recovering "managed" speed if constraints are subsequently released.

(c)  For many people, the term ECON Mach (for a flight managed at a given costindex) is synonymous with slower speeds solely for the sake of fuel economy. This is totally wrong as we know : hourly costs referred to earlier on, when combined with today's low fuel colts, will systematically lead to cost indices that are over and above long-range cruise cost indices.
More clarification of this point will be given in Figure 32 below.
Figure 32.

 

7.4 Airbus family Long-Range Cruise (LRC) cost indices
LRC speeds (that give a specific range equating to 99% of Maximum Range Cruise (MRC)) being a function of aircraft gross weight and flight level, the corresponding cost index is also variable as shown in Figure 33.

However, assuming that the aircraft should always be flying at about its optimum flight level (between OPT FL -2000ft and OPT FL +2000ft), calculations confirmed that the cost index values in the following table should systematically return a Mach Number close to long-range cruise Mach and for each aircraft type.
As a summary, it can be recalled that the Optimum Mach Number is a MN which remains greater than MRC and lower than LRC over the entire range of a typical cruise operation in terms of gross weight and altitude. The Optimum Flight Level for this Optimum MN is the FL which provides the greatest specific range at given gross weight. The optimum FL increases with decreasing gross weights, as illustrated in the FCOM.

On the A300-600/A310 family the Optimum Mach Number is set at 0.79 (GE) or at
0.80 (PW) and the Optimum Flight Level (all temperatures) as a function of Max. Altitude (ISA + 10/ISA + 20, n=1.3/1.4g) is provided in FCOM 2.17.30.
On the A320/A330/A340 Families Optimum Mach Number is presented in FCOM tables as ECON Mach versus cost index, altitude and wind as calculated by the FMG(E)C and to be found in FCOM 3.05.15. Appendix 5 shows an example for the A340-313E.
Table 14. LRC cost Index

Cost Index (kg/min) (100 lbs/hr) 
A300/A310  70 SPERRY FMS 60 SMITHS FMS  150 SPERRY FMS 130 SMITHS FMS 
A320  f(GW, FL) refer to Appendix 6 
A330 GE1A2 PW4168/4164 RR772  40 30 40  53 40 53 
A340-200/300 CFM 5C4 5C3 5C2  90 80 80  120 106 106 
A340-300E (High Gross Weight) CFM 5C4  80  106 
A340 FMGC L7 CFM 5C4 5C3 5C2  50 50 50  67 67 67 
　
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