getting to grips with the cost index(16)

(b) At a given flight level
The curves below show cruise speed variations at different cost indices as a function of aircraft gross weight. They give a useful indication of ECON Mach variation at a fixed flight level and for a range of gross weights.

Figure 27. ECON cruise Mach = f(CI)
A320/CFM56

Figure 29. ECON cruise Mach = f(CI)
A340-313/CFM56-5C4

These figures clearly show that ECON cruise Mach stays fairly constant throughout the flight for representative cost indices as discussed in Section 3, as well as for representative weights and flight levels.
Moreover, one should notice that, for low cost indices, a small cost index; increment has a far-reaching influence on ECON Mach (2 or 3 points) and hence on flight time, especially for the A340 as can be confirmed by means of specific range curves in Section 3.3.b.

7.2 Cost Index - fuel consumption relationship
The following figure illustrates the block fuel increment for a range of practical cost index values for each Airbus model. Increment levels are approximate and it is considered that engine type has no influence on the . trip fuel.

To summarize, we can say that there is no advantage whatsoever gained by flying at low cost indices (i.e below LRC cost indices) since fuel gains are not at all meaningful when traded far time, especially for the A340.
This finding will be more precisely highlighted in section 10.2.b with . time/. fuel tables facilitating trade-off appreciations.

7.3 Cruise "managed" versus cruise "selected"
(a)  Flying at a given cost index rather than at a given Mach Number provides the added advantage of always benefiting from the optimum Mach Number as a function of aircraft gross weight, flight level and head/tailwind component.

This means ECON Mode ("managed" speed) can save fuel relative to fixed Machschedules ("selected" speed) and for an identical block time.
By way of example, the following table points to potential fuel savings in "managed" speed versus "selected" speed. It applies to all Airbus models and may be used to interpolate to the trip length at least for a rough operational assessment.
Table 13. Potential fuel savings

Aircraft type(Takeoff weight)  Range(nm)  Type of cruise  Trip fuel (kg)  Flight lime  Fuel economy(k9) 
A310-324 (130 t)  2000  CI = 50  19560 19630  4h 26 min  70 
MACH 0.81 
A320-211 (65 t)  1000  CI = 30  5830 5860  2h 22 min  30 
MACH 0.78 
A330-342 (190 t)  4000  CI = 80  45750 45850  8h 42 min  100 
MACH 0.82 
A340-311 (250 t)  6000  CI = 150  84000 84500  12h 56 min  500 
MACH 0.82 

Albeit possibly negligible on a single flight, this can be rather meaningful on a yearly basis when extended to a whole fleet (see Figure 31 on the following page).
　
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