An error of 5 US$/min in the time cost computation which leads to a cost index error of 20 (fuel price = 0.25 US$/kg) can increase the trip cost from 0.2 up to almost 2% (especially for the A320) depending on the cost index range and the aircraft model.
Although possibly negligible on a single flight, this becomes rather meaningful on a yearly basis when applied to a whole fleet.

4.2 Trip cost variations at fixed time-cost
This section shows the importance of adapting the cost index to each airline route sector, that is to say to each fuel price sector.
The following graphs illustrate trip cost variations according to different fuel prices and for each Airbus model, depicting similar curves to those in 4.1 above.
Figure 7. .
 Trip cost = f(CI)
A300/A310(All models)

Figure 8. .
 Trip cost = f(CI)
A319/A320/A321(All models)

Figure 9. .
 Trip cost = f(CI)
A330
(All models)

A340
(All models)

In a similar manner, we notice that the lower the fuel price, the higher the cost index corresponding to the minimum trip cost (different minima for each fuel price value).
Moreover, we also appreciate that a fuel price variation of 0.25 U$/USG may lead to a trip cost increase of up to 1% if not properly taken into account.
To sum up, and considering average time-related cost for each aircraft type, we can say that fuel prices are indeed rather influential in cost index determination especially when their value exceeds 1 US$/USG (see preceding graphs).

5. CLIMB PERFORMANCE VERSUS COST INDEX
5.1 Cost index - climb profile relationships
Let us consider the influence of the cost index on the climb profiles shown in the following graph. We can readily appreciate how the FMS computes the Top of Climb (TOC) as a function of the cost index.

Fuel and Time to "Distance" (e.g. 150 or 200nm) is also a good indicator
We notice that the higher the cost index :
-
the shallower the climb path (the higher the speed),

-
the longer the climb distance,

-
the farther the Top of Climb (TOC),

In order to be more accurate, we have to review the cost index influence on climb for each aircraft type, and this is done in the following two sections.

5.2 Variation of climb parameters with the cost index
The following Table 6 shows the different relevant accurate climb parameters (time, speed, fuel, distance...) computed by in-flight performance software (not FMS computation) for the A300, A310, A320, A330 and A340. The A340 case is considered more particularly in the next section because its climb CAS is not a cost index function.

 Table 6. Climb parameters to FL330
(ISA conditions, no wind)
(250kt up to FL 100)

Climb parameter to FL 330 (ISA condition, no wind, 250kt up to FL 100)
AIRCRAFT TYPE (T/OFF weight)  COST INDEX (Kg/min)  ONLY CLIMB SEGMENT  CLIMB WITH CRUISE SEGMENT  CAS/MACH  RATE at TOC (ft/min) 
　
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