getting to grips with the cost index(31)

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(e.g. FCOM Optimum Mach or LRC or any Mach Number).
The cost index should, however, not be manipulated as a speed control tool by varying the value on the CDU. Moreover, any overemphasis on the fuel economy side may be accompanied by costly repercussions on the time-related cost, hence jeopardizing total trip cost.
This Appendix proposes a technique for the assessment of a Default Cost Index value, using the graphs published in the FCOM :
- 
2.02.19 - page 34 (Sperry - Honeywell)

- 
2.02.19 - page 32 (Smiths).

Adapting a Default Cost Index can - in many cases where no calculated value is available be quite adequately defined for
.
the whole flight or for a given flight phase

.
for any desired speed :

.  given cruise Mach Number or LRC
.
given descent IAS (separate graphs). Step .

.  
Determine an average cruise gross weight (e.g. weight at cruise middle point) for the route, the area of operation or for the entire network.

.  
Using the CRUISE ALTITUDE CHART in FCOM 2.17.30 (page 20 for A310, page 14 for A300-600), determine the optimum altitude for this gross weight.

Step .

.  Using the average cruise gross weight and corresponding optimum altitude, determined in Step ., enter the FMS graph in 2.02.19, as follows :
–  
plot the above values in the FL / GROSS WEIGHT lower graph,

–  
from this point, draw a vertical line.

 

Step .

.  Using the desired MN (Optimum MN, LRC or any MN) enter the graph from the right side in the ECON Mach scale and draw a horizontal line (do not consider any cruise wind component).
Step .

.  
At the intersection between the vertical line (drawn in Step .) and the horizontal line (drawn in Step é), read the default cost index value, e.g. :

–  
C l = 30,
Default Cost Index resulting in an ECON MN (without wind) =
FCOM Optimum MN,

–  
Cl = 70, Default Cost Index resulting in an ECON MN (without wind) = LRC MN.

 

.  
Interpolate, as required, between the cost index values of the two adjacent Cost Index curves.

 

APPENDIX 3

A340-200 AND -300 COST INDEX CONVERSIONS FOR LOAD 6
PENDING LOAD 7 RELEASE

1. Assessing A340 specific range variation versus Mach Number
As requested by launching customers, the A340 has been designed for a cruise Mach Number of 0.82 at 35 000ft. Even at Mach 0.83, the payload-range capability of the A340 satisfies most mission requirements. Reduction of cruise Mach to 0.81 or 0.80 brings an increase of range capability for very long stretches. It appears that the curve representing specific range variations versus Mach Number is quite flat.
Because of the flat characteristic of the Specific Range curve, assessing accurately the Maximum Range point and, hence, the MRC and LRC Mach numbers is more difficult for the FMS software. The initial software resulted in lower-than-anticipated MN values.
　
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