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- from current position up to 150nm ahead : actual encountered wind,
- further up, a wind evolving linearly towards the wind inserted by the pilot into the FMS at that flight level.
However, the cost index would have to be changed in flight if the encountered winds were becoming so great that it could result in a missed hub connection upon arrival. It should be done after checking the fuel predictions on the secondary flight plan in the FMS with the new cost index value.
By iteration on the A300, A310 and A320, this recommendation could be followed on the A330 and A340 via the time constraint option.
(b) Changing the cost index in the case of fuel problems should be done asfollows
The objective is to avoid having to make a refueling stop. Select a lower cost index than the actual one in case of negative or pessimistic fuel predictions (extra fuel/extra time <0 in the FUEL PRED page in the FMS) due to strong winds encountered or ATC rerouting, restrictions or expected holding at arrival.

Important : this should be done first on the secondary flight plan and after checking fuel predictions before entering the adapted value (found by iteration until obtaining extra time/extra fuel>0 in the FUEL PRED page in the FMS) in the primary flight plan to avoid unnecessary thrust variations.
That is why the quickest strategy is to check the fuel predictions first with the LRC cost index (see Section 7.4) and select CI=0 only if there is a fuel concern.
(c)
Changing the cost index for speed control should never be done except inthe case of fuel problems (LRC or MRC) as just explained.

(d)
For  a fuel-critical route, setting a zero cost index may be envisagedexceptionally provided all mandatory route reserves can then be maintained.

 

10. CONCLUSION
The cost index is a simple and effective tool when it is appropriately used by an airline. This means airlines should have a thorough knowledge of costs in order to optimize operating economics. This is the single and only purpose of the cost index, keeping in mind that wrong utilization and/or wrong calculation of it leads inevitably to cost penalties. These penalties pertain to overall costs and not just to fuel costs ; apparent overconsumption caused by the cost index may sometimes be attributed to the need to save expensive flying time.
Therefore, one should always bear in mind that the cost index trades off both fuel and time provided they are properly assessed.
All of the above should not hide the fact that aircraft performance is rather variable when depending on the cost index : speed and rate of climb, Mach as a function of gross weight, flight level and cruise winds. Its output performance may also lead to incompatibilities with ATC constraints. The development of FANS (Future Air Navigation System) with CNS - ATM and new FMS avionics should prompt a more appropriate utilization of the cost index and certainly a more dedicated optimization of flight economics. Lateral track clearances and improved altitude allocation should certainly enable better use to be made of the cost index concept.
　
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