Would you please send your comments and remarks to the following contact point at Airbus. The topic of the cost index has been the subject of so much correspondence / communication, agreement / disagreement, action / inaction in recent years that we value your contributions very much. These will be taken into account in the following issues to be edited.

2. INTRODUCTION : COST INDEX DEFINITION AND DETERMINATION

The fundamental rationale of the cost index concept is to achieve minimum trip cost by means of a trade-off between operating costs per hour and incremental fuel burn. In essence, the cost index is used to take into account the relationship between fuel-and time-related costs. As a matter of fact, this underlying idea had already been introduced with the Performance Data Computer (PDC), the predecessor of the Flight Management System (FMS).
2.1 Trip cost
Without having to resort to complicated mathematics we can readily appreciate that the total cost of a specific trip is the sum of fixed and variable costs :
C = CF x .
F + CT x .
T + Cc
with CF = cost of fuel per kg
CT = time-related cost per minute of flight
Cc = fixed costs independent of time
.F = trip fuel
.T = trip time
In order to minimize C or the total trip cost we therefore need to minimize the variable cost :
CF x .F + CT x .T
For a given sector and period, the fuel price may be assumed to be a fixed value.
Let us consider a cost function τ
 = C/CF = .F + CT/CF x .T with CT/CF = CI (defined as the cost index)
Over a certain stage length .S this means : τ
 (1 nautical mile) = 1/SR + CI x 1/V
with SR being the specific range at weight, altitude and other conditions SR = .S/.F (nautical miles per kg)
with V being the ground speed to cover .S stage nautical miles including winds V = aM + Vc (Vc as the average head or tail wind component)
For a given sector, minimum trip cost is therefore achieved by adopting an operational speed that properly proportions both fuel- and time-related costs.
For a given cost index Mach Number (MN) variations will actually compensate for fluctuations in wind (see 7.5).

2.2 Time-related costs
Time-related costs contain the sum of several components :
- hourly maintenance cost (i.e. excluding cyclic cost as shown in Figure 1),
- flight crew and cabin crew cost per flight hour :
*  Even for crews with fixed salaries, flight time has an influence on crew cost. On a yearly basis, reduced flight times can indeed lead to :
.  
normal flight crews instead of reinforced ones,

.  
lower crew rest times below a certain flight time (i.e. better crew availability on some sectors),

.  
better and more efficient use of crews.

- marginal depreciation or leasing costs (i.e. the cost of ownership or aircraft rental) for extra flying per hour, not necessarily a fixed calendar time cost, but possibly a variable fraction thereof.
　
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