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 Direct Operating Costs (DOC) are made up of fixed, flight-time related and fuel-consumption related costs. As a result, for a given trip, DOC can be expressed as:
 

 
 
where CC = fixed costs
 CF = cost of fuel unit      .F = trip fuel
 CT = time related costs per flight hour .T = trip time
 
 As DOCs are calculated per nautical mile, it is possible to plot fuel-related costs, flight-time related costs, and direct operating costs based on Mach number .

 

 Minimum fuel costs correspond to the Maximum Range Mach number. The minimum DOC corresponds to a specific Mach number, referred to as Econ Mach (MECON).
 

 
 
The MECON value depends on the time and fuel cost ratio. This ratio is called cost index (CI), and is usually expressed in kg/min or 100lb/h:
 

 
 
Depending on the cost index, the predicted aircraft and atmospheric conditions, the optimum altitude and the economic Mach number are computed. From then on, fuel consumption depends only of the chosen cost index.
 
The following chart shows the economic Mach number variation with flight level for different cost indices. 

This shows the general trend, common with all aircraft, of increasing 
economic Mach number with flight level. 
The charts also show large economic  Mach number changes with flight level for low cost indices, whereas it is rather constant for high cost indices. The economic Mach is very sensitive to the cost index when flying below the optimum altitude.
 
The effect of weight variation at a fixed flight level is shown below.
 

 
 
The charts show that for high cost indices, the economic Mach number stays fairly constant throughout the flight. Nevertheless, for a low cost index, the economic Mach number reduces significantly as the weight reduces. This is quite normal as low cost indices favor fuel consumption at the expense of time. Moreover, we notice that for low cost indices, a small cost index increment has a far-reaching influence on the economic Mach number, and hence on flight time. These trends are typical of all aircraft.
 
 
 
 
 
 
5.3.5.2 TIME/FUEL RELATIONSHIP

 
To know whether the fuel economies at low cost indices are worthwhile, the impact of cost  index on time has to be considered. The following graph show both trip fuel and time for different flight levels and cost indices. The shape of this chart is typical of all types.
 

 
As it can be seen, it is not really advantageous to fly at very low cost indices as fuel savings are not significant compared to time loss. Although using slightly higher fuel, a slightly higher cost index gives significant time gains.
　
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