Advanced Instrumentation
Cost Index
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Cost Index - Introduction
In essence, the cost index is used to take into account the
relationship between fuel and time related costs.
All modern Flight Management Systems has the ability to
calculate performance and fuel burn based on a Cost
Index number.
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Time-related costs
Time-related costs contain the sum of several
components :
hourly maintenance cost
flight crew and cabin crew cost per flight hour
marginal depreciation or leasing costs (i.e. the cost of ownership
or aircraft rental)
In addition to the above time-related costs, extra cost
may arise from overtime, passenger dissatisfaction,
hubbing or missed connections.
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Time-related costs
With time-related costs, the faster the aircraft is flown,
the more money is saved.
This is because the faster the aircraft is flown, the more
miles time-related components can be used and the more
miles can be flown and produced between inspections.
However, if the aircraft is flown faster to reduce
timerelated costs, fuel burn increases and money will be
lost in turn.
On the other hand, to avoid over-consumption of fuel,
the aircraft should be flown more slowly. To solve this
dilemma, the FMS uses both ingredients, and is therefore
able to counterbalance these cost factors and to help
select the best speed to fly, therefore called ECON (i.e.
minimum
cost)
speed.
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Cost Index formula
To counterbalance these cost factors, we use this
formula:
𝐶"#$%
𝐶! =
𝐶&'%(
Where CI is a dimentionless ratio between time related
cost per hour and fuel burn per hour.
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Cost Index formula
This gives us the following relationship on time versus
fuel and Cost Index:
𝐶! = 0
MINIMUM FUEL CONSUMPTION
MAXIMUM RANGE
(𝐶" small, 𝐶# large)
ECON
MODE
𝐶! = MAX
MINIMUM TIME
MAXIMUM SPEED
(𝐶" large, 𝐶# small)
The cost index effectively provides a flexible tool to
control fuel burn and trip time between these two
extremes.
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CI – Climb profile relationship
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),
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CI – Cruise speed relationship
In general, we can say that, at a given cost index :
The higher the flight level, the higher the ECON Mach,
The higher the aircraft gross weight, the higher the ECON Mach.
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CI – Decent profile relationship
We notice that the higher the cost index :
The steeper the descent path (the higher the speed) ,
The shorter the descent distance,
The later the top of descent (TOD).
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Use of Cost Index
The Cost Index number for a particular flight is usually
determined by the Company. And will be part of the SOP.
And will be dispatched as part of the Operational Flight
Plan
By setting the CI in the FMS, the FMS will calculate the
most economical (ECON) performance parameter for the
flight:
Optimum flight level
Optimum climb speed/rate
Optimum cruise speed
Optimum decent speed/rate
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Questions
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