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5.3.5 MANAGED MODE
The flight management system (FMS) optimizes the flight plan for winds,
operating costs and suggests the most economical cruise altitude and airspeed,
depending on the cost index chosen by the airline. An airline that wants to save
fuel has to choose a low cost index. The next part intends to highlight the impact
of the cost index on fuel consumption and on trip time. More complete information
can be found in the “Getting to Grips with Cost Index” brochure.
5.3.5.1 ECONOMIC MACH NUMBER
Long-range Cruise Mach number was considered as a minimum
fuel regime. If we consider the Direct Operating Cost instead, the Economic
Mach number (MECON), can be introduced.
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:
DOC C C F C T C F T = + .Δ + .Δ
where CC = fixed costs
CF = cost of fuel unit ΔF = trip fuel
CT = time related costs per flight hour ΔT = trip time
As DOCs are calculated per nautical mile, it is possible to plot fuelrelated
costs, flight-time related costs, and direct operating costs based on Mach
number .
Getting to grips with Fuel Economy IN FLIGHT PROCEDURES
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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).
ECON
ECON
weight constant FL M
constant weight M
= ⇒
FL = ⇒
􀃊 􀃊
􀃌 􀃌
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:
F
T
C
C
=
Cost of fuel
Cost Index (CI) = Cost of time
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.
Economic Cruise Mach Number
A310-324 ISA 130000kg
0.73
0.74
0.75
0.76
0.77
0.78
0.79
0.8
0.81
0.82
290 300 310 320 330 340 350 360 370 380
Flight Level
ECON Mach
Optimum
F/L
Maximum
F/L
0
20
40
60
80
100
Cost Index
5 - IN FLIGHT PROCEDURES Getting to grips with Fuel Economy
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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.
Economic Cruise Mach number
A310-324 ISA F/L 350
0.7
0.72
0.74
0.76
0.78
0.8
0.82
90 95 100 105 110 115 120 125 130 135 140
Aircraft Weight - tonnes
ECON Mach
0
20
40
60
80
100
Cost
Index
Getting to grips with Fuel Economy IN FLIGHT PROCEDURES
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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.
For instance, for the A319, increasing the cost index from 0 to 20 reduces
the block time by 15 minutes (5%) for a fuel burn increase of only 200kg (2%) on
a 2000nm sector.
A319
FL390
FL370
　
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