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FL350
FL330
FL310
FL290
FL390
FL370
FL350
FL330
FL310
FL290
FL390
FL370
FL350
FL330
FL310
FL290
9000
9500
10000
10500
11000
11500
12000
12500
13000
4.2 4.4 4.6 4.8 5 5.2 5.4
time (hours)
fuel consumption (kg)
CI=0
CI=20
CI=40
CI=60
CI>100
5 - IN FLIGHT PROCEDURES Getting to grips with Fuel Economy
- 56 -
5.3.6 EFFECT OF SPEED INCREASE ON MANAGED MODE
Flying at a given cost index rather than at a given Mach number provides the
added advantage of always benefiting from the optimum Mach number as a
function of aircraft gross weight, flight level and head/tailwind components.
This means the ECON mode (“managed” mode) can save fuel relative to
fixed Mach schedules (“selected” mode) and for an equivalent time.
One can wonder whether selecting a higher Mach number than the one
chosen by the FMS has a significant impact on fuel consumption. Imagine an
aircraft flying at flight level 370, in managed mode and at the optimum weight of
FL370. The FMS computes the optimum speed based on cost index, temperature
and wind. If the pilot selects another (higher) Mach number, the fuel consumption
will increase.
The following tables show the effect of such a speed increase.
We notice that although decreasing block times, the increase of Mach
number above the Optimum speed can result in significant increases in fuel burn.
Pilots hence have to be patient and should not change the Mach number even
when under the impression that the aircraft does not fly fast enough.
Moreover, when possible, the managed mode must be kept.
Economic Mach No + 0.005 Economic Mach No + 0.01
Fuel Penalty ΔTime Fuel Penalty ΔTime
Aircraft Sector Kg % Min Kg % Min
A300-605 2000 Nm 110 0.4 1 230 0.9 3
A310-324 2000 Nm 90 0.4 1 430 2.0 8
A318 1000 Nm 30 0.5 1 60 1.0 1
A319 1000 Nm 20 0.2 1 40 0.6 2
A320 1000 Nm 20 0.3 1 40 0.7 2
A321 1000 Nm 10 0.1 1 30 0.4 1
A330 4000 Nm 150 0.3 3 330 0.6 6
A340-212 6000 Nm 390 0.5 5 790 0.9 10
A340-313E 6000 Nm 380 0.4 5 900 1.0 10
A340-500 6000 Nm 1050 0.9 5 2540 2.1 9
A340-600 6000 Nm 820 0.7 4 2060 1.8 9
Getting to grips with Fuel Economy IN FLIGHT PROCEDURES
- 57 -
5.4 DESCENT
5.4.1 INTRODUCTION
Depending on the descent law, flight paths do vary in steepness.
Indeed, the higher the speed law, the steeper the flight path.
Descent profiles.
Descents are normally performed in three phases on a constant IAS/Mach
descent speed schedule, as follows:
• Constant Mach number is maintained until the crossover altitude
• Constant indicated air speed is maintained down to 10000ft
• 250 KT indicated air speed (IAS) is maintained below flight level 100, until
the aircraft decelerates for landing
The engine thrust is normally set to flight idle for the descent and the speed
is controlled by the aircraft attitude. In these conditions higher weights increase
the descent distance because of the reduction of descent gradient (which equals
[thrust-drag]/weight in stabilized flight). This also increases the descent fuel.
However a descent from high altitudes at low weight may lead to a gradient
of descent that results in an excessive cabin rate of descent. In these cases the
rate of descent is reduced by application of power, until a flight idle descent can
be continued. This results in what is known as the re- pressurization segment, and
this can reverse the weight-descent distance relationship.
To correctly evaluate the effects of descent techniques, cruise and
descent flight must be viewed in relation to each other. A short descent distance
for example extends the cruise distance. One has therefore to consider in addition
to the descent, a small portion of the cruise to the same distance.
High speed
Low speed
Cruise level
TOD
5 - IN FLIGHT PROCEDURES Getting to grips with Fuel Economy
- 58 -
5.4.2 THE EFFECT OF DESCENT TECHNIQUES ON FUEL BURN
An evaluation has been made of the fuel burn to a constant distance, and
this now shows that the higher weights use less fuel. Lower speeds, although
requiring more fuel for the descent only requires less total fuel because of the
longer descent distance. This is shown in the following chart.
At a fixed weight, the following chart shows that the minimum fuel occurs
at a descent speed of 240kias to 280kias, dependant on flight level.
However there is a significant time penalty at these speeds.
Effect of Descent Technique on Fuel and Climb for 115nm
　
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