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A310-324 ISA 110000Kg
17.5
18.0
18.5
19.0
19.5
20.0
20.5
21.0
21.5
300 350 400 450 500 550 600 650
Fuel - kg
Time - min
240
250
260
300
320
330
3
13
33
54
74
100
390 370 Initi3a5l 0Flight Level330 310
Cost
Index
Descent
Speed KIAS
280
Effect of Descent technique on Fuel and Time for 115nm
A310-324 ISA F/L 350
17.5
18.0
18.5
19.0
19.5
20.0
20.5
380 400 420 440 460 480 500 520 540
Fuel - kg
Time - min
90t
110t
130t
260
280
300
320
Aircraft
Weight
Descent
Speed kias
Getting to grips with Fuel Economy IN FLIGHT PROCEDURES
- 59 -
Note that the effect of the descent Mach number is very dependant on cruise
flight level and descent speed, but is relatively small compared to the descent
speed effect, and is not fully investigated here.
These descent charts are typical of the other Airbus aircraft. Generally they
show a minimum fuel speed of 260 to 280 kts for flight level 310, reducing to
240kts for flight level 390. The exceptions are the A318, A319, A320 and A330,
which show the minimum fuel at 240kias for all flight levels which is slightly lower
than the other aircraft at FL310.
Appendix B presents some examples of these descent charts for other Airbus
aircraft.
The following tables show, for various Airbus aircraft, the descent time and
fuel variations for a fixed distance, from FL 350, relative to a 300kias reference
speed.
* A300/A310/A320 330kias A330/A340 340kias
Type ΔFuel – kg
240KT 260 KT 280 KT 300 KT 320 KT 330/340KT
*
A300 -55 -60 -30 0 25 35
A310 -55 -60 -30 0 25 40
A318, 319, 320 -50 -40 -20 0 20 25
A321 -35 -40 -20 0 20 35
A330 -110 -105 -60 0 50 70
A340–200/300 -70 -90 -50 0 50 75
A340-500/600 -125 -130 -70 0 70 100
Type ΔTime – minutes
240 KT 260 KT 280 KT 300 KT 320 KT 330/340KT
*
A300 2.7 1.5 0.6 0 -0.4 -0.6
A310 2.4 1.4 0.6 0 -0.4 -0.6
A320 family 2.6 1.4 0.6 0 -0.4 -0.6
A330 3.5 2.0 0.8 0 -0.6 -0.8
A340–200/300 3.2 1.8 0.8 0 -0.6 -0.8
A340-500/600 3.3 1.9 0.8 0 -0.6 -0.8
5 - IN FLIGHT PROCEDURES Getting to grips with Fuel Economy
- 60 -
Comparing these tables to the equivalent climb comparison tables in
chapter 5.2.4, it can be noticed that descent techniques often have a
greater effect on fuel and time than climb techniques.
5.4.3 MANAGED MODE DESCENT
The FMS computes the Top Of Descent (TOD) as a function of the cost index.
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.
Descent performance is a function of the cost index; the higher the cost
index, the higher the descent speed. But contrary to climb, the aircraft gross
weight and the top of descent flight level appear to have a negligible effect on the
descent speed computation.
It can be noticed that time to descent is more dependant on cost indices
than the time to climb.
On the Effect of Descent Technique on Fuel and Time chart, the cost index
has been annotated for each speed. It can be seen that the minimum fuel is at a
CI of 0, and the minimum time occurs with a high CI, as would be expected.
For the A300, A310 and A320 family the speed at zero cost index is about
250kias. For the A330/340 it is about 270kias. Max speed normally corresponds
with a high cost index of 60 to 120. Once more it can be seen how, in the
managed mode, the cost index is used to choose the balance between fuel burn
and flight time.
5.4.4 EARLY DESCENT
If the aircraft begins its descent too early, the aircraft would leave its optimal
flight level, where fuel consumption is at its best, and would have to cruise at a
lower altitude to arrive at the same point.
Cruise Flight Level
•
TOD
Early Descent
FL 100
Getting to grips with Fuel Economy IN FLIGHT PROCEDURES
- 61 -
Two descent situations were simulated:
• Descent commenced 15nm (or about 2 minutes) early followed by a leveloff
at FL100.
• Cruise continued from the early descent point until the optimum start of
descent, followed by the descent
At 10000ft, the cruise speed could be selected between LRC and max speed.
If in managed mode, one could continue at the same cost index, or select the
250kias below 10000ft limiting speed. The following table compares the two
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