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MMR speed, the MLRC speed also decreases with decreasing weight, at constant
altitude.
A more detailed explanation of this can be found in “Getting to Grips with
Aircraft Performance”
5 - IN FLIGHT PROCEDURES Getting to grips with Fuel Economy
- 48 -
The following chart shows the typical variation of the Long Range Cruise
Mach Number with aircraft weight for various flight levels. Also plotted on this
chart is the optimum altitude line. This shows that there is not much variation in
the long-range cruise mach number at these altitudes.
It would therefore be possible to fly a constant Mach number procedure
instead of the variable LRC speed procedure. In order to save fuel however, the
exact LRC speed should be maintained.
Long Range Cruise Mach Number
A330-243 ISA
0.72
0.74
0.76
0.78
0.8
0.82
0.84
160 170 180 190 200 210 220 230 240
Aircraft Weight (1000kg)
Mach Number
270
290
310
330
350
370
390
410
Flight
Optimum
Altitude
The Long Range Cruise Speed can be found in the Cruise tables in the FCOM.
Getting to grips with Fuel Economy IN FLIGHT PROCEDURES
- 49 -
5.3.4 WIND INFLUENCE
Wind can have a significant influence on fuel burns. Nowadays,
meteorological forecasts are very reliable and its integration into the FMS provides
accurate information to crews. Hence the latter best perform flight planning with a
view towards fuel savings.
The effect of the wind on trip time and fuel is shown on the following chart,
which gives fuel consumption and time for a 2000nm sector, with respect to flight
levels, Mach number and wind (tailwind positive) for a fixed take-off weight.
This plot graphically shows the magnitude of the significant changes in fuel
consumption and time due to winds. FCOM Tables show the equivalent still air
distances for any ground distance/wind combination.
However the winds can affect the performance optimization as well as
changing the effective still air distance. The MMR (or MLRC) value varies with
headwind or tailwind, due to changes in the SR.
The effect of a tailwind is to increase the ground speed, and therefore the
SR, by the ratio of ground speed to airspeed. A given wind speed therefore has a
larger effect at the lower airspeeds, which changes the optimum speed.
A321
FL 350
FL 330
FL 310
FL 290
FL 330
FL 350
FL 310
FL 290
11000
12000
13000
14000
15000
16000
17000
18000
3.5 3.7 3.9 4.1 4.3 4.5 4.7 4.9 5.1 5.3 5.5 5.7
Time (hour)
Fuel consumption (kg)
M 0.8
M 0.78 M 0.76
M 0.8
M 0.78
M 0.76
M 0.8
M 0.78
M 0.76
M 0.8
M 0.78
M 0.8 M 0.76
M 0.78
M 0.76
-60 kts
-30 kts
30 kts
60 kts
0 kts
5 - IN FLIGHT PROCEDURES Getting to grips with Fuel Economy
- 50 -
The following chart shows the Maximum Range Mach number versus wind
variations.
MMR and wind influence
This shows that
Tailwinds increase the specific range and lower the speeds
Headwinds decrease the specific range and raise the speeds.
The wind speed can be different at different altitudes. For a given
weight, when cruise altitude is lower than optimum altitude, the specific range
decreases. Nevertheless, it is possible that, at a lower altitude with a favorable
wind, the ground specific range improves. When the favorable wind difference
between the optimum altitude and a lower one reaches a certain value, the
ground-specific range at lower altitude is higher than the ground-specific range at
optimum altitude. As a result, in such conditions, it is more economical to cruise
at the lower altitude.
There is information in the most FCOM’s (not A300/310) to indicate the
amount of favorable wind, necessary to obtain the same ground-specific range at
altitudes different from the optimum. If the wind is more favorable then it is
beneficial to fly lower. The following shows such a page:
Given weight
and Pressure
Altitude
Getting to grips with Fuel Economy IN FLIGHT PROCEDURES
- 51 -
IN FLIGHT PERFORMANCE
CRUISE
3.05.15 P 7
SEQ 020 REV 24
WIND ALTITUDE TRADE FOR CONSTANT SPECIFIC RANGE
GIVEN : Weight : 68000 kg (150 000 lb)
Wind at FL350 : 10 kt head
FIND : Minimum wind difference to descend to FL310 : (26 − 3) = 23 kt
RESULTS : Descent to FL310 may be considered provided the tail wind at this
altitude is more than (23 − 10) = 13 kt.
5 - IN FLIGHT PROCEDURES Getting to grips with Fuel Economy
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