getting to grips with fuel economy(26)

 
Thus delaying a climb to a higher altitude has a significant impact on fuel burn. 
 
 
 
 
 
 
 
 
 
5.3.2.4 OPTIMUM ALTITUDES ON SHORT STAGES

 
For short stages, the choice of cruise flight level is often restricted due to the necessary climb and descent distance. Airbus philosophy assumes a minimum 5 minute cruise sector, because a climb followed immediately by the descent is not appreciated by pilots, passengers or ATC. 
If the stage length is of sufficient length that the optimum flight level can be reached, but the cruise is of short duration, then the benefits at this flight level will be marginal. It may even be worthwhile to cruise at one flight level lower, as the increased climb consumption offsets any reduced cruise consumption.
In the FCOM there is a chart showing the optimum altitude on a short stage. An example is shown below.

 
5.3.3 CRUISE SPEED OPTIMISATION

 
 
Having been given a flight level which may be a requested optimum altitude or one imposed by air traffic control, speed is the only remaining parameter that requires selection. The following picture shows the variation of Specific Range with Mach Number for different aircraft weights at a fixed altitude.
 

 
 
The Mach number, which gives the best specific range, can be determined. It is called the maximum range cruise Mach (MMR). Nevertheless, for practical operations, a long-range cruise procedure is defined which gains a significant increase in speed compared to MMR with only a 1% loss in specific range. Like the 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”
 
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.
 

 
The Long Range Cruise Speed can be found in the Cruise tables in the FCOM.
 
 
 
 
 
 
 
 
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.
　
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