getting to grips with fuel economy(5)

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This graph, which is for the A310-203, shows the advantage of flying at aft CG. Also shown are the optimum altitude lines and these show the effects of CG to be constant at these altitudes, with almost no variation with aircraft weight. Other aircraft have similar shape curves with similar optimum altitude characteristics (except the A320 family). The following table summarizes the effect of CG on specific range at the optimum altitude :
 
Aircraft Type
 Aft CG(35-37%)
 Fwd CG(20%)
 
A300-600
 +1.7%
 -0.9%
 
A310
 +1.8%
 -1.8%
 
A330
 +0.5%
 -1.3%
 
A340
 +0.6%
 -0.9%
 

 
For the A300/A310 reference CG is 27% and aft CG is 35%.
For the A330/A340 reference CG is 28% and aft CG is 37%.
At  maximum altitude, the change in fuel consumption given in the table is larger by up to 1%. However no benefit is obtained, as the specific range (SR) is lower at aft CG at maximum altitude than at mid CG at optimum altitude.
For aircraft that are not fitted with automatic center of gravity management, not all these advantages may be realized because of the normal forward and rearward shift of CG in flight due to fuel burn. In addition loading these aircraft at max fuel to an aft CG could prove difficult.
The A320 family does not show the same SR variation with CG as the other aircraft. The aft CG produces worst SR at FL290, crossing over to show an improvement at higher flight levels. The SAR variation is much smaller also. This is due to a complex interaction of several aerodynamic effects. The SAR can be considered effectively constant with CG position. Loading is therefore not critical for fuel economy for the A320 family.
In order to assess the overall impact of CG variation on fuel burn, it must be assessed on a complete sector. The following table shows increases in fuel consumed with a more forward CG. It is expressed as  kg per 1000nm sector per 10% more forward CG for the max variation case (high weight, high flight level) with no in flight CG shift. The fuel increment in kg is also given for the Forward (20%) position, compared with the Aft (35 or 37%) position, for a typical sector. 
 
Fuel Burn Increase with a more Forward CG
 
Aircraft types
 Fuel increment KG/1000nm/10%CG
 Typical Sector distance (nm)
 Fuel increment per sector (kg)
 
A300-600
 240
 2000nm
 710
 
A310
 110
 2000nm
 330
 
A319/A320/A321
 Negligible
 1000nm
 Negligible 
 
A330-200
 70
 4000nm
 480
 
　
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