It should be noted that the influence of airspeed on optimum altitude is not very significant in the range of normal cruise speeds.
 
 In order to minimize fuel burn, the aircraft should therefore be flown at the optimum altitude. However this is not always possible. Performance limitations such as rate of climb or available cruise thrust can lead to a maximum altitude below the optimum, as can buffet limitations. At low weights, the optimum altitude may be above the maximum certificated altitude. In addition, Air Traffic Control restrictions can affect the flown flight level.
The following table shows the specific range penalty of not flying at optimum altitude, assuming a cruise Mach No of 0.8. It should be noted that each airframe/engine combination has different values. It should be noted that these are average values and there are slight variations with different weight/optimum altitude combinations.
 
 
Specific Range Penalty for not flying at Optimum Altitude
 
Aircraft
 +2000ft
 -2000ft
 -4000ft
 -6000ft
 
A300B4-605
 2.0%
 0.9%
 3.4%
 9.3%
 
A310-324
 1.9%
 1.4%
 4.4%
 9.3%
 
A318-111
 0.7%
 1.6%
 5.0%
 10.0%
 
A319-132
 1.0%
 3.0%
 7.2%
 12.2%
 
A320-211
 **
 1.1%
 4.7%
 9.5%
 
A320-232
 1.4%
 2.1%
 6.2%
 12.0%
 
A321-112
 2.3%
 1.4%
 4.6%
 15.2%
 
A330-203
 1.8%
 1.3%
 4.2%
 8.4%
 
A330-343
 3.0%
 1.0%
 3.2%
 7.2%
 
A340-212
 1.4%
 1.5%
 4.0%
 8.0%
 
A340-313E
 1.5%
 1.6%
 5.2%
 9.5%
 
A340-642
 1.6%
 0.6%
 2.2%
 5.1%
 

 
** Above Maximum Altitude
 
Generally if one flies within 2000ft of optimum altitude, then the specific range is within about 2% of the maximum. However fuel burn-off is an important consideration. 
Consider an A340-313E at a weight such that the optimum altitude is 33000ft. If the aircraft flies at FL 310 the SR penalty is 2.1% for the weight considered. However after a fuel burn of 20800kg, during which the aircraft would have traveled 1400nm the optimum altitude increases to 35000ft and the penalty is now 5.2%. 
　
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本文鏈接地址：getting to grips with fuel economy(22)