This table shows that flying at crossover altitude increases the fuel burn significantly for a relatively small reduction in block time.
 
 
 
 
5.3.2.2  CRUISE OPTIMISATION WITH STEPPED CLIMB

5.3.2.2.1 Introduction

 
It has been shown that flying at non-optimum altitudes can cause significant fuel penalties, and that the effect of fuel burn is to increase the optimum altitude. The ideal scenario would be to follow the optimum altitude as in the climbing cruise, but A.T.C. constraints, performance and buffet limits do not make this possible.  However, by changing the cruise level with step climbs, as the aircraft gets lighter the aircraft will remain as close as possible to the optimum altitude. 
5.3.2.2.2 Choice of Profile

Several parameters such as weather conditions, ATC requirements, may influence any decision made by the crew with regard to three fundamental priorities: maneuverability, passenger comfort, and economics.
This pertains to the choice of the cruise flight level that can be made according to the three following climb profiles as shown below for an A340-642:

The Low profile initiates the step climb at the weight where the next available flight level is also the optimum flight level at that weight. Consequently the flight levels are always at or below the optimum. This has the advantage of better maneuverability margins and generally a better speed as it is closer to the crossover altitude. 
 
The high profile initiates the step climb at the weight where the next available flight level is also the maximum flight level at that weight. The flight levels are mainly above the optimum and the aircraft will have decreased maneuverability and fly slower.
The mid profile initiates the step climb at the weight where the specific range at the next available flight level is better than that at the current flight level. This enables the flight profile to remain as close as practically possible to the optimum flight level. It is this technique that is recommended for best fuel economy, and is also very close to that required for best economics.
It is interesting to note that, in this case,  the Mid profile step climb is made 1140nm before the Low Profile step climb and 1520nm after the High profile step climb. 
The situation changes with odd flight levels:

 
Because of the different available flight levels, the step climbs are initiated some 1500nm further than the even flight level step climb points. However the relative merits of each profile remains the same.
With Reduced Vertical Separation Minima (RVSM) the difference between flight levels reduces from 4000 to 2000ft and this enables the aircraft profile to remain much closer to the optimum. In addition the high profile (depending on the aircraft) remains much higher than the optimum, increasing the fuel penalty. This profile is shown on the following page.
　
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本文鏈接地址：getting to grips with fuel economy(24)