getting to grips with fuel economy(23)

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There is also an effect on block time due to the different altitudes. The true air speed increases/decreases 4kts, or just under 1% for each 2000ft lower/higher cruise altitude.
5.3.2.1 CROSS-OVER ALTITUDE VERSUS OPTIMUM ALTITUDE

 
It has been previously shown that the TAS is the maximum at the crossover altitude. One can wonder whether it is profitable to stay at this altitude, instead of climbing to the first optimum altitude. 
Assuming the standard climb laws, the crossover altitude can be derived. The standard speed laws are tabulated in paragraph 5.2.2.
 The next table shows the effect of flying at the crossover altitude instead of  optimum flight levels. The 1st optimum flight level has been chosen for the short sectors, whereas longer sectors assume step climbs with FL 310, 350 and 390 being available. This assumes ISA conditions and a take-off weight for a typical sector with max passengers and some freight (2500kg for the A320 family and 5000kg for the other aircraft).
 
Aircraft type
 Sector Distance  
 Cross-over altitude
 Optimum Flight Levels
 Gained time (min)
 Increase in fuel consumption 
 
A300B4-605R
 2000nm
 29000 ft
 310/350
 7
 1190kg
 
A310-324
 2000nm
 30000ft
 350/390
 3
 2160kg
 
A318-111
 1000nm
 29000 ft
 370
 3
 740kg
 
A319-112
 1000nm
 29000 ft
 370
 3
 650kg
 
A320-214
 1000nm
 29000 ft
 350
 2
 580kg
 
A320-232
 1000nm
 29000 ft
 340
 2
 440kg
 
A321-211
 1000nm
 29000 ft
 330
 2
 350kg
 
A330-203 
 4000nm
 31000 ft
 350/390
 9
 5040kg
 
A330-223
 4000nm
 31000 ft
 350/390
 9
 5780kg
 
A330-343
 4000nm
 31000 ft
 350/390
 10
 6380kg
 
A340-212
 6000nm
 29000 ft
 310/350/390
 17
 10900kg
 
A340-313
 6000nm
 29000 ft
 310/350/390
 14
 8410kg
 
A340-313E
 6000nm
 29000 ft
 310/350/390
 17
 9310kg
 
A340-500/600
　
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