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Due to its particular climb speeds, the A340 cross-over altitude (well above the others) corresponds exactly to the first optimum FL for high take-off weights, whereas the optimum altitude is well above the cross-over altitude for all the other aircraft.
This leads us to consider the cost strategy case of the A340 in Section 6.3 herebelow because of its pre-eminent TAS advantage at cross-over altitude (around FL320).
On the other Airbus models this TAS advantage is never predominant compared to the accompanying fuel increment. This is due to the significant altitude difference (from 6000ft up to 9000ft) between optimum and cross-over altitude. By way of example we can indicate respectively the A320 and A330 families as consuming some 1000kg extra for a time gain of 5 to 8 minutes when climbing straight to the crossover altitude on typical and respective stage lenghts of 2000 and 3000 Nm.
In short, there is every reason for us to take the time factor into account in initial step climb management. The concern for fuel will receive priority thereafter.
To summarize this we can say that :
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On models other than the A340, the OPT altitude being higher than the cross-over
altitude, the emphasis must immediately be placed on fuel economy (i.e, following scenario . from the outset even for the very first cruise flight level).
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On the A340 models the OPT altitude being the same as the cross-over altitude, the emphasis should rather be placed on time economy as no real advantage can be gained from fuel economy until further into the flight.
6.3 Best cost strategy : A340 application
(a) Climb capability
In practice and contrary to some opinions, the A340 does climb straight to higher levels than its immediate competitor.
Europe
Source : EUROCONTROL
The following Table 9 provides a good approximation of optimum and maximum flight level for a Mach Number of 0.82 (< ISA +10).
Table 9. A340-311/CFM56-5C2
GROSS WEIGHT (t) OPT FL MAX FL (FMS)
250 310 330
230 330 350
220 350 370
200 370 390
180 390 410
170 410 410 (max certified)
Optimum altitude is also a function of the cost index but its influence will only be significant for values in excess of 200 (see Figure 17). 中國航空網 m.k6050.com 航空翻譯 www.aviation.cn 本文鏈接地址：getting to grips with the cost index(13)
