getting to grips with aircraft performance 如何掌握飞机性能 2(24)

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2.2.4. Descent at Minimum Cost
The cost index aims at lowering direct operating costs for a given flight. For given cost index, an optimum descent Mach (MachECON) and an optimum descent speed (IASECON) are calculated by the FMGS as a function of the aircraft’s weight.

The descent is then carried out in managed mode, based on the following MACH/IAS law:
MachECON / IASECON / 250 kt
To minimize overall fuel consumption during flight, a low cost index must be used. As the descent phase is performed at idle thrust, it is advantageous to maximize its duration, from a fuel consumption standpoint. This is achieved at a low descent speed, which depends on the aircraft type (e.g. 250 knots for the A320 family). In any case, the descent speed must remain above green dot.
CI = 0 .
 IASECON = Minimum descent speed (depends on A/C type)
On the other hand, a high cost index is required when the overall flight time needs to be reduced for cost reasons. In this case, the descent must be as fast as possible (i.e. at the maximum rate of descent speed). It is obtained at a speed, which is generally limited to VMO – 10 kt in normal operations .
CI = CImax .
 IASECON = VMO – 10 kt
2.2.5. Emergency Descent
An emergency descent has to be carried out, in case of a cabin pressurization failure, the aim being to reach FL100 as soon as possible due to oxygen constraints. For this reason, MMO/VMO is the best speed schedule, as it enables the quickest possible rate of descent. This rate can even be increased by extending the airbrakes, if needed (refer to “En route Limitations” chapter).
2.3. FCOM Descent Table
Figure H8 shows an example of an A320 FCOM descent table:

 IN FLIGHT PERFORMANCE DESCENT  3.05.30 P 2 
SEQ 110  REV 31 

R
DESCENT -M.78/300KT/250KT 
IDLE THRUST NORMAL AIR CONDITIONING ANTI-ICING OFF  ISA CG=33.0%  MAXIMUM CABIN RATE OF DESCENT 350FT/MIN 
WEIGHT (1000KG)  45  65  IAS (KT)
FL  TIME (MIN)  FUEL (KG)  DIST. (NM)  N1  TIME (MIN)  FUEL (KG)  DIST. (NM)  N1 
390  16.1  181  102  73.0  17.0  125  103  IDLE  241 
370  14.6  152  90  74.0  16.2  121  98  IDLE  252 
350  12.9  115  77  75.9  15.6  118  93  IDLE  264 
330  11.7  90  69  IDLE  14.9  114  88  IDLE  277 
310  11.2  87  65  IDLE  14.3  110  83  IDLE  289 
290  10.8  84  61  IDLE  13.7  107  79  IDLE  300 
270  10.2  81  57  IDLE  12.9  103  73  IDLE  300 
250  9.6  77  53  IDLE  12.1  98  67  IDLE  300 
　
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