getting to grips with aircraft performance 如何掌握飞机性能 2(25)

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240  9.3  75  50  IDLE  11.7  95  64  IDLE  300 
220  8.6  71  46  IDLE  10.9  90  59  IDLE  300 
200  8.0  67  42  IDLE  10.1  84  53  IDLE  300 
180  7.4  63  38  IDLE  9.3  79  48  IDLE  300 
160  6.8  58  34  IDLE  8.5  73  43  IDLE  300 
140  6.1  53  30  IDLE  7.6  66  37  IDLE  300 
120  5.5  49  26  IDLE  6.8  60  32  IDLE  300 
100  4.8  44  22  IDLE  5.9  53  27  IDLE  300 
50  1.8  17  8  IDLE  2.2  21  9  IDLE  250 
15  .0  0  0  IDLE  .0  0  0  IDLE  250 
CORRECTIONS  LOW AIR CONDITIONING  ENGINE ANTI ICE ON  TOTAL ANTI ICE ON  PER 1° ABOVE ISA 
TIME  .  +10%  +10%  . 
FUEL  . 2 %  + 60 %  + 75 %  + 0.60 % 
DISTANCE  .  + 10 %  + 10 %  + 0.60 % 
Figure H8: A320 Descent Table Example

 

2.4. Cabin Descent
The cabin pressure rate is optimized during descent, so that it reaches the landing field pressure + 0.1 psi just prior to landing.
Depending on the initial cabin and destination airport altitudes, the FMGS calculates the necessary cabin descent time. This time is obtained from the selected cabin rate of descent, defaulted to –350 feet per minute in the FMGS, but which can be modified up to a maximum of –750 feet per minute.

As soon as the cabin descent time is longer than the aircraft descent time, a repressurization segment is necessary, during which the aircraft vertical speed is limited to permit cabin repressurization (Figure H9).
Altitude

segment
The above A320 descent table (Figure H8) shows that to descend from FL390 at a weight of 45 tons, the N1 parameter must be maintained at 73%, from the start of the descent, in order to limit aircraft vertical speed.
Note that, in some particular cases (landing at high altitude airports) , the cabin pressure at cruise level is higher than the pressure at the landing airport. Therefore, the cabin pressure has to decrease during descent, which means that the cabin’s vertical speed is positive while the aircraft’s vertical speed is negative.
3. HOLDING
3.1. Holding Speed
When holding is required, it is generally flown on a “race track pattern”, composed of two straight legs plus two 180 degree turns. As the aircraft is turning around, the distance covered is not the primary objective. On the contrary, the knowledge of the maximum holding time (maximum endurance) is a determining factor for any diversion decision. As a result, it is important, during holding, to try to minimize fuel consumption versus time as much as possible, or to simply minimize fuel flow (kg or lb per hour).
　
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