getting to grips with aircraft performance 如何掌握飞机性能 2(21)

H03C -04 A320-231 V2500 21100000C5KG330 0 018590 0 0 2 1.0 500.0 300.0 1 03 FCOM-N0-03-05-10-003-000

2.4. Cabin Climb
As the cabin is pressurized, a cabin pressurization system adjusts cabin altitude to provide passengers with a comfortable flight.
During normal operations, the cabin altitude is limited to a maximum value, which depends on the aircraft type. The purpose of this is to limit differential pressure .P (between the inside and outside) to a maximum value. For instance:
.
A320 family : Max cabin altitude = 8,000 feet , .Pmax = 556 hPa (8.06 PSI)

.
A340-200/300 : Max cabin altitude = 7,350 feet , .Pmax = 593 hPa (8.6 PSI)

Cabin altitude varies according to a preprogrammed law, in order to reach the scheduled cabin altitude at the top of climb defined by the FMGS cruise FL. For fly-by-wire aircraft, the cabin rate of climb is limited to 1,000 feet per minute.

In the above Figure (G7): When the FMGS cruise level is FL250, the cabin altitude remains at 3,050 feet during the cruise phase at this altitude.

H. DESCENT / HOLDING
1. FLIGHT MECHANICS
1.1. Definitions
The following Figure (H1) shows the different forces which applied on an aircraft in descent.

. 
For angle definitions, refer to the “Climb” chapter.

. 
The rate of descent (RD) represents the vertical component of the aircraft’s speed. It is negative and expressed in feet per minute.

1.2. Descent Equations
While climb is due to excess thrust, descent is, on the other hand, caused by a lack of thrust. Therefore, the descent gradient and the rate of descent, which depend on the difference (Thrust – Drag), are negative.
1.2.1. Descent Gradient (γ
)
As seen in the “Climb” chapter, the gradient can be expressed as:
1 In order to simplify, the thrust vector is represented parallel to the aircraft longitudinal axis.

Thrust -Drag
(1)
γ
rad =
Weight
Descent is carried out at the Flight Idle thrust (i.e. at a thrust close to zero). Consequently:
Drag
γ
rad =.
(2)
Weight
By introducing L/D (the Lift to Drag ratio), and as the weight value is close to the lift one (Lift = Weight.cosγ), the descent angle becomes:
1
γ
rad =.
(3)
Which gives, in percent:
100
γ
(%)=.
(4)
Conclusion: At a given weight, the magnitude of the descent gradient is minimum when the drag is minimum, or when the lift-to-drag ratio is maximum. The minimum descent angle speed is, therefore, green dot speed.
1.2.2. Rate of Descent (RD)
The Rate of Descent (RD) corresponds to the vertical component of the TAS.
(5) RD = TAS sinγ ≈ TAS γ (sinγ≈γrad as γ is small)
　
中国航空网 www.aero.cn
航空翻译 www.aviation.cn
本文链接地址：getting to grips with aircraft performance 如何掌握飞机性能 2(21)