getting to grips with aircraft performance 如何掌握飞机性能 2(23)

As a conclusion, in the standard descent speed range:
Weight . . descent gradient ì rate of descent .
1.3.4. Wind Effect
As shown in Figure H5 below, the air descent gradient (γa) remains unchanged, whatever the wind component. So, the fuel and time necessary to descend from the Top Of Descent (T/D) to the final level remain unchanged.

Headwind .
.  Rate of descent . Fuel and time from T/D . Flight path angle .
γ
g.
.

.γ
Ground distance from T/D .

Tailwind .
.  Rate of descent . Fuel and time from T/D . Flight path angle .
γ
g.
.

.γ
Ground distance from T/D .

2. DESCENT IN OPERATION
2.1. Thrust Setting
The standard engine rating for descent is “Flight Idle Thrust”. For fly-by-wire aircraft, the thrust throttle position doesn’t change when autothrust is engaged. The throttles remain on the “CL” (climb) gate for the entire flight (Figure H6). The engine-monitoring computer, or FADEC (Full Authority Digital Engine Control), adjusts the thrust level to the required value.
In case of an altitude constraint or a repressurization segment (see ”Cabin Descent”), the aircraft’s vertical speed may have to be limited during descent. This is achieved at a thrust called “Adapted Thrust”. The adapted thrust may vary between flight idle thrust and maximum cruise thrust. It is delivered by the engines, when autothrust is engaged, as soon as the aircraft descent speed plus one of the two descent parameters (gradient or rate) have to be maintained at fixed values.

2.2. Descent Speeds
2.2.1. Descent at Given MACH/IAS Law
A descent is generally operated at a constant Mach Number and Indicated Air Speed (IAS). For instance, a standard descent profile for the A320 family is:
M0.78 / 300 kt / 250 kt

TAS variations during descent are illustrated in Figure H7. For more details, refer to the “Climb” chapter.

2.2.2. Descent at Minimum Gradient (Drift Down)
The descent gradient at green dot speed is at its minimum. Descending at green dot speed enables the highest possible altitude to be maintained over the longest distance.
A green dot speed descent is of no interest in normal operations, as it requires a too much time. On the other hand, it is of great interest in case of an engine failure during cruise over a mountainous area, since it offers more escape solutions than any other speed. A green dot speed descent with one engine inoperative is called a drift down procedure (refer to the “En route Limitations” chapter).
2.2.3. Descent at Minimum Rate
The minimum rate of descent speed is lower than green dot. As a result, a descent at minimum rate is of no interest in operations, compared to a descent at green dot. Indeed, the time needed to reach a given altitude is longer than at green dot, whereas the distance covered is shorter. For this reason, and as a general rule, it is not beneficial to descend at a speed lower than green dot.
　
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