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. 

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. 

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 . 

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  

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). 

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.