TAS decreases: The climb gradient and rate of climb increase, as kinetic energy is converted into potential energy. 

2.1.3. Climb Ceiling 

The climb could continue until leveling off (i.e. when the rate of climb is close to zero). Nevertheless, as it would be both time and fuel consuming to reach the desired flight level, so the FMGS limits the climb to a maximum altitude. This maximum altitude is generally obtained when the rate of climb is equal to 300 feet per minute. 

2.2. Climb Speeds 

2.2.1. Climb at Given IAS/MACH Law 

A climb is generally operated at a constant Indicated Air Speed (IAS) and Mach Number. For instance, a standard climb profile for the A320 family is: 

250 kt / 300 kt / M0.78 

The climb phase is, therefore, divided into 3 phases (Figure G5): 

Below 10,000 feet: Climb at constant IAS = 250 knots. The speed is limited by Air Traffic Control (ATC) laws. 

Above 10,000 feet: Climb at constant IAS = 300 knots (limited to M0.78). At 10,000 feet, the aircraft accelerates to a more optimum climb speed (300 knots), which is maintained as long as the mach number remains under 

Above the crossover altitude: Climb at constant Mach = M0.78. The crossover altitude is the altitude where 300 knots IAS is equal to M0.78. Above this altitude, a constant ratio between the TAS and the sound velocity must be maintained to avoid high speed buffeting. 

2.2.2. Climb at Maximum Gradient 

The climb gradient at green dot speed is at its maximum. Climbing at green dot speed enables a given altitude to be achieved over the shortest distance. 

Green dot speed is computed by the Flight Management System based on aircraft weight, and is indicated on the Primary Flight Display (PFD) as soon as the aircraft is in clean configuration. This speed can, consequently, be easily flown in manual mode. Green dot is the target speed, in case of an engine failure after takeoff. 

2.2.3. Climb at Maximum Rate 

Climbing at the maximum rate of climb speed enables a given altitude to be reached in the shortest time. 

The maximum rate of climb speed is not indicated on the PFD. Nevertheless, a climb at maximum rate can be carried out in managed mode (refer to “Climb at minimum cost”). 

2.2.4. Climb at Minimum Cost 

As seen in the “Cruise” chapter, the cost index aims at lowering direct operating costs. As a result, for a given cost index, an optimum climb speed (IASECON) and an optimum climb mach number (MachECON) are calculated by the FMGS as a function of the aircraft’s weight. The climb is then carried out in managed mode, based on the following IAS/Mach law: 

250 kt / IASECON / MachECON 

To minimize overall fuel consumption during flight, a low cost index must be used. As the climb phase is fuel consuming, it is advantageous to minimize climb duration. This is achieved at the maximum rate of climb speed. 

IASECON = Maximum rate of climb speed 

On the other hand, a higher cost index provides a higher climb speed, thus lowering the rate of climb. But the distance covered during the climb is longer, so the cruise phase and total flight time are reduced. The maximum climb speed is generally limited to VMO -10 knots. 

2.3. FCOM Climb Table 

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.