3.1.2. Wind Influence 

The MMR (or MLRC or MECON) value varies with headwind or tailwind, due to changes in the ground SR. Figure F12 shows the Maximum Range Mach number versus wind variations. 

The wind force can be different at different altitudes. For a given weight, when cruise altitude is lower than optimum altitude, the specific range decreases (Figure F8). Nevertheless, it is possible that, at a lower altitude with a favorable wind, the ground specific range improves. When the favorable wind difference between the optimum altitude and a lower one reaches a certain value, the ground-specific range at lower altitude is higher than the ground-specific range at optimum altitude. As a result, in such conditions, it is more economical to cruise at the lower altitude. 

Figure F13 indicates the amount of favorable wind, necessary to obtain the same ground-specific range at altitudes different from the optimum: 

 IN FLIGHT PERFORMANCE CRUISE  3.05.15  P 7  

SEQ 020  REV 24  

WIND ALTITUDE TRADE FOR CONSTANT SPECIFIC RANGE 

GIVEN : Weight : 68000 kg (150 000 lb) 

Wind at FL350 : 10 kt head FIND : Minimum wind difference to descend to FL310 : (26 . 3) = 23 kt RESULTS : Descent to FL310 may be considered provided the tail wind at this 

altitude is more than (23 . 10) = 13 kt. 

3.2. Maximum Cruise Altitude 

3.2.1. Limit Mach Number at Constant Altitude 

Each engine has a limited Max-Cruise rating. This rating depends on the maximum temperature that the turbines can sustain. As a result, when outside temperature increases, maximum thrust decreases (see Figure F14). 

3.2.2. Maximum Cruise Altitude 

On the other hand, when an aircraft flies at a given Mach number, the higher the altitude, the more the thrust must be increased. The maximum cruise altitude is defined for a given weight, as the maximum altitude that an aircraft can maintain at maximum cruise thrust when the pilot maintains a fixed Mach number. 

Figure F15: Maximum Altitudes at Maximum Cruise Thrust 

From Figure F15, it can be deduced that: 

At m1, the maximum altitude is PA1 for temperatures less than ISA + 10 

At m2, the maximum altitude is PA2 for temperatures less than ISA + 10, but PA1 for temperatures equal to ISA + 20. 

Maximum cruise altitude variations can be summed up as: 

Figure F16 illustrates how maximum and optimum altitudes are shown in an A330 FCOM: 

3.3. En route Maneuver Limits 

3.3.1. Lift Range 

In level flight, lift balances weight and, when CL equals CLmax, the lift limit is reached. At this point, if the angle of attack increases, a stall occurs. 

At a given weight, depending on the lift limit equation, each CLmax.M2 value corresponds to a static pressure (Ps) value. That is, a pressure altitude (PA). Therefore, there is a direct relationship between CLmax.M2 and PA.

Figure F18: Lift Area Definition 

3.3.2. Operating Maneuver Limitations 

3.3.2.1. Buffet phenomenon 

Concerning the low Mach number limit, when speed decreases, the angle of attack must be increased in order to increase the lift coefficient, which keeps the forces balanced. 

In any case, it is not possible to indefinitely increase the angle of attack (AoA). At a high AoA, the airflow separates from the upper wing surface. If the AoA continues to increase, the point of airflow separation is unstable and rapidly fluctuates back and forth. Consequently, the pressure distribution changes constantly and also changes the lift’s position and magnitude. This effect is called buffeting and is evidenced by severe vibrations. 

When the AoA reaches a maximum value, the separation point moves further ahead and total flow separation of the upper surface is achieved. This phenomenon leads to a significant loss of lift, referred to as a stall. 

The high Mach number limit phenomenon is quite different. In fact, at high speed, compressibility effects produce shock waves on the upper wing surface. When Mach number, and/or AoA increase, the airflow separates from the upper surface behind the shock wave, which becomes unstable and induces buffeting of the same type as encountered in the low speed case. 

3.3.2.2. Buffet limit When maneuvering, the aircraft is subject to a load factor expressed as: 

During turns, the load factor value mainly depends on the bank angle, as shown in Figure F21. In fact, in level flight, n = 1/cos(bank angle). 

At a given pressure altitude (Ps) and given weight (mg), one load factor corresponds to each CL max M2. Therefore, a curve representing load factor versus Mach number will have the same shape as the one observed in Figure F17.