A330/A340 机组训练手册 FCTM Flight Crew Training Manual 2(71)

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influence of the CG position
 
IMPACT ON TAKEOFF PERFORMANCE
TAKEOFF SPEED AND ASSOCIATED DISTANCES
The aircraft operating speeds are referenced to the stall speed, e.g. V2 >1.13 VS1G. If the CG moves forward ▸ VS1G increases ▸ V2 increases ▸ TOD increases (or the runway-limited TOW decreases).
ROTATION MANEUVER
An aft CG gives the aircraft a nose up attitude, that helps the rotation. A forward CG results in a
nose heavy situation, and a difficult rotation. It is "heavier", therefore longer, at forward CG.
CLIMB GRADIENT
As mentioned above, a forward CG is counteracted by a greater THS downward force.
This creates drag, that, in addition to the drag caused by the lift increase, degrades aircraft performance, for example, the climb gradient.
For example:
A340 Performance decrement with CG full forward, in comparison with CG 26 % CONF. 3, Packs On, ISA, Zp=0
 CG 26 % CG full forward 
TOD 3 165 m 3 241 m 76 m
5 % Climb gradient limiting weight 257.6 t 256.2 t 1.4 t

IMPACT ON IN-FLIGHT PERFORMANCE
A forward CG causes a nose up counter moment, that degrades fuel consumption, because of increased induced drag. For low fuel consumption, it is better to have the CG as far aft as possible.
 
SUPPLEMENTARY INFORMATION
CENTRE OF GRAVITY
 
Fuel increment 1 000 nm stage length CG location at 20 % (reference at 35 %)
Aircraft type Fuel increment (kg/lb)
A330 220/485
A340 380/838

The result summarizes the worst cases, i.e. it considers an heavy aircraft at a high flight level. Airbus has developed a trim tank transfer system, that controls the aircraft CG. When the airplane
is in cruise, the system optimizes the CG to save fuel by reducing the drag on the airplane. The system either transfers fuel to the trim tank (aft transfer), or from the trim tank (forward transfer). This movement of fuel changes the CG. The flight crew can also manually select a forward fuel transfer. The Fuel Control and Management Computer (FCMC) calculates the CG of the aircraft, based on the ZFWCG and fuel distribution. It then compares the result to a target value. From this
calculation, the FCMC determines the quantity of fuel to be moved aft or forward in flight (usually one aft fuel transfer is carried out during each flight).
IMPACT ON LANDING PERFORMANCES
For landing, the aircraft operating speeds are referenced to the stall speed, i.e. Vapp > 1.23 VS1G If the CG moves forward ▸ VS increases ▸ Vapp increases ▸ Landing Distance increases (or the Maximum Landing Weight, limited by landing distance, decreases).
INFLUENCE OF THE CG ON THE STRUCTURE AND HANDLING CHARACTERISTICS
Applicable to: ALL
TAKEOFF AND LANDING
During the flights phases, that are critical and constraining, the CG must stay within certified limits for structural and handling qualities considerations (when in direct law). The following CG-certified envelope has been designed, in accordance with these limits:
　
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