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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:
A330/A340 FLIGHT CREW TRAINING MANUAL
SUPPLEMENTARY INFORMATION
CENTRE OF GRAVITY
A330/A340 FLEET SI-050 P 4/10
FCTM 17 JUN 09
CG envelope
A330/A340 FLIGHT CREW TRAINING MANUAL
SUPPLEMENTARY INFORMATION
CENTRE OF GRAVITY
A330/A340 FLEET SI-050 P 5/10
FCTM 17 JUN 09
A330/A340 FLIGHT CREW TRAINING MANUAL
SUPPLEMENTARY INFORMATION
CENTRE OF GRAVITY
A330/A340 FLEET SI-050 P 6/10
FCTM 17 JUN 09
A330/A340 FLIGHT CREW TRAINING MANUAL
SUPPLEMENTARY INFORMATION
CENTRE OF GRAVITY
A330/A340 FLEET SI-050 P 7/10
FCTM 17 JUN 09
A330/A340 FLIGHT CREW TRAINING MANUAL
SUPPLEMENTARY INFORMATION
CENTRE OF GRAVITY
A330/A340 FLEET SI-050 P 8/10
FCTM 17 JUN 09
The CG envelope must also allow passengers to move in the cabin. Therefore, when the takeoff
CG and landing CG envelope have been determined, the in flight envelope is deducted from
takeoff/landing CG envelope by adding a 2 % margin, provided that all the criteria for the handling
characteristics are met.
Note: In approach with flaps extended, there is a nose down moment. This is counteracted by
THS nose up setting. The further forward the CG is, the more the THS nose up setting is
required. This can result in a THS stall, particularly in cases of push over when the pilot
pushes hard on the sidestick, in reaction to a significant speed decrease. This limits the
forward CG during approach.
IN FLIGHT
On a fly-by-wire aircraft, in direct law, the handling characteristics are affected by the location of
the CG, in the same way as in a mechanically controlled aircraft.
STABILITY ISSUE – THE AERODYNAMIC CENTER OR NEUTRAL POINT
In case of a perturbation or a gust, the aircraft is considered as stable, if it tends to revert toward
its previous status. The aerodynamic center or neutral point is the point where there is an
increase (or decrease) of lift, when the aircraft angle-of-attack changes.
Aircraft longitudinal stability
The gust illustrated here causes an increase in the angle-of-attack, therefore an additional lift.
The nose down moment, due to the lift increase, causes a decrease in the angle-of-attack. The
aircraft is stable.
If the CG is behind the aerodynamic center, the increase in lift creates a nose up moment, that
adds to the initial nose up moment caused by the gust. The aircraft is unstable.
The CG must be forward of the aerodynamic center for stability.
MANEUVERING CRITERIA – MANEUVER POINT
Depending on where the CG is, a deflection of the elevator causes a sharper, or less aircraft
maneuver. In other words, the CG has a direct influence on the maneuverability of the aircraft.
A330/A340 FLIGHT CREW TRAINING MANUAL
SUPPLEMENTARY INFORMATION
CENTRE OF GRAVITY
A330/A340 FLEET SI-050 P 9/10
FCTM 17 JUN 09
If a very small deflection of the elevator causes "a lot of g", the efficiency of the elevator is
very high. The aircraft is considered to be very sensitive to maneuver. The maneuver point is
the CG, for which the elevator is infinitely effective. The CG must obviously be as far forward
from the maneuver point as possible. This distance is defined by a maneuverability criteria,
that determines that "at least 1 ° of elevator deflection is required to pull 1 g load factor". This
condition defines the aft CG limit in terms of maneuverability.
　
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