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4.1.1 INTRODUCTION
The gross weight is the sum of the dry operating weight, payload and fuel
and acts as one force through the center of gravity (CG) of the aircraft. The
balance chart allows the determination of the overall center of gravity of the
airplane taking into account the center of gravity of the empty aircraft, the fuel
distribution and the payload. It must be ensured that the center of gravity is
within the allowable range referred to as the center of gravity envelope.
A more forward center of gravity requires a nose up pitching moment
obtained through reduced tail plane lift, which is compensated for by more wing
lift. This creates more induced drag and leads to an increase in fuel consumption.
It is better to have the center of gravity as far aft as possible. As a rearward shift
in CG position deteriorates the dynamic stability of the aircraft, the CG envelope
defines an aft limit.
4.1.2 AUTOMATIC CENTER OF GRAVITY MANAGEMENT
AIRBUS has created a trim tank transfer system that controls the center of
gravity of the airplane. This system is installed on some A300 and A310 aircraft
and all A330 and A340 aircraft. When an airplane with a trim tank is in cruise, the
system optimizes the center of gravity position to save fuel by reducing the drag
on the airplane. The system transfers fuel to the trim tank (aft transfer) or from
the trim tank (forward transfer). This movement of fuel changes the center of
gravity position. The crew can also manually select forward fuel transfer.
The Fuel Control and Management Computer (FCMC) calculates the center of
gravity of the airplane from various parameters including input values (Zero Fuel
Weight or Gross Take-off Weight and the associated CG) and the fuel tank
contents. It continuously calculates the CG in flight. From this calculation, the
FCMC decides the quantity of fuel to be moved aft or forward in flight to maintain
the CG between the target value and 0.5% forward of the target band.
Usually one initial aft fuel-transfer is carried out late in the climb to bring the
CG within this band. During the flight there are several smaller forward
movements as the fuel burn moves the CG more aft. Finally a forward transfer is
made as the aircraft nears its destination to bring the CG within the landing CG
range.
Getting to grips with Fuel Economy 4 - PRE-FLIGHT PROCEDURES
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4.1.3 INFLUENCE ON FUEL CONSUMPTION
The following graph shows the change in fuel consumption, expressed in
terms of specific range (nm per kg of fuel), for both a Forward (20%) and an Aft
(35%) CG position compared to a mid CG position of 27% at cruise Mach.
This graph, which is for the A310-203, shows the advantage of flying at aft
CG. Also shown are the optimum altitude lines and these show the effects of CG
to be constant at these altitudes, with almost no variation with aircraft weight.
Other aircraft have similar shape curves with similar optimum altitude
characteristics (except the A320 family). The following table summarizes the
effect of CG on specific range at the optimum altitude :
Aircraft Type Aft CG(35-37%) Fwd CG(20%)
A300-600 +1.7% -0.9%
A310 +1.8% -1.8%
A330 +0.5% -1.3%
A340 +0.6% -0.9%
For the A300/A310 reference CG is 27% and aft CG is 35%.
For the A330/A340 reference CG is 28% and aft CG is 37%.
Specific Range variation with CG position
-3%
-2%
-1%
0%
1%
2%
3%
290 310 330 350 370 390 410 430
Flight Level
SR Variation
140t
140t
130t
130t
110t
110t
90t
90t
Weight 35%
27%
20%
CG
OPT FL
OPT FL
4 - PRE-FLIGHT PROCEDURES Getting to grips with Fuel Economy
- 10 -
At maximum altitude, the change in fuel consumption given in the table is
larger by up to 1%. However no benefit is obtained, as the specific range (SR) is
lower at aft CG at maximum altitude than at mid CG at optimum altitude.
For aircraft that are not fitted with automatic center of gravity management,
not all these advantages may be realized because of the normal forward and
rearward shift of CG in flight due to fuel burn. In addition loading these aircraft at
max fuel to an aft CG could prove difficult.
The A320 family does not show the same SR variation with CG as the other
aircraft. The aft CG produces worst SR at FL290, crossing over to show an
improvement at higher flight levels. The SAR variation is much smaller also. This
is due to a complex interaction of several aerodynamic effects. The SAR can be
considered effectively constant with CG position. Loading is therefore not critical
for fuel economy for the A320 family.
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