We can notice that the difference in take off weights is all the more important with short runways and an elevated terrain. For example:
OAT 20°C
Pressure Altitude (Zp) 0ft
Runway length 3000m
CG position Basic Alternate
Takeoff Weight (kg) 266013.9 264207.8
.TOW = 1806.1kg
. Conclusion
Therefore, by using the performances calculated for the alternate CG (A340) or for the basic CG (A320), the operator can increase the aircraft takeoff weight, thus the payload or the range of the aircraft. On the other hand, the envelope is narrower, thus leading to more constraints on the loading aspect.
WEIGHT AND
BALANCE
WEIGHT AND
BALANCE
A. GENERALITIES
3.3. Impact on in-flight performance
Let‘s revert to the impact of the CG position on the forces applied on the aircraft and to the consequence it has on aircraft performance.
Reminder: In order to keep the airplane level, the Trimmable Horizontal Stabilizer (THS) creates a downward force. This additional force creates a pitch-up moment that counters the pitch-down moment due to the aircraft weight but also a drag increase which magnitude depends on the aircraft CG position.
a) AFT CG POSITION
Small balance-arm
b) FWD CG POSITION
The further forward the CG, the greater the counter moment necessary to keep the flight level.
This is due to the increasing balance arm between lift and weight.
In the case of a forward CG position, the THS is set to an aircraft nose-up position that creates
important lift degradation therefore creating important drag. This drag will lead to an increase in
fuel consumption.
The further aft the CG, the lower the fuel consumption.
A. GENERALITIES
c) Influence on fuel consumption
This part refers to the Airbus —Getting to Grips with Fuel Economy“ Brochure.
During cruise, on all the wide-body Airbus aircraft, the FCMC (Fuel Control and Management Computer) on the A330 and the A340 or the CGCC (Center of Gravity Control Computer) on the A300-600 and the A310 controls the CG position by transferring fuel in the trim tank. The aircraft therefore flies with an aft CG, thus giving better in-flight performance. However, a failure can prevent the fuel transfer and CG position control.
The following graphs show the influence of the CG position on the aircraft specific range depending on the aircraft weight. The data are expressed in Specific Range (SR) variations at cruise mach with a center of gravity of 20% and 35%. The SR with a CG position at 27 % is taken as a reference. For the other aircraft, the curves all have a similar shape as these ones:
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