getting to grips with fuel economy(4)

. Excess Takeoff weight

. Flight Planning

. Ways of taxiing to save fuel

. Auxiliary Power Unit 

. Fuel Tankering

. Aerodynamic Deterioration

4.1 CENTER OF GRAVITY POSITION

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.
 
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.
　
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