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In order to assess the overall impact of CG variation on fuel burn, it must be
assessed on a complete sector. The following table shows increases in fuel
consumed with a more forward CG. It is expressed as kg per 1000nm sector per
10% more forward CG for the max variation case (high weight, high flight level)
with no in flight CG shift. The fuel increment in kg is also given for the Forward
(20%) position, compared with the Aft (35 or 37%) position, for a typical sector.
Fuel Burn Increase with a more Forward CG
Aircraft types Fuel increment
KG/1000nm/10%CG
Typical Sector distance
(nm)
Fuel increment per
sector (kg)
A300-600 240 2000nm 710
A310 110 2000nm 330
A319/A320/A321 Negligible 1000nm Negligible
A330-200 70 4000nm 480
A330-300 90 4000nm 600
A340-200 90 6000nm 900
A340-300 80 6000nm 800
A340-500 150 6000nm 1550
A340-600 130 6000nm 1300
Getting to grips with Fuel Economy 4 - PRE-FLIGHT PROCEDURES
- 11 -
4.2 TAKEOFF WEIGHT
4.2.1 INTRODUCTION
Another way to save fuel is to avoid excess take-off weight, which consists of
the operating empty weight of the aircraft plus the payload plus the fuel.
In addition accurate knowledge of weight is an important factor needed to
ensure that fuel burn predictions are met. This gives pilots confidence in the flight
plans thus reducing the tendency to carry excess fuel.
4.2.2 OVERLOAD EFFECT
The specific range, flying at given altitude, temperature and speed depends
on weight. The heavier the aircraft, the higher the fuel consumption.
In addition, fuel savings can be made during climb since the aircraft would
reach its optimal flight level earlier if it were lighter.
The effect of overloading with respect to the in-flight weight is shown on the
following graph, for an excess load of 1% of MTOW (2600kg) in cruise for an
A340-313 This shows the increase in specific range penalty with both weight and
altitude. Maximum and optimum altitudes are shown together with selected sub
optimum flight levels representing the choice of a FL below the Optimum instead
of above it. For example, at 220t the optimum altitude is just under FL 350. If we
select FL 330 1% extra MTOW will decrease the specific range by just under 1.2%
Specific Range Penalty for Excess Weight of 1% MTOW
A340-313 ISA MN 0.82
0.00%
0.50%
1.00%
1.50%
2.00%
2.50%
120 140 160 180 200 220 240 260 280 300
Aircraft Weight - Tonnes
Decrease in SR
Optimum
Altitude
Maximum
Altitude
Selected Sub
Optimum FL
FL410 FL390
FL370
FL350 FL330
FL310
FL290
4 - PRE-FLIGHT PROCEDURES Getting to grips with Fuel Economy
- 12 -
The characteristic curves for the other aircraft types have a similar shape.
Calculating the weight effect on specific range on all Airbus aircraft in accordance
with the lower boundary of typical flight levels gives an average reduction of 1%
of SR for a weight increase of 1% of Maximum Take-off Weight. The scatter in this
value is generally within .2%.
At the higher altitudes, obtainable at lower weights, the previous picture shows
that the SR reduction can increase to 1.5%
Overloading affects not only the trip fuel but also the reserves and requires
increased fuel uplift for a specific mission. The following table shows the effect of
1 tonne/1000nm and also 1% of basic MTOW for a typical sector, both at optimum
altitude, assuming maximum passengers and some freight.
Although the A320 family show considerably lower fuel burn penalties than
the other aircraft, the total fuel penalty is of a similar order due to the high
number of sectors per day. It can readily be seen that a 1% weight penalty has a
significant impact on fuel costs when looked at on a yearly basis for a fleet of
aircraft.
4.2.3 AIRCRAFT OPERATING WEIGHT
The operating empty weight of an aircraft is defined as the manufacturer’s
weight empty plus the operator’s items. The latter include the flight and cabin
crew and their baggage, unusable fuel, engine oil, emergency equipment, toilet
chemicals and fluids, galley structure, catering equipment, seats, documents, etc.
The OEW of new aircraft, even in the same fleet, can vary significantly, due
to specification changes, build differences and normal scatter. Also aircraft
Aircraft
types
Payload Weight
Increase
Stage Fuel Penalty
1000nm/t
Fuel penalty
per sector
Extra
Reserves
A300-600 31000 kg 1705 kg 2000 Nm 93 kg 320 kg 100 kg
A310-300 26560 kg 1500 kg 2000 Nm 59 kg 240 kg 90 kg
A318 14650 kg 640 kg 1000 Nm 31 kg 30 kg 30 kg
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