getting to grips with fuel economy(6)

 
 
 
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%

 
 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.
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
 
　
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