getting to grips with fuel economy(27)

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The effect of the wind on trip time and fuel is shown on the following chart, which gives fuel consumption and time for a 2000nm sector, with respect to flight levels, Mach number and wind (tailwind positive) for a fixed take-off weight.

 
 This plot graphically shows the magnitude of the significant changes in fuel consumption and time due to winds. FCOM Tables show the equivalent still air distances for any ground distance/wind combination.
However the winds can affect the performance optimization as well as changing the effective still air distance. The MMR (or MLRC) value varies with headwind or tailwind, due to changes in the SR.
The effect of a tailwind is to increase the ground speed, and therefore the SR, by the ratio of ground speed to airspeed. A given wind speed therefore has a larger effect at the lower airspeeds, which changes the optimum speed.
 
 
 
The following chart shows the Maximum Range Mach number versus wind variations.
 
 

 
MMR and wind influence
 
 
This shows that
 
Tailwinds    increase  the specific range and lower the speeds 
Headwinds decrease the specific range and raise the speeds.
 
 The wind speed can be different at different altitudes. For a given weight, when cruise altitude is lower than optimum altitude, the specific range decreases. Nevertheless, it is possible that, at a lower altitude with a favorable wind, the ground specific range improves. When the favorable wind difference between the optimum altitude and a lower one reaches a certain value, the ground-specific range at lower altitude is higher than the ground-specific range at optimum altitude. As a result, in such conditions, it is more economical to cruise at the lower altitude.
 
 There is information in the most FCOM’s (not A300/310) to indicate the amount of favorable wind, necessary to obtain the same ground-specific range at altitudes different from the optimum. If the wind is more favorable then it is beneficial to fly lower. The following shows such a page: 
Given weightand PressureAltitude

 
 

 
 
 
 
 
5.3.5  MANAGED MODE

The flight management system (FMS) optimizes the flight plan for winds, operating costs and suggests the most economical cruise altitude and airspeed, depending on the cost index chosen by the airline. An airline that wants to save fuel has to choose a low cost index. The next part intends to highlight the impact of the cost index on fuel consumption and on trip time. More complete information can be found in the “Getting to Grips with Cost Index” brochure.
5.3.5.1 ECONOMIC MACH NUMBER

 
 Long-range Cruise Mach number was considered as a minimum fuel regime. If we consider the Direct Operating Cost instead, the Economic Mach number (MECON), can be introduced.
　
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