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5.3 CRUISE
5.3.1 INTRODUCTION
The cruise phase is the most important phase regarding fuel savings. As it is the longest for long haul aircraft, it is possible to save a lot of fuel. So discipline must be exercised particularly in this phase.
The two variables that most influence cruise fuel consumption are the cruise speed (IAS or Mach Number) and the altitude or flight level. The following shows their influence on a single sector assuming standard climb and descent procedures.
The correct selection of the cruise parameters is therefore fundamental in minimizing fuel or operating cost. This chart shows the normal laws that aircraft consume less fuel when flown slower or when flown higher. However there are limits to these laws. Flying lower than the maximum range speed will increase the block fuel, as will flying higher than an optimum altitude.
5.3.2 CRUISE ALTITUDE OPTIMISATION
In examining SR changes with the altitude at a constant Mach number, it is apparent that, for each weight, there is an altitude where SR is maximum. This altitude is referred to as “optimum altitude”.
Optimum Altitude Determination at Constant Mach Number
When the aircraft flies at the optimum altitude, it is operated at the maximum lift to drag ratio corresponding to the selected Mach number.
High Speed Polar Curve
When the aircraft flies at high speed, the polar curve depends on the indicated Mach number, and decreases when Mach increases. So, for each Mach number, there is a different value of (CL/CD)max, that is lower as the Mach number increases.
M = 0.86
M = 0.82
Pressure Altitude
M <0.76
M=0.84
When the aircraft is cruising at the optimum altitude for a given Mach, CL is fixed and corresponds to (CL/CD)max of the selected Mach number. As a result, variable elements are weight and outside static pressure (Ps) of the optimum altitude. The formula expressing a cruise at optimum altitude is:
In the FCOM Flight Planning Chapters the optimum altitude is shown on the Cruise Level chart for 2 or more speeds. This chart also shows the Maximum Altitudes as limited by performance and buffet. A typical FCOM chart showing the variation of optimum altitude with weight for one speed is shown below.
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