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We notice that although decreasing block times, the increase of Mach number above the Optimum speed can result in significant increases in fuel burn. Pilots hence have to be patient and should not change the Mach number even when under the impression that the aircraft does not fly fast enough.
Moreover, when possible, the managed mode must be kept.
5.4 DESCENT
5.4.1 INTRODUCTION
Depending on the descent law, flight paths do vary in steepness.
Indeed, the higher the speed law, the steeper the flight path.
GD
Descent profiles.
Descents are normally performed in three phases on a constant IAS/Mach descent speed schedule, as follows:
. Constant Mach number is maintained until the crossover altitude
. Constant indicated air speed is maintained down to 10000ft
. 250 KT indicated air speed (IAS) is maintained below flight level 100, until the aircraft decelerates for landing
The engine thrust is normally set to flight idle for the descent and the speed is controlled by the aircraft attitude. In these conditions higher weights increase the descent distance because of the reduction of descent gradient (which equals [thrust-drag]/weight in stabilized flight). This also increases the descent fuel.
However a descent from high altitudes at low weight may lead to a gradient of descent that results in an excessive cabin rate of descent. In these cases the rate of descent is reduced by application of power, until a flight idle descent can be continued. This results in what is known as the re- pressurization segment, and this can reverse the weight-descent distance relationship.
To correctly evaluate the effects of descent techniques, cruise and descent flight must be viewed in relation to each other. A short descent distance for example extends the cruise distance. One has therefore to consider in addition to the descent, a small portion of the cruise to the same distance.
TOD
Cruise level
High speed
Low speed
5.4.2 THE EFFECT OF DESCENT TECHNIQUES ON FUEL BURN
An evaluation has been made of the fuel burn to a constant distance, and this now shows that the higher weights use less fuel. Lower speeds, although requiring more fuel for the descent only requires less total fuel because of the longer descent distance. This is shown in the following chart.
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