getting to grips with fuel economy(18)

 
This time difference plot has the same characteristics for all Airbus aircraft, with the best time being obtained at the highest climb speed and max climb Mach number. Note that although a slow climb speed gets the aircraft to cruise altitude earlier, this requires more acceleration to cruise speed and more cruise to a given distance, making it slower overall.
The fuel difference plot characteristics vary with aircraft type. The A310, A321 and A330 show similar characteristics to the A300 with a best fuel climb speed of about 290 to 300 knots. 
The A318, A319 and A320 show better fuel burn at the lower speed range (260 to 280 knots) 
The A340 shows better fuel burn at the higher speed range (310-330 knots). 
The A310 and A340 are similar to the A300 in showing minimum fuel at a max climb Mach number of 0.78. In fact 0.8 is better for the A340-500/600. 
However the A320 family and A330 benefit from the lower Mach No of 0.76.
Thus the A320 family benefits from low climb speeds and the A340 from high climb speeds. This difference arises from the different behavior during climb of twin-engine and four-engine aircraft. Indeed, twin-engine aircraft have a higher thrust than four engine aircraft, as they must satisfy some take-off climb requirements with only one engine operative, compared with 3 engines operative on the quads. This enables them to have a higher rate of climb than four engine aircraft and reach cruise flight levels quicker.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
5.2.3 CORRELATION OF FUEL BURN & TIME WITH CLIMB TECHNIQUE

 
 
The following chart shows the differences in fuel and time to climb and cruise to a fixed distance with varying climb speed and max climb Mach number relative to the standard technique. 

 
 This chart shows that the fastest technique (330/0.82) uses the least time (-3.2%) and the most fuel (+1.5%) whereas the slowest technique (270/0.76) uses the most time (+4.5%) and nearly the most fuel (+1.4%). The least fuel is obtained using a 300/0.78 climb technique. Variation of climb technique can cause a total variation of 1.5% and climb time by 8% for this aircraft.
 
Also plotted on the charts are lines representing the speeds selected by the FMGS for various cost indices (CI). The left hand point of each line represents a CI of zero (fuel cost priority) and the right hand point represents a CI of 100 (flight time priority). It should be noted how the FMGS line approximates to the lower boundary of the time - fuel difference plot.
 
The chart on the following page is for the A320 and shows completely different characteristics. 
　
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